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Estimate of the aerodynamic roughness parameters over an incomplete canopy cover of cotton
Abstract
Wind speed and temperature data at five levels were used to estimate the roughness length, z(om), and displacement height, d(o), for a sparsely covered cotton field with an average plant height of 32 cm and 1-m width furrows approximately 16 cm in depth. Five near-neutral cases produced an estimate of z(om) = 0.066 m (+/- 0.043) and d(o) = 0.31 m (+/- 0.20). In terms of obstacle height, h (i.e., furrows and plants), this yields z(om)/h approximately =0.14 and d(o)/h approximately =0.65. Confidence in the estimates of the roughness parameters was obtained with a comparison of the sensible heat flux, H, determined by K-theory relationships for gradients of wind speed and temperature in the surface layer and by measurements using the eddy correlation technique. The root mean square error was approximately 30 W m−2 between the two methods.
... But an excess resistance (kB -1 ) due to the replacement of aerodynamic temperature by a radiometric one is often used as a corrective term along with the combination equations in estimating the H. This "radiometric" kB -1 is different from its aerodynamic counterpart and has been a subject of several studies in the past (e.g., Kustas et al., 1989;Lhomme et al., 1994aLhomme et al., , b, 1997Lhomme et al., , 2000Chehbouni et al., 1997;Troufleau et al., 1997). ...
... 2 Theoretical background and literature review 50 Kustas et al. (1989), in a study of the energy balance of sparse vegetation composed of bushes and bare soil in arid climate using airborne infrared thermometer observations, concluded that the value of kB -1 may range from 1 to 10. They proposed parameterization of the excess resistance, kB -1 , as a function u z (T r -T a ). ...
... They proposed parameterization of the excess resistance, kB -1 , as a function u z (T r -T a ). Troufleau et al. (1997), however, found that the parameterization of kB -1 as proposed by Kustas et al. (1989) to be valid only for high H values. Stewart et al. (1994) also reported values of kB -1 ranging from 3.4 to 12.4 for eight semiarid sites. Lhomme et al. (1994a, b), on the other hand, used a statistical relationship that linked radiative-air temperature differences to soil-foliage temperature differences in order to estimate H over sparse millet crop and Sahelian fallow savannah. ...
... Some studies, however, showed that there could be large differences between T r and T o . This problem becomes critical especially when H has to be estimated using radiometric surface temperature measurements over sparse or heterogeneous canopies (Choudhury et al., 1986;Huband and Monteith, 1986) leading to erroneous, usually greater, estimates of H (Kustas et al., 1989;Kalma and Jupp, 1990;Stewart et al., 1994) for unstable conditions. This has led some researchers to conclude that the radiometric surface temperature measurements are not adequate to result in reasonably accurate estimates of H (e.g., Hall et al., 1992). ...
... This r r arises due to the replacement of T o by T r and is parameterized as a function of a dimensionless factor kB −1 . This 'radiometric' kB −1 has been a subject of several studies (e.g., Kustas et al., 1989;Stewart et al., 1994;Lhomme et al., 1997Lhomme et al., , 2000Troufleau et al., 1997). Little, if any, convergence has been reached on a commonly-accepted way of determining the 'radiometric' kB −1 and it remains largely a fitting parameter . ...
... Little, if any, convergence has been reached on a commonly-accepted way of determining the 'radiometric' kB −1 and it remains largely a fitting parameter . Kustas et al. (1989), in a study of the energy balance for sparse vegetation composed of bushes and bare soil in an arid climate using airborne infrared thermometer observations, concluded that kB −1 may range from 1 to 10. They proposed parameterization of the excess resistance, kB −1 , as a function of u z (T r -T a ), where u z is the horizontal wind speed at the canopy surface and T a the air temperature. ...
Competition for fresh water between agriculture and domestic and industrial uses is increasing worldwide. This is forcing subsistence and commercial agriculture to produce more with less water. Consequently, it is crucial to properly and efficiently manage water resources. This requires accurate determination of crop water loss into the atmosphere, which is greatly influenced by the exchange of energy and mass between the surface and the atmosphere. Measurement of these exchange processes can best be accomplished by micrometeorological methods. However, most micrometeorological methods are very expensive, difficult to set up, require extensive post-data collection corrections and/or involve a high degree of empiricism. This review discusses estimation of evapotranspiration using relatively inexpensive micrometeorological methods in temperature-variance (TV), surface renewal (SR) and mathematical models. The TV and SR methods use high frequency air temperature measurements above a surface to estimate sensible heat flux (H). The latent heat flux (λE), and hence evapotranspiration, is calculated as a residual of the shortened surface energy balance using measured or estimated net radiation and soil heat flux, assuming surface energy balance closure is met. For crops with incomplete cover, the disadvantage of these methods is that they do not allow separation of evapotranspiration into soil evaporation and plant transpiration. The mathematical models (single- and dual-source) involve a combination of radiation and resistance equations to determine evapotranspiration from inputs of automatic weather station observations. Single-source models (Penman-Monteith type equations) are used to determine evapotranspiration over homogeneous surfaces. The dual-source models, basically an extension of single-source models, determine soil evaporation and plant transpiration separately over heterogeneous or sparse vegetation. These mathematical models have also been modified to accommodate inputs of remotely-sensed radiometric surface temperatures that enable estimation of evapotranspiration on a regional and global scale.
... Most algorithms 2 K.L. Driese, W.A. Reiners/Agricultural and Forest Meteorology 88 (1997) [1][2][3][4][5][6][7][8][9][10][11][12][13][14] used by models to describe turbulent transfer of heat and moisture use empirically derived estimates of two parameters, roughness length (zO) and displacement height (d), that describe the roughness characteristics of a surface (Abtew et al., 1989), but estimates of these parameters for sparsely vegetated natural vegetation communities are scarce and difficult to obtain. While a relatively rich literature describes turbulence above and within crop canopies ( i.e., Jacobs and Van Boxel, 1988;Kustas et al., 1989), fewer studies (i.e., Menenti and Ritchie, 1994) have examined less homogeneous natural systems. The result is that models incorporating natural land surfaces often use somewhat crude estimates of aerodynamic roughness parameters which in many cases are extrapolated from crop studies or are based on a few data points. ...
... Alternatively, some researchers have used relationships between easily measured characteristics of the roughness elements (e.g., plant height, spacing) to estimate d and/or z,,. Values of d/h, (h, = canopy height cm)) from literature range from 0.35 for cotton (Hatfield, 1989) to a range of 0.61-0.80 for maize (Uchijima and Wright, 1964;Jacobs and Van Boxel, 1988;Kustas et al., 1989). Mathias et al. (1990) report a rough estimate of 0.66 from a review of literature. ...
... Relationships between z0 and plant characteristics are more complex, and less consistent in the literature. Menenti and Ritchie (1994) report z,, = 0.33 hko7 for a natural semi-arid system while others report zo/h, values of 0.04 for sparse sorghum (Azevedo and Verma, 19861, 0.14 for small cotton (Kustas et al., 1989) and 0.8 for cotton of intermediate foliage density (Hatfield, 1989). Mathias et al. (1990) and Garratt (1992) report z,,/h, = 0.1 as a rough estimate from a review of literature although Garratt notes that the ratio can range from 0.02 to 0.2 for natural surfaces based on an extensive review of the literature. ...
Estimates of aerodynamic roughness length (z0) were calculated at nine sites for natural sagebrush (Artemisia tridentata spp.), saltbush (Atriplex nuttallii) and greasewood (Sarcobatus vermiculatus) plant communities in two semi-arid basins in Wyoming, USA. Estimates were based on wind and temperature profiles measured above the plant canopies during summer (August) of 1994 and fall (September and October) of 1995. Values of z0 were estimated for periods of near-neutral stability (fully forced convection) for four scenarios of displacement height (d) at each site, and for a fifth scenario based on using the low level drag coefficient for constraining friction velocity (). Vegetation canopy height, fractional cover, shrub density, average canopy area (per shrub), and average biomass/area were measured for all sites. Additionally, leaf and plant area indices (LAI, PAI) were measured at three of the sites, and average shrub height was measured at the six sites. Roughness lengths calculated using an iterative method with unconstrained averaged 0.01 m for the saltbush sites, 0.02 m for the sagebrush sites, and 0.07 m for the greasewood site. z0/canopy height ratios () averaged 0.04, 0.04 and 0.13, respectively, for the three shrub types, although the average is misleading for saltbush, which showed considerable variation between sites. Roughness length appears to be related to shrub structure, as expressed by the dominant species, and by shrub density at the sites, although differences were large depending on which calculation method was used. When was constrained, calculated z0 and were smaller and less consistent in terms of relationships to vegetation structure, suggesting that further constraints on the iterations may be necessary. The results highlight the importance of improving aerodynamic roughness parameterization of natural vegetation communities.
... Soil heat flux was assumed to be only a fraction of solar irradiance reaching the soil surface, and this fraction was diurnally varied. Kustas et al. (1989) found that between 9:00 to 16:00 hours the ratio of soil heat flux to solar irradiance reaching the soil surface was 0.35. Jones (1991) stated that the ratio of soil heat flux to solar irradiance could vary from 0.02 for dense canopies to more than 0.30 for sparse canopies. ...
... It would be difficult to characterise the large variability of microclimatological parameters within canopies by only using the meteorological data collected above the canopies as was done in this study. Some studies indicated that air-flow and temperature distributions for row crops were likely to differ from profiles in vegetation randomly distributed in aerial space (Arkin and Perrier, 1974;Graser et al., 1987), especially when furrow depth (height) and plant spacing was large relative to vegetation height and amount of cover (Graser et al., 1987;Kustas et al., 1989). ...
The primary purpose of this study was to model the partitioning of evapotranspiration in a maize-sunflower intercrop at various canopy covers. The Shuttleworth-Wallace (SW) model was extended for intercropping systems to include both crop transpiration and soil evaporation and allowing interaction between the two. To test the accuracy of the extended SW model, two field experiments of maize-sunflower intercrop were conducted in 1998 and 1999. Plant transpiration and soil evaporation were measured using sap flow gauges and lysimeters, respectively. The mean prediction error (simulated minus measured values) for transpiration was zero (which indicated no overall bias in estimation error), and its accuracy was not affected by the plant growth stages, but simulated transpiration during high measured transpiration rates tended to be slightly underestimated. Overall, the predictions for daily soil evaporation were also accurate. Model estimation errors were probably due to the simplified modelling of soil water content, stomatal resistances and soil heat flux as well as due to the uncertainties in characterising the 2 micrometeorological conditions. The SW's prediction of transpiration was most sensitive to parameters most directly related to the canopy characteristics such as the partitioning of captured solar radiation, canopy resistance, and bulk boundary layer resistance.
... Classically, both parameters are estimated from multi-level measurements of wind speed over a homogeneous surface (Lo 1976;Garratt 1978;de Bruin and Moore 1985;Kustas et al. 1989). This led to a body of literature summarizing values for different surfaces (Wieringa 1993). ...
... Jacobs and van Boxel (1988) found z 0 = 0.26(h − d) for a maize canopy. However, these rules of thumb should be modified if the surface of interest is not dense and uniform, in which case a site-specific determination is required (Kustas et al. 1989;Lloyd et al. 1992). ...
We applied three approaches to estimate the zero-plane displacement
$d$
through the aerodynamic measurement height
$z$
(with
$z = z_{m}- d$
and
$z_{m}$
being the measurement height above the surface), and the aerodynamic roughness length
$z_{0}$
, from single-level eddy covariance data. Two approaches (one iterative and one regression-based) were based on the universal function in the logarithmic wind profile and yielded an inherently simultaneous estimation of both
$d$
and
$z_{0}$
. The third approach was based on flux–variance similarity, where estimation of
$d$
and consecutive estimation of
$z_{0}$
are independent steps. Each approach was further divided into two methods differing either with respect to the solution technique (profile approaches) or with respect to the variable (variance of vertical wind and temperature, respectively). All methods were applied to measurements above a large, growing wheat field where a uniform canopy height and its frequent monitoring provided plausibility limits for the resulting estimates of time-variant
$d$
and
$z_{0}$
. After applying, for each approach, a specific data filtering that accounted for the range of conditions (e.g. stability) for which it is valid, five of the six methods were able to describe the temporal changes of roughness parameters associated with crop growth and harvest, and four of them agreed on
$d$
to within 0.3 m most of the time. Application of the same methods to measurements with a more heterogeneous footprint consisting of fully-grown sugarbeet and a varying contribution of adjacent harvested fields exhibited a plausible dependence of the roughness parameters on the sugarbeet fraction. It also revealed that the methods producing the largest outliers can differ between site conditions and stability. We therefore conclude that when determining
$d$
for canopies with unknown properties from single-level measurements, as is increasingly done, it is important to compare the results of a number of methods rather than rely on a single one. An ensemble average or median of the results, possibly after elimination of methods that produce outliers, can help to yield more robust estimates. The estimates of
$z_{0}$
were almost exclusively physically plausible, although
$d$
was considered unknown and estimated simultaneously with the methods and results described above.
... Classically, both parameters are estimated from multi-level measurements of wind speed over a homogeneous surface (Lo 1976;Garratt 1978;de Bruin and Moore 1985;Kustas et al. 1989). This led to a body of literature summarizing values for different surfaces (Wieringa 1993). ...
... Jacobs and van Boxel (1988) found z 0 = 0.26(h − d) for a maize canopy. However, these rules of thumb should be modified if the surface of interest is not dense and uniform, in which case a site-specific determination is required (Kustas et al. 1989;Lloyd et al. 1992). ...
The displacement height d and roughness length z0 are parameters of the
logarithmic wind profile and as such these are characteristics of the
surface, that are required in a multitude of meteorological modeling
applications. Classically, both parameters are estimated from
multi-level measurements of wind speed over a terrain sufficiently
homogeneous to avoid footprint-induced differences between the levels.
As a rule-of thumb, d of a dense, uniform crop or forest canopy is 2/3
to 3/4 of the canopy height h, and z0 about 10% of canopy height in
absence of any d. However, the uncertainty of this rule-of-thumb becomes
larger if the surface of interest is not "dense and uniform", in which
case a site-specific determination is required again. By means of the
eddy covariance method, alternative possibilities to determine z0 and d
have become available. Various authors report robust results if either
several levels of sonic anemometer measurements, or one such level
combined with a classic wind profile is used to introduce direct
knowledge on the friction velocity into the estimation procedure. At the
same time, however, the eddy covariance method to measure various fluxes
has superseded the profile method, leaving many current stations without
a wind speed profile with enough levels sufficiently far above the
canopy to enable the classic estimation of z0 and d. From single-level
eddy covariance measurements at one point in time, only one parameter
can be estimated, usually z0 while d is assumed to be known. Even so,
results tend to scatter considerably. However, it has been pointed out,
that the use of multiple points in time providing different stability
conditions can enable the estimation of both parameters, if they are
assumed constant over the time period regarded. These methods either
rely on flux-variance similarity (Weaver 1990 and others following), or
on the integrated universal function for momentum (Martano 2000 and
others following). In both cases, iterations over the range of possible
d values are necessary. We extended this set of methods by a
non-iterative, regression based approach. Only a stability range of data
is used in which the universal function is known to be approximately
linear. Then, various types of multiple linear regression can be used to
relate the terms of the logarithmic wind profile equation to each other,
and derive z0 and d from the regression parameters. Two examples each of
the two existing iterative approaches, and the new non-iterative one are
compared to each other and to plausibility limits in three different
agricultural crops. The study contains periods of growth as well as of
constant crop height, also allowing for an examination of the relations
between z0, d, and canopy height. Results indicate that estimated z0
values, even in absence of prescribed d values, are fairly robust,
plausible and consistent across all methods. The largest deviations are
produced by the two flux-variance similarity based methods. Estimates of
d, in contrast, can be subject to implausible deviations with all
methods, even after quality-filtering of input data. Again, the largest
deviations occur with flux-variance similarity based methods. Ensemble
averaging between all methods can reduce this problem, offering a
potentially useful way of estimating d at more complex sites where the
rule-of-thumb cannot be applied easily. Martano P (2000): Estimation
of surface roughness length and displacement height from single-level
sonic anemometer data. Journal of Applied Meteorology 39:708-715.
Weaver HL (1990): Temperature and Humidity flux-variance relations
determined by one-dimensional eddy correlation. Boundary-Layer
Meteorology 53:77-91.
... The formulation of H using the definition of T o requires an additional resistance called the excess resistance and denoted by r r in Eq. (12). Following many authors [49,50], it is surmised that the aerodynamic resistance to heat transfer (r ah ) is greater than aerodynamic resistance for momentum transfer (r a ). Consequently, the roughness length for heat transfer (z oh ) is lower than the roughness length for momentum transfer (z om ). ...
... The z om depends on various factors such as wind direction, vegetation height, canopy cover, vegetation type, and row spacing. Estimating these factors using an empirical equation as a function of NDVI might be an over simplification; however, such estimates are reasonably accurate for uniform cover and fairly flat terrains [50]. Although remote sensing observations provide vegetation information, estimation of roughness height remains a challenge for regional modeling of turbulent transport because of highly variable topographic and canopy structures, and wind behaviors. ...
... d and z 0 can be estimated from the wind profile measurement data (Robinson, 1962; Stearns, 1970; Lo, 1977; Kanemasu et al., 1979) Most of the researchers estimated d and z 0 from the data measured under " near neutral stability " conditions and assumed that the estimated d and z 0 values were also valid under stable and unstable conditions (e.g. Hatfield, 1989; Kustas et al, 1989b; Matthias et al, 1990; Jacovides et al, 1992; Tolk et al, 1995; Sauer et al, 1996; Driese and Reiners, 1997). At natural situations, however, most of the atmospheric stratifications were either unstable or stable and appeared neutral only at the transitional period. ...
... Thom (1975) suggested that the neutral stability (fully forced convection) exists only when Ri ≤ 0.01 (Ri is the Richardson Number). Some researchers used Ri ≤ 0.015 as the criterion for neutral stability (Kustas et al., 1989; Mathias et al., 1990). A more relaxed criterion of Ri ≤ 0.1 was used by Hatfield et al. (1985) and Driese and Reiners (1997). ...
... This process is critical for the subsequent numerical calculation of H in SEB models that use dT, as its accuracy is closely related to quantifying the energy balance at the hot and cold endmembers (Trezza, 2006;Allen et al., 2007;Singh and Irmak, 2011;Singh et al., 2012). Secondly, roughness characteristics near the surface where the heat fluxes originate are parameterised by z0m, which depends on several factors, such as wind direction, height and type of the vegetation cover (Kustas et al., 1989b). Estimation of z0m only with an exponential relationship, as a function of vegetation indices, may be an oversimplification (Kustas et al., 1989a;Paul et al., 2013). ...
... where the ratio of soil heat flux to net radiation Γ c = 0.315 for a full-vegetation canopy [37] and Γ s = 0.05 for bare soil [38]. An interpolation was conducted between these limiting cases by applying the fractional canopy coverage, f c . ...
Water-use efficiency (WUE) is a crucial physiological index in carbon–water interactions and is defined as the ratio of vegetation productivity to water loss. The variation in climatic variables and drought have the most significant effects on WUE and evapotranspiration (ET). Nevertheless, how WUE varies with climate factors and drought processes in the Tianshan Mountains (TMS) is still poorly understood. In the present work, we analyzed the spatiotemporal variations in WUE, and investigated the correlations between WUE, climate factors, and drought, in the study area. The results showed that, in the TMS during 2000–2020, annual net primary productivity (NPP) ranged from 147.9 to 189.4 gC·m−2, annual ET was in the range of 212.5–285.8 mm, and annual WUE ranged from 0.66 to 0.78 gC·kg−1·H2O. Both NPP and ET exhibited an increasing trend with some fluctuation, whereas WUE showed the opposite tendency during the study period. The obtained results demonstrated that the decrease in WUE was primarily because of the increase in ET. There were obvious differences in WUE, under different land-use types, caused by NPP and ET. However, the interannual variation in WUE showed small fluctuations and the dynamic process of WUE in each land-use type showed good consistency. Temperature and wind speed had a positive influence on WUE in the middle and eastern regions of the TMS. Precipitation also played a mainly positive role in enhancing WUE, especially on the northern slope of the TMS. There was strong spatial heterogeneity of the correlation coefficient (0.68, p < 0.05) between WUE and the temperature vegetation drought index (TVDI). Moreover, the slopes of WUE and TVDI showed good consistency in terms of spatial distribution, suggesting that drought had a significant impact on ecosystem WUE. This work will enhance the understanding of WUE variation, and provide scientific evidence for water resource management and sustainable utilization in the study area.
... This process is critical for the subsequent numerical calculation of H in SEB models that use dT, as its accuracy is closely related to quantifying the energy balance at the hot and cold endmembers (Trezza, 2006;Allen et al., 2007;Singh and Irmak, 2011;Singh et al., 2012). Secondly, roughness characteristics near the surface where the heat fluxes originate are parameterised by z0m, which depends on several factors, such as wind direction, height and type of the vegetation cover (Kustas et al., 1989b). Estimation of z0m only with an exponential relationship, as a function of vegetation indices, may be an oversimplification (Kustas et al., 1989a;Paul et al., 2013). ...
... Values of the zero plane displacement (d) and roughness length (z 0 ) for the experimental crop were selected from previous studies for a variety of canopies. For a sparse cotton field, when the plant height was 0.67 m, z 0 and d were suggested to be 0.36 h and 0.3 h [50]. However, h cannot be the only vegetative characteristic, particularly for sparse crop rows [51]. ...
Accurate classification of multilayered plants is vital to understanding the interaction of each canopy in a greenhouse environment and designing plant models based on the irradiation, canopy temperature, transpiration, and heat flux by the leaf area index (LAI). Based on the measurements from a greenhouse in operation, plant models for each LAI are discussed in this study. If the heat flux between plants and air can be accurately predicted through plant models using LAI, the heating and cooling load in various virtual greenhouses with densely planted crops can be predicted. To enhance the measurement accuracy, a temperature and humidity sensor with an aspirated shield, an infrared canopy sensor, and CO2 sensor were installed. The plant environment was measured with a portable pyranometer, porometer, ceptometer, and anemometer. The measurements were inputted to the plant models, and the canopy temperature was calculated. The canopy temperature from the models was evaluated for reliability by comparing it with field measurements (R2 = 0.98 and RMSE = 0.46). The results indicated that the big leaf model is suitable when the air circulation layer is larger than the canopy size, but when physical properties of the plant change band affect the LAI, as in a greenhouse, a multi-layer model should be considered.
... Single-source or bulk transfer schemes for modeling sensible heat flux (H) often employ an additional resistance term (R ex ) to relate T 0 to T s [18][19][20]. Appropriately calibrated, one-source models have shown satisfactory estimates of surface energy fluxes [21][22][23]. ...
... Single-source or bulk transfer schemes for modeling sensible heat flux (H) often employ an additional resistance term (R ex ) to relate T 0 to T s [18][19][20]. Appropriately calibrated, one-source models have shown satisfactory estimates of surface energy fluxes [21][22][23]. ...
... Single-source or bulk transfer schemes for modelling H treat soil and canopy as a single flux source and often employ an additional resistance term (R AH , usually dependent on the Stanton number kB −1 ) because heat transport is less efficient than momentum transport from land surface (see e.g., Garratt and Hicks [29] or Verhoef et al. [30]). Appropriately calibrated, one-source energy balance (OSEB) models have shown satisfactory estimates of surface energy fluxes in heterogeneous landscapes [31][32][33][34]. However, due to the difficulty in robustly and parsimoniously parametrizing R AH for OSEB schemes at different landscapes, climates, and observational configurations [35], the two-source energy balance (TSEB) modelling approach was developed [36]. ...
The Sentinel-2 and Sentinel-3 satellite constellation contains most of the spatial, temporal and spectral characteristics required for accurate, field-scale actual evapotranspiration (ET) estimation. The one remaining major challenge is the spatial scale mismatch between the thermal-infrared observations acquired by the Sentinel-3 satellites at around 1 km resolution and the multispectral shortwave observations acquired by the Sentinel-2 satellite at around 20 m resolution. In this study we evaluate a number of approaches for bridging this gap by improving the spatial resolution of the thermal images. The resulting data is then used as input into three ET models, working under different assumptions: TSEB, METRIC and ESVEP. Latent, sensible and ground heat fluxes as well as net radiation produced by the models at 20 m resolution are validated against observations coming from 11 flux towers located in various land covers and climatological conditions. The results show that using the sharpened high-resolution thermal data as input for the TSEB model is a sound approach with relative root mean square error of instantaneous latent heat flux of around 30% in agricultural areas. The proposed methodology is a promising solution to the lack of thermal data with high spatio-temporal resolution required for field-scale ET modelling and can fill this data gap until next generation of thermal satellites are launched.
... The aerodynamic resistance (ra) is defined by the variation of the wind speed above a rough surface. It is determined according to a general model often used by climatologists to describe the wind velocity profile at the upper boundary layer of a plant canopy [41,42]. According to Ref. [28], the aerodynamic resistance (ra) which takes into account the very small deviations between the ambient air temperature and the foliage temperature can be calculated by equation (18): ...
This article presents the numerical simulation of the energy effect of a solar mask created by a green wall on the thermal performance of a building envelope in a temperate climate in order to integrate it into thermal calculation codes. The document describes primarily the adaptation of a simplified prediction model. The simplified model was developed and validated on the basis of tests carried out with an experimental controlled occultation device by integrating all external parameters influencing heat exchanges (conduction, convection and radiation). A first validation of the numeric resolution algorithm of the preliminary model was realised. Then the model was supplemented by considerations related to the characteristics of the selected plants: ivy and Virginia creeper, such as the semi-transparent appearance of foliage plants and their geometric distribution. The numerical prediction was very close to the experimental values, and the mean error was about 1 °C. This difference makes it possible to validate the developed model.
... Linearity between these geometrical parameters (h v , d 0 and z 0m ) is always retrieved in the formulations given by Moran or Brutsaert [20,38], or also by Su et al. [18], among others [19,59,60]. Moreover, Su et al. [18] demonstrated that the Massman's [9] kB −1 model is particularly sensitive to the vegetation height, with a relative error up to 46% of the mean measured H when using 150% of the h v reference value. ...
The parameterization of heat transfer based on remote sensing data, and the Surface Energy Balance System (SEBS) scheme to retrieve turbulent heat fluxes, already proved to be very appropriate for estimating evapotranspiration (ET) over homogeneous land surfaces. However, the use of such a method over heterogeneous landscapes (e.g., semi-arid regions or agricultural land) becomes more difficult, since the principle of similarity theory is compromised by the presence of different heat sources at various heights. This study aims to propose and evaluate some models based on vegetation geometry partly developed by Colin and Faivre, to retrieve the surface aerodynamic roughness length for momentum transfer ( z 0 m ), which is a key parameter in the characterization of heat transfer. A new approach proposed by the authors consisted in the use of a Digital Surface Model (DSM) as boundary condition for experiments with a Computational Fluid Dynamics (CFD) model to reproduce 3D wind fields, and to invert them to retrieve a spatialized roughness parameter. Colin and Faivre also applied the geometrical Raupach’s approach for the same purpose. These two methods were evaluated against two empirical ones, widely used in Surface Energy Balance Index (SEBI) based algorithms (Moran; Brutsaert), and also against an alternate geometrical model proposed by Menenti and Ritchie. The investigation was carried out in the Yingke oasis (China) using very-high resolution remote sensing data (VNIR, TIR & LIDAR), for a precise description of the land surface, and a fine evaluation of estimated heat fluxes based on in-situ measurements. A set of five numerical experiments was carried out to evaluate each roughness model. It appears that methods used in experiments 2 (based on Brutsaert) and 4 (based on Colin and Faivre) are the most accurate to estimate the aerodynamic roughness length, according to the estimated heat fluxes. However, the formulation used in experiment 2 allows to minimize errors in both latent and sensible heat flux, and to preserve a good partitioning. An additional evaluation of these two methods based on another k B − 1 parameterization could be necessary, given that the latter is not always compatible with the CFD-based retrieval method.
... Pour les canopées végétales uniformes, la hauteur de déplacement (d 0 ) et la longueur de rugosité (z 0m ) peuvent être approximés connaissant uniquement la hauteur de la canopée à l'aide de corrélations empiriques (Kustas et al., 1989;Stearns, 1970). ...
This study was conducted in the framework of the National Program "ANR-VegDUD Project : Role of vegetation in sustainable urban development, an approach related to climatology, hydrology, energy management and environments" (2010 -2013). It deals with the experimental and numerical modeling of green roofs and green facades to evaluate their thermohydric effects on buildings and urban microclimates. A physical model describing the thermal and water transfer mechanisms within the vegetated building envelopes has been developed. The model’s program has been implemented in a building simulation program. Using this tool, we are able to predict the impact of green roofs and green facades on building energy performance. This approach is extended to the street canyon in order to assess the microclimatic interaction in building simulation. An experimental mockup modeling an urban scene at reduced scale is designed to study the impact of different types of green roofs and walls. The comparison of the measurements carried out on vegetated buildings and streets with the reference highlights the hygrothermal and radiative impacts of vegetated buildings envelopes. In addition, these experimental data are used to verify and validate the reliability of developed tools. The results show that thermal and water transfers are strongly coupled. Hence, the thermal behavior of green roofs and green walls depend on the water availability within the growing medium. In summer and winter, measurements and numerical simulations show that green envelopes improve the energy efficiency of buildings and reduce the urban heat island.
... Linearity between these geometrical parameters (h v , d 0 and z 0m ) is always retrieved in the formulations given by Moran (1990) and Brutsaert (1982), or also by Su et al. (2001), among others (Covey, 1963;Kustas et al., 1989a;De Vries et al., 2003). Moreover, Su et al. (2001) demonstrated that the Massman (1999) kB 1 model is particularly sensitive to the vegetation height, with a relative error up to 46% of the mean measured H when using 150% of the h v reference value. ...
La paramétrisation du transfert de chaleur par télédétection, basée sur le schéma SEBS, s'est déjà avérée très adaptée pour l'estimation de l'évapotranspiration (ET) sur des surfaces naturelles homogènes. Cependant, l'utilisation d'une telle méthode pour des paysages hétérogènes (e.g. régions semi-arides ou surfaces agricoles) est plus délicate, puisque le principe de la théorie de la similarité est compromis par la présence de différentes sources de chaleur et de hauteurs variées. Dans un premier temps, cette thèse a pour objectif de proposer et d'évaluer différents modèles basés sur la géométrie de la végétation qui permettent d'estimer la longueur de rugosité pour le transfert de quantité de mouvement à la surface (z0m), cette dernière étant un paramètre clé dans la caractérisation du transfert de chaleur. En revanche, une telle investigation ne peut être menée qu'à une petite échelle et à l'aide de données de télédétection très haute résolution permettant ainsi une description très détaillée de la surface. Ensuite, le second aspect de ce travail est de caractériser le transfert de chaleur dans le cas d'études régionales. Puis, la capacité de SEBS à estimer les flux de chaleur turbulents à de grandes échelles spatiales et temporelles sera évaluée. Pour ce faire, l’approche multi-échelle de SEBS (MSSEBS) a été implémentée afin de traiter une zone de 2,4 millions km2, incluant le Plateau du Tibet et l’amont des principaux fleuves d’Asie du sud-est. La combinaison de données horaires de température de surface FY-2 avec un rayonnement net journalier et des paramètres de surface avancés, permet de produire une série temporelle d’ET sur le Plateau du Tibet pour la période 2008-2010, et à une fréquence journalière.
... Pour les canopées végétales uniformes, la hauteur de déplacement (d 0 ) et la longueur de rugosité (z 0m ) peuvent être approximés connaissant uniquement la hauteur de la canopée à l'aide de corrélations empiriques (Kustas et al., 1989;Stearns, 1970). ...
... Large variations in the ratio of z 0m /h over different land surfaces are found in the literature. For example, z 0m /h is 0.04 for sparse sorghum (Azevedo and Verma, 1986); 0.14 for small cotton (Kustas et al., 1989), and 0.8 for cotton of intermediate foliage density (Hatfield, 1989). Matthias et al.(1990) and Garratt (1992) found z 0m /h = 0.1 as a rough estimate from a review of literature even though Garratt noted that the ratio can range from 0.02 to 0.2 for natural surfaces. ...
... This height changes according to the geometry, spacing, and arrangement of roughness elements on the Earth's surface (Lettau, 1969;Garratt, 1992;Raupach, 1992Raupach, , 1994, ranging from 0.0002 to 0.005 m for water and bare soil surfaces, respectively, to ] 2 m in urban settings (Wieringa, 1993). Empirical research has established that, in homogeneous terrain with closely spaced roughness elements (i.e., where a skimming wind-flow regime is induced), z o is proportional to the roughness element height (h), although large variations in the ratio of z o /h over different land surfaces are found in the literature (Azevedo and Verma, 1986;Hatfield, 1989;Kustas et al., 1989;Mathias et al., 1990;Garratt, 1992). ...
In this research note we show that airborne imaging light detection and ranging (LiDAR) is capable of estimating the aerodynamic roughness height (z o) in a mixed grassland prairie. This is accomplished by establishing empirical relations between wind profile derived estimates of z o and vegetation height measurements from the airborne LiDAR data. We show that up to 65% of the variation of z o can be explained by LiDAR estimates of vegetation height and that up to 76% of the variation can be explained by the height variability of vegetation. We also show the effects of different filter sizes used to identify the ground surface and the top of the vegetation in the LiDAR point cloud data. Overall, results from this investigation are encouraging for future spaceborne LiDAR missions, especially in terms of the potential for providing new insight on spatial and temporal patterns of z o . Résumé. Dans cette note de recherche, on montre que le LIDAR (« light detection and ranging ») imageur aéroporté permet d'estimer la hauteur de rugosité aérodynamique (z o) dans une prairie herbeuse mixte. La procédure consiste à établir des relations empiriques entre les estimations dérivées des profils de vent z o et les mesures de la hauteur de la végétation issues des données LIDAR aéroportées. On montre que plus de 65% de la variation de z o peut être expliquée par les estimations LIDAR de hauteur de la végétation et que plus de 76% peut être expliquée par la variabilité de la hauteur de la végétation. On montre également les effets des différentes dimensions de filtres utilisées pour identifier la surface du sol et le sommet de la végétation dans les données de nuages de points LIDAR. Globalement, les résultats de cette recherche sont encourageants dans le contexte des futures missions LIDAR satellitaires, particulièrement en termes de leur potentiel à fournir un éclairage nouveau sur les patrons spatiaux et temporels de z o .
... This is true even when the same type of vegetation cover is studied, such as Hatfield et al. (1985), who measured z o 0.25H, where H is the height of the plants (cotton in this case); Stanhill (1969), who measured z o 0.13H; and Matthias et al. (1990), who measured z o 0.04H. The uncertainties associated with d or z o for a given set of observations can also be quite significant, such as Kustas et al.'s (1989) cotton observations of d 0.31 m(0.20 m) and z o 0.006 m(0.043 m). These estimations have uncertainties that are roughly 65% of the estimations for either d or z o . ...
A new method for estimating the roughness parameters (displacement height d, momentum roughness length zo, and friction velocity u), under adiabatic conditions, is introduced in this paper. The goals of this method are to obtain an unambiguous best fit for the roughness parameters and, perhaps, to reduce the present large uncertainties in these estimations via screening of the wind profile observations using a standard quality-of-fit measure. There are a number of advantages to this new method. Not only does it produce better fits (less error between predicted and observed values) than the two most commonly used methods but it also does so more effectively when the observations are of high quality. The standard wind profile equation is rewritten in a manner that allows linear least squares analysis to be performed unambiguously to determine the best fit values for dand zo for any given u. The u is scanned to find the true best fit for all three parameters. The quality of fit, Q, of the parameters to the wind profile observations is also found. This quality of fit test is used beyond the normal test for adiabaticity to screen out the low-quality observations. Using only high-quality observations to estimate the roughness parameters should reduce the uncertainties. The purposes of this paper are to introduce the method, to illustrate how it improves the fit, and to make it known that a FORTRAN program that performs this analysis is available by contacting the author for the World Wide Web address.
... Even with this limitation, a several-level wind speed profile is required to get reasonable fitting statistics for the time-dependent u * at each measurement time. In addition, near-neutral conditions must be somehow selected from the dataset (Kustas et al. 1989). As noted by Sozzi et al. (1998), this procedure can have the additional shortcoming of combining data from different heights, that is, different source areas for the meteorological variables. ...
The problem of finding joint values of both the roughness length z0 and the displacement height d is discussed in the context of the Monin-Obukhov similarity law for the wind speed profile. When focused on single-level datasets from one sonic anemometer (i.e., wind velocity, Reynolds stress, and sensible heat flux datasets at one height), it has been shown that this problem can be reduced to a simpler least squares procedure for one variable only. This procedure is carried out over a proper function of the data, representing the relative uncertainty of the roughness length, z0/z0. This function is minimized with respect to d, giving a direct estimate of d, z0, and their statistical uncertainty. The scheme is tested against two datasets in homogeneous and nonhomogeneous surface conditions.
... Studies have found that only a small fraction of wind profiles are suitable for the determination of z 0 (e.g. Hatfield, 1989;Kustas et al., 1989;Matthias et al., 1990). LiDAR is currently the only technology able to estimate z 0 over large, heterogeneous surfaces. ...
The aerodynamic roughness length (z0) is a key variable for the parameterization of momentum, mass and heat exchanges between land surfaces and the atmosphere. Its estimation however is complicated due to the large number of input variables such as height and arrangement of roughness elements on the surface, and its measurement relies on complex micrometeorological instrumentation that is typically unavailable. One remote sensing technology well suited to measuring the height of objects is light detection and ranging (LiDAR). This study demonstrates the use of pre- and post-harvest LiDAR data to quantify the aerodynamic roughness of a post-harvest forest surface. LiDAR data were acquired before and after clearcut harvesting of a 77-ha Douglas-fir dominated site on Vancouver Island, for which a micrometeorological tower provided direct year-long measurements of shear or Reynolds stress (i.e., momentum flux) and windspeed, thus permitting the independent assessment of z0 using the logarithmic wind profile equation.
The LiDAR data were used to estimate z0 based on the standard deviation of roughness element heights within the source areas of the micrometeorological tower. Estimated z0 from the LiDAR analysis compared well to z0 calculated using the micrometeorological measurements. The standard deviation of roughness element height estimated from the LiDAR analysis resulted in z0 = 0.13 ± 0.01 m (mean ± SD) for neutral atmospheric stability conditions and z0 = 0.13 ± 0.01 m for all stability conditions. The value of z0 calculated using the logarithmic wind profile equation was 0.13 ± 0.13 m for neutral conditions and 0.12 ± 0.30 m for all stability conditions after applying diabatic profile corrections. The results from this study demonstrate the potential of using LiDAR data to estimate z0 across large areas and in complex situations where direct measurements of z0 are impossible.
... In meteorological and environmental sciences, the aerodynamic resistance to heat transfer is often modeled according to logarithmic models of wind profiles, taking into account the roughness of the plant canopies [23][24][25][26]. Therefore, r a can be expressed as: ...
This paper describes a dynamic model of transient heat and mass transfer across a green roof component. The thermal behavior of the green roof layers is modeled and coupled to the water balance in the substrate that is determined accounting for evapotranspiration. The water balance variations over time directly impact the physical properties of the substrate and the evapotranspiration intensity. This thermal and hydric model incorporates wind speed effects within the foliage through a new calculation of the resistance to heat and mass transfer within the leaf canopy. The developed model is validated with experimental data from a one-tenth-scale green roof located at the University of La Rochelle. A comparison between the numerical and the experimental results demonstrates the accuracy of the model for predicting the substrate temperature and water content variations. The heat and mass transfer mechanisms through green roofs are analyzed and explained using the modeled energy balances, and parametric studies of green roof behavior are presented. A surface temperature difference of up to 25 degrees C was found among green roofs with a dry growing medium or a saturated growing medium. Furthermore, the thermal inertia effects, which are usually simplified or neglected, are taken into account and shown to affect the temperature and flux results. This study highlights the importance of a coupled evapotranspiration process model for the accurate assessment of the passive cooling effect of green roofs.
... where u* is the friction velocity [ [52]. Equation (14) converts meteorological wind data to the local wind velocities for sensible heat flux calculations at the vegetation level. ...
... the Windbreak Array [42] The well-known logarithmic wind profile function (1) has been widely used in modeling wind flows above flat surfaces, including vegetated surfaces such as cropland and forest, to describe the relationship among wind speed, roughness length, and momentum flux in the boundary layer [Businger et al., 1971; DeBruin and Moore, 1984; William et al., 1989; Mölder et al., 1999]. The parameters u * , d, and z 0 were found to be different over various surfaces, and some studies have been conducted on this issue [Rao et al., 1974; Lo, 1990; Bergeron and Abrahams, 1992]. ...
1] As vegetative windbreaks become established on a large scale in agricultural ecosystems, understanding the influence of windbreak networks on the momentum budget of the atmospheric boundary layer becomes important. The authors conducted a wind tunnel experiment to study the variation of wind speed profile and surface shear stress of wind flow passing from an open surface to another with parallel windbreaks. Five spacing (L = 5, 10, 15, 20, 30 h, wherein h is the windbreak height) windbreak arrays with moderate porosity (aerodynamic porosity a = 0.501) were used in the experiments. Both near-floor and over-array wind speed measurements showed that airflow will approach equilibrium state behind a special windbreak of the array, varying from 4th to 9th windbreak when the spacing change from 30 to 5 h. Within the range of L/h values investigated, arrays with narrower spacing cause higher friction velocity and roughness length, which were up to 2.26 and nearly 100 times those observed over open floor, respectively. A semiempirical momentum budget model is developed on the arrayed surface to estimate windbreak drag and shear stress on the protected floor. Windbreak drag accounts for more than 80% of shear stress on the arrayed surface, and the shear stress on protected floor is less than 20% when L/h < 40 based on the model estimation. The sum of the two estimated components agrees well with the estimates obtained from over-array wind profiles.
... The mean VPD was computed using the corresponding instantaneous wet and dry bulb temperatures and the standard psychrometer equation. CWSI is based on the fact that the canopy-air temperature difference is linearly related to the air vapor pressure deficit (VPD) (Jackson et al. 1981; Kustas et al. 1989) as per the equation below. ...
In eastern India, cultivation of winter maize is getting popular after rainy season rice and farmers practice irrigation scheduling
of this crop based on critical phenological stages. In this study, crop water stress index of winter maize at different critical
stages wase determined to investigate if phenology-based irrigation scheduling could be optimized further. The components
of the energy budget of the crop stand were computed. The stressed and non-stressed base lines were also developed (between
canopy temperature and vapor pressure deficit) and with the help of base line equation, [(T
c−T
a)=−1.102 VPD−3.772], crop water stress index (CWSI) was determined from the canopy-air temperature data collected frequently
throughout the growing season. The values of CWSI (varied between 0.42 and 0.67) were noted just before the irrigations were
applied at critical phenological stages. The soil moisture depletion was also measured throughout the crop growing period
and plotted with CWSI at different stages. Study revealed that at one stage (silking), CWSI was much lower (0.42–0.48) than
that of recommended CWSI (0.60) for irrigation scheduling. Therefore, more research is required to further optimize the phenology-based
irrigation scheduling of winter maize in the region. This method is being used now by local producers. The intercepted photosynthetically
active radiation and normalized difference vegetation index over the canopy of the crop were also measured and were found
to correlate better with leaf area index.
... Large variations in the ratio of z 0m /h over different land surfaces have been found in the literature. For example, z 0m /h is 0.04 for sparse sorghum (Azevedo and Verma, 1986); 0.14 for small cotton (Kustas et al., 1989 ), and 0.8 for cotton of intermediate foliage density (Hatfield, 1989). Mathias et al., 1990) and Garratt (1992) reported z 0m /h = 0.1 as a rough estimate from a review of literature although Garratt noted that the ratio can range from 0.02 to 0.2 for natural surfaces based on an extensive review of the literature. ...
Aerodynamic roughness length (z
0m) is a key factor in surface flux estimations with remote sensing algorithms and/or land surface models. This paper calculates
z
0m over several land surfaces, with 3 years of experimental data from Xiaotangshan. The results show that z
0m is direction-dependent, mainly due to the heterogeneity of the size and spatial distribution of the roughness elements inside
the source area along different wind directions. Furthermore, a heuristic parameterization of the aerodynamic roughness length
for heterogeneous surfaces is proposed. Individual z
0m over each surface component (patch) is calculated firstly with the characteristic parameters of the roughness elements (vegetation
height, leaf area index, etc.), then z
0m over the whole experimental field is aggregated, using the footprint weighting method.
Improvement of evapotranspiration (ET) estimates using remote sensing (RS) products based on multispectral and thermal sensors has been a breakthrough in hydrological research. In large-scale applications, methods that use the approach of RS-based surface energy balance (SEB) models often rely on oversimplifications of the aerodynamic resistances. The use of these SEB models for Seasonally Dry Tropical Forests (SDTF) has been challenging due to incompatibilities between the assumptions underlying those models and the specificities of this environment, such as the highly contrasting phenological phases or ET is mainly controlled by soil–water availability. We developed a RS-based SEB model from a one-source bulk transfer equation, called STEEP. Our model uses the Plant Area Index to represent the woody structure of the plants in calculating the moment roughness length. In the aerodynamic resistance for heat transfer, the parameter kB-1 was included, correcting it with RS soil moisture. Besides, the remaining λET in endmembers pixels was quantified using the Priestley-Taylor equation. We implemented the STEEP algorithm on the Google Earth Engine platform, using worldwide free data. Four sites with eddy covariance data located in the Caatinga, the largest SDTF in South America, in the Brazilian semiarid region, were used to evaluate the STEEP model. Our results show that STEEP based on the specific characteristics of the SDTF increased the accuracy of ET estimates without requiring any additional climatological information. This improvement is more pronounced during the dry season, which in general, ET for these SDTF is overestimated by traditional SEB models, as happened in our research with the SEBAL. The STEEP model had similar or superior behaviour and performance statistics relative to global ET products (MOD16 and PMLv2). This work contributes to an improved understanding of the drivers and modulators of the energy and water balances at local and regional scales in SDTF.
This book is a collection of recent developments, methodologies, calibration and
validation techniques, and applications of thermal remote sensing data and derived
products from UAV-based, aerial, and satellite remote sensing. A set of 15 papers written
by a total of 70 authors was selected for this book. The published papers cover a wide
range of topics, which can be classified in five groups: algorithms, calibration and
validation techniques, improvements in long-term consistency in satellite LST,
downscaling of LST, and LST applications and land surface emissivity research.
The primary purpose of this study was to model the partitioning of radiation capture and evapotranspiration in a two-crop (maize and sunflower) intercropping system. Two field experiments were conducted in 1998 and 1999. Detailed canopy architecture was measured, and transpiration and soil evaporation were measured using sap flow gauges and lysimetres, respectively. One-(lD) and two-dimensional (2D) models were developed for modelling radiation attenuation in one dimension (vertical) and two dimensions (vertical and horizontal), respectively. The simpler 1D model was slightly more accurate than the more complex 2D model, where the mean errors (95 % error range in brackets) for estimating the fractional radiation interception were 0.01 (-0.09-0.11) and 0.04 (-0.13-0.06), respectively. Nevertheless, the hourly simulations by the 2D model followed the measured diurnal trend of total radiation capture more closely than those by the 1D model. The Shuttleworth-Wallace evapotranspiration equation was extended and applied to intercropping systems. Its mean prediction error for transpiration was near zero (-0.01 mm h 1), and its accuracy was not affected by plant growth stages, but simulated transpiration during high measured transpiration rates was underestimated. There were also larger errors in predictions by both models for daily soil evaporation than for plant transpiration.
This chapter is devoted to the introduction of some geographical and meteorological information involved in the numerical modeling of wind fields and solar radiation. First, a brief description of the topographical data given by a Digital Elevation Model and Land Cover databases is provided. In particular, the Information System of Land Cover of Spain (SIOSE) is considered. The study is focused on the roughness length and the displacement height parameters that appear in the logarithmic wind profile, as well as in the albedo related to solar radiation computation. An extended literature review and characterization of both parameters are reported. Next, the concept of atmospheric stability is introduced from the Monin–Obukhov similarity theory to the recent revision of Zilitinkevich of the Neutral and Stable Boundary Layers (SBL). The latter considers the effect of the free-flow static stability and baroclinicity on the turbulent transport of momentum and of the Convective Boundary Layers (CBL), more precisely, the scalars in the boundary layer, as well as the model of turbulent entrainment.
In the framework of AMMA program (African Monsoon Multidisciplinary Analysis), the aim of the present work is to provide some elements to better understand and quantify surface processes that impact on sensible and latent heat fluxes over sahelian landscapes. An approach using remote sensing and a SVAT model (Soil-Vegetation-Atmosphere Transfer) has been developed at different spatial scales. The SEtHyS_Savannah model has been adapted to semi-arid environments. The first step has been to calibrate parameters then to validate the model locally using AMMA measurements acquired over a millet and a fallow fields in Niger. Then specific methodologies have been used and developed in order to apply and validate the model at the scale of the AMMA/Niger super-site. Thus SPOT high resolution data have been used to estimate the landuse and the vegetation characteristics over the area. ASAR/ENVISAT data, SEVIRI/MSG and MODIS products have been used to propose a first validation of surface soil moisture and land surface temperature which are two key variables of the hydric and energetic budgets. The model simulations show good agreement with the ground truth measurements and the remote sensing products over the AMMA/Niger super site . This thesis provides multiple outlooks for the study of land-atmosphere interactions and surface hydric budget changes due to climate changes and anthropic pressure.
This paper reviews the methods developed to infer surface fluxes from satellite data, with or without the aid of other meteorological information. These methods are based on physical modelling or on statistical relations between satellite measurements and surface parameters. However,their accuracy and their possible applications differ from each other. In particular, the surface radiation budget can be obtained over sea and over particular land areas with rather good accuracy. Some of these methods could constitute the basis for future use in atmosphere models (for climate modelling or meteorological forecasting), but the quality of most of them has still to be assessed. They are therefore rarely directly used to test atmospheric models (weather prediction or climate global circulation models). A preferred approach consists of comparisons between satellite data (either direct measurements or well-established retrieved parameters) and the same parameters derived from models.
Aerodynamic roughness (z0) is a widely used parameter describing the effective roughness of a surface to fluid flow, commonly measured with wind profiling. Wind profiling is time-consuming, equipment intensive, spatially limited, and requires specific wind conditions. To solve this, satellite and airborne remote sensing and ground-based proxies have been developed to measure and relate physical surface roughness to profile-measured aerodynamic roughness. However, for un-vegetated settings, most satellite and airborne remote sensing proxies are, generally, incapable of estimating sub-meter roughness and ground-based measurements are spatially limited and time-consuming. This paper presents a new method for estimating physical roughness using terrestrial laser scanning (TLS) point cloud data. TLS data provide a centimeter-scale, three-dimensional, spatially contiguous representation of a surface. At seven sites with different roughness conditions (silt playa to boulder covered) we compared TLS metrics of surface roughness to wind profile estimates of z0. From point clouds, the mean height (hTLS) and root-mean squared height (RMSH) of roughness elements were calculated. Manual measurements of clast dimensions, height, and density were used to guide point cloud processing. Results indicate a strong positive linear relation between z0 and hTLS (r2 = 0.99, p < 0.001), and between z0 and RMSH (r2 = 0.96, p < 0.001). This suggests that TLS measurements of physical roughness could serve as a proxy for z0 based on empirical relations developed with wind profiling in un-vegetated terrain. With further testing, TLS could improve operational parameterizations of z0 for use in large scale atmosphere-surface models.
The ultimate goal of this dissertation was to produce maps of surface evaporation for agricultural areas based on Landsat Thematic Mapper (TM) spectral data. This achievement was dependent upon successful attainment of four intermediate goals: (1) Enhancement of TM thermal spatial resolution; (2) Atmospheric correction of TM visible and near-IR spectral data; (3) Atmospheric correction of TM thermal data; and (4) Remote estimation of crop aerodynamic properties. A statistical technique was developed to combine low-resolution (120 m) TM thermal data (TM6) with higher resolution (30 m) TM reflective data based on the relation between TM6 and the TM red and near-IR wavebands. This method was successful in improving the visible appearance of the TM6 image and retaining the original thermal spectral information over diverse agricultural landscapes. Several atmospheric correction procedures were examined to determine which techniques could provide the ease and accuracy necessary for the remote ET model. The Lowtran7 radiative transfer code was chosen for correction of TM visible and near-IR data (TM1-TM4) because it provided adequate accuracy (+/-0.02 reflectance, 1 sigma RMS) and easy application. For TM6, results using the Lowtran7 code with a variety of atmospheric models were unsatisfactory. However, a simple linear regression of measured surface temperatures (T _{rm s}) and TM6 digital numbers provided estimates of T_{rm s} to within +/-1.2 ^circC of measured values. Though the procedure was accurate, it required concurrent ground -based measurements of T_{rm s } and would obviously be inconvenient if it were used on an operational basis. Reasonable estimates of aerodynamic parameters were made for an alfalfa canopy from remote measurements of red and near-IR reflectance. The uncertainty in sensible heat flux density associated with the error in remote estimates of aerodynamic resistance was +/-25%. Since these results were probably crop-specific and possibly site-specific, more data sets of this nature will need to be collected for other crops to determine a universal relation between remotely sensed data and aerodynamic properties. Data from the satellite-based TM sensor and ground -based meteorological instruments were combined to produce maps of latent heat flux density (LE: a function of evaporation rate (E) and heat of vaporization (L)) for Maricopa Agricultural Center, Arizona. The satellite-based estimates of LE differed from coincident ground-based measurements, using a Bowen -ratio apparatus, by 4% in cotton and -6% in alfalfa. These results were within the suggested accuracy goal of +/-11%.
The zero-plane displacement (d) and the roughness length (z0) of sparse shrubs in semi-arid region were determined by the conventional (statistical) method during the HAPEX-Sahel experiment. Measurements of the wind profile at four levels above of surface (3.0, 4.1, 5.3 and 8.5 m) and eddy correlation turbulent fluxes determined at 9 m were used. The method was applied in neutral atmospheric conditions, with stability parameter z < |0.0325|, wind speed > 1 m s-1 in the lower level and the friction velocity > 0.1 m s-1 determined by eddy correlations. Approximately 4.3% of the observation satisfied these restrictions, but only 1% had adequate fetch conditions. The value of d was equal to 1.09 ± 0.14 m and it represented about 53% of the height of the vegetation (h) and z0 = 0.184 ± 0.017 m = 0.089h. The data selection criteria used was satisfactory, but the applicability of the method is restricted by the difficulties in observing adiabatic conditions and adequate fetch. Alternative estimates with a single height of measurement above the inertial sublayer gave physically inconsistent estimates of d and z0.
Aerodynamic parameters (displacement height d and roughness length for momentum z(o)), as well as friction velocity u. and temperature scaling parameter T-., were estimated from the Alpilles energy balance measurement data. Air temperature and wind speed measurements at two heights and sensible heat flux data were needed in the estimation. Application of this method to two data sets measured in different ways for the temperature gradients found that the estimation of d and z(o) was applicable only when the temperature gradients were measured differentially. Sensitivity tests showed that the estimated d and z(o) were sensitive to the temperature gradients. However, both of the data sets produced good estimation of it, and T-.. The main advantage of this method is that it is applicable not only under near-neutral conditions but also under strong unstable conditions.
A new method to estimate the effective aerodynamic roughness of a complex landscape is presented. High-resolution elevation profiles measured with an airborne laser altimeter have been used to compute geometrical parameters of three landscape elements of the Walnut Gulch experimental watershed, Arizonia. Mean crop height, its standard deviation, and the frequency distribution (as a function of penetration depth) of plant elements hit by the laser beam have been calculated for short segments (1 m) measured over intervening grass between shrubs. Longer segments have been applied to obtain the mean height and spacing of taller shrubs and trees. Finally, laser profiles covering the entire watershed gave the mean amplitude and wavelength of hillocks and ridges. The aerodynamic roughness length of the intervening grass is estimated using the ratio of standard deviation to vegetation height, corrected for instrument noise, times mean height. An effective roughness length which parameterizes the total stress due to grass and taller shrubs and trees is calculated first. Finally, a watershed-scale effective roughness length is calculated combining the amplitude and wavelength of hillocks and ridges with the roughness length obtained in the previous step.
A two-source energy balance model developed to use directional radiometric surface temperature for estimating component heat fluxes from soil and vegetation has had several recent modifications to account for some of the unique properties associated with sparse canopies. Two of these changes involve the algorithms predicting the divergence of net radiation inside the canopy and how to account for clumped vegetation, which affects both the wind and radiation penetration inside the canopy and radiative temperature partitioning between soil and vegetation components. Model results with and without these modifications are compared using data collected from a sparsely vegetated row crop of cotton (Gossypium hirsutum L. cv. Delta Pine 77). It is suggested that these two new algorithms be incorporated in any two-source model applied to sparse canopies.
Conventional methods that use point measurements to estimate evapotranspiration are representative only of local areas and cannot be extended to large areas because of heterogeneity of landscape. To overcome this difficulty, remote sensing has proven to be the most suitable approach for large area estimation of evapotranspiration because remote sensing data can provide representative parameters such as radiometric surface temperature, albedo and vegetation index. The heterogeneity is more prominent in the Indus Basin as more than 80% of farmers have land holdings less than 4 ha, and within these holdings, two or three different crops are usually grown. The limitation of most remote sensing based procedures to estimate evapotranspiration is the measurements of large number of crop‐specific and climatic parameters, which are not only difficult to obtain but also require considerable field work, equipment and therefore involve much expenditure. The purpose of this study was to present a simple methodology that requires minimum ground observations for estimation of evapotranspiration to test the reliability of Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) data for estimation of evapotranspiration in the Indus Basin. The methodology is based on surface energy balance to estimate sensible and latent heat fluxes by combining remotely sensed data from ASTER with common meteorological data. The various components of surface energy balance were computed during satellite overpass and 24‐h integrated fluxes were derived for the full ASTER scene acquired over the lower Rechna doab region of the Indus Basin. The surface brightness temperatures were derived from thermal band 13 and NDVI from two VNIR bands of the ASTER. Evapotranspiration values from the maize field in the Indus Basin, as estimated using ASTER data at Shahkot, Jaranwala and Satiana locations, were estimated as 2.05, 2.77 and 2.32 mm day, respectively. The estimated evapotranspiration was compared with evapotranspiration computed at three different locations using CROPWAT software and was found to be in close agreement.
This work is aimed at deriving canopy component (soil and foliage) temperatures from remote sensing measurements. A simulation study above sparse, partial and dense vegetation canopies has been performed to improve the knowledge of the behaviour of the composite radiative temperature and emissivity. Canopy structural parameters have been introduced in the analytical parameterization of the directional canopy emissivity and directional canopy radiance: namely, the leaf area index (LAI), directional gap fraction and angular cavity effect coefficient. The parameterization has been physically defined allowing its extension to a wide range of Leaf Inclination Distribution Functions (LIDF). When single values are used as leaves and soil temperatures, they prove to be retrieved with insignificant errors from two directional measurements of the canopy radiance (namely at 0 degrees and 55 degrees from nadir), provided that the canopy structure parameters are known. A sensitivity study to the different parameters shows the great importance of the accuracy on LAI estimation (an accuracy of 10 per cent is required to retrieve the leaves temperature with an accuracy better than O.5 deg K, the same requirement being 5 per cent for the retrieval of soil temperature). The radiometric noise is important too, but its effects may be limited by using very different angles for the measurements: for 0 degrees and 55 degrees, the effect of a Gaussian noise (NE Delta T = 0.05 deg K) is lower than 0.5 deg K on the retrieved soil and foliage temperatures). Uncertainties on the leaf and soil emissivities (Delta epsilon = 0.01) cause little errors in the retrieval (lower than 0.5 deg K). If the inclination dependence of the leaves temperature is considered, a 1 deg K error is observed in the retrieved soil and foliage temperatures. This error is due to the fact that the effective foliage temperature varies with the view angle (a few 10(-1) deg K at 55 degrees), which implies errors in the inversion scheme. This effect may be corrected for by using an angular corrective term delta depending only on the off-nadir angle used.
A network of meteorological stations was installed during the Monsoon '90 field campaign in the Walnut Gulch experimental watershed. The study area has a fairly complex surface. The vegetation cover is heterogeneous and sparse, and the terrain is mildly hilly, but dissected by ephemeral channels. Besides measurement of some of the standard weather data such as wind speed, air temperature, and solar radiation, these sites also contained instruments for estimating the local surface energy balance. The approach utilized measurements of net radiation (Rn), soil heat flux (G) and Monin-Obukhov similarity theory applied to first- and second-order turbulent statistics of wind speed and temperature for determining the sensible heat flux (H). The latent heat flux (LE) was solved as a residual in the surface energy balance equation, namely, LE = −(Rn + G + H). This procedure (VAR-RESID) for estimating the energy fluxes satisfied monetary constraints and the requirement for low maintenance and continued operation through the harsh environmental conditions experienced in semiarid regions. Comparison of energy fluxes using this approach with more traditional eddy correlation techniques showed differences were within 20% under unstable conditions. Similar variability in flux estimates over the study area was present in the eddy correlation data. Hence, estimates of H and LE using the VAR-RESID approach under unstable conditions were considered satisfactory. Also, with second-order statistics of vertical velocity collected at several sites, the local momentum roughness length was estimated. This is an important parameter used in modeling the turbulent transfer of momentum and sensible heat fluxes across the surface-atmosphere interface.
The zero-plane displacement (d) and the roughness length (z0) of sparse shrubs in semi-arid region were determined by the conventional (statistical) method during the HAPEX-Sahel experiment. It was used measurements of the wind profile at four levels above of surface (3.0, 4.1, 5.3 and 8.5 m) and eddy correlation turbulent fluxes determined at 9 m. The method was applied in neutral atmospheric conditions with stability parameter ζ < |0.0325|, wind speed > 1 m s−1 in the lower level, and the friction velocity > 0.1 m s−1 determined by eddy correlations. Approximately 4.3% of the observation satisfied these restrictions, but only 1% had adequate fetch conditions. The value of d was equal to 1.09 ± 0.14 m and it represented about 53% of the height of the vegetation (h), and z0 = 0.184 ± 0.017 m = 0.089h. The data selection criteria used was satisfactory, but the applicability of the method is restricted by the difficulties in observing adiabatic conditions and adequate fetch. Alternative estimates with a single height of measurement above the inertial sublayer gave physically inconsistent estimates of d and z0.
The potential feedback on global atmospheric and climate change of climate-driven changes in terrestrial vegetation is examined by systematically relating the surface exchanges of energy, mass and momentum to two dimensions of vegetation, structure and taxonomy, such that the significance of climate driven changes in these characteristics can be assessed. A detailed quantitative understanding of this feedback is an important prerequisite to realistic and dynamic representations of the Earth's surface within general circulation and biological models (GCMs and GBMs). Without realistic representations of terrestrial vegetation within these models, any forecasts of future climates by these models must be suspect.Several general conclusions are drawn. The first is that the indirect feedbacks, those associated with the clouds and aerosols of the planetary boundary layer, appear to be very powerful but as yet their behaviour and connections with the underlying surface are both poorly understood and captured within GCMs.The physical structure of vegetation, the disposition of biomass in 3-D, is the characteristic that most strongly influences the exchange of momentum (via aerodynamic roughness) and solar radiation (via albedo). Vegetation structure and species composition determine the most important of the mass exchanges, evapotranspiration. Of all of the surface exchanges, the parameterization of evapotranspiration (E
) and the simulation of the water balance over time is the most critical.Lastly, the problems of scaling and spatial heterogeneity, the sub-grid variability of the modellers, looms as a difficult, but not insoluble, problem. It remains a critical problem however, and the detailed parameterization of the various big leaf models stands in absurd contrast to the simplistic generalization of the spatial heterogeneity of terrestrial landscapes.Plant ecologists can contribute to the task of improving the representation of vegetated landscapes within GCMs. There is need to simply and unify the way in which vegetation can be grouped at landscape scales. A classification that is based on function rather than phylogeny is required. The definition of Vegetation Functional Types (VFTs) would expedite research on both the impact of, and feedback on, climate change.
Maize (Zea mays L.) and alfalfa (Medicago sativa L.) were simultaneously irrigated in two adjoining plots with the same sprinkler solid-set system under the same operational and technical conditions. The Christiansen's uniformity coefficient (CUC) and the wind drift and evaporation losses (WDEL) were assessed from the irrigation depth (IDC) collected into pluviometers above each crop. A network of pluviometers was located above the maize canopy. Two networks of pluviometers were located above the alfalfa, one above the canopy and the other at the same level as that above the maize. The latter was used to analyze the effects of the water collecting plane. The wind velocity (WV) profile was measured above each crop using anemometers located at three levels. Both the CUC and the WDEL differed between maize and alfalfa.The crops modified both the wind velocity above the canopy and the water interception plane. Both effects were related to the height of the crops (h).When h increased, the water interception plane increased, and the overlap of the sprinklers decreased. The CUC of the IDC increased with the overlap. Because h was greater for maize than for alfalfa, the CUC was noticeably smaller for maize.The WV greatly decreased in proximity to the canopy. The WV at the level of the nozzles was smaller above the maize because the top of the canopy was closer to the nozzles than it was for alfalfa. However, the CUC of the IDC mainly depended on the WV at higher levels, where the WV was similar above both maize and alfalfa. The logarithmic wind profile overestimated the vertical variation of the WV in the space where the sprinklers distributed the water.The WDEL was greater above the maize than above the alfalfa. This finding was related to the underestimation of the IDC above maize, especially under windy conditions, because the pluviometers were located very close to the nozzles.
Hatfield, J.L., Perrier, A. and Jackson, R.D., 1983. Estimation of evapotranspiration at one time-of-day using remotely sensed surface temperatures. Agric. Water Manage., 7: 341–350.
Observations from two towers situated in flat, tree covered terrain (zSUP0 lying between 0.4 and 0.9m) have been used to investigate the flux profile relations in the height range z/zo from 50 to 85, where z is the height above the zero plane displacement (from author's abstract)
The surface roughness parameter, z0, can be estimated with different techniques. These techniques are analyzing the mean wind profile, estimating the surface drag coefficient and using the universal functions according to the Monin-Obukhov similarity theory. The first two techniques mentioned have been applied over more or less homogeneous terrain. As a result, it appears that the drag method is very sensitive to shear stress measuring errors and to local changes and irregularities in terrain.
Unstressed baselines of the crop water stress index for cotton under full ground cover have slopes about twice those under partial canopy cover. These results were consistent between four strains and one commercial variety and were caused by a more aerodynamically rough canopy under partial ground cover. The roughness length and displacement height values for partial ground cover were much larger than those currently based only on height. These results show that partial canopy is a very complex system and values of crop water stress index would have to be interpreted very carefully.
In order to check TAKEDA's theory (equation (2)), observations of wind velocity profiles were made over the sorgo canopy in August 1967. Furthermore, to test the theory another set of observations were made over the corn canopy in Tanashi, Tokyo, in July and August 1968. The author wants to know if the theory can be applied to all wind velocity profiles over different kinds of plant canopies at different stages of growth and development.The wind velocity profiles were measured with five small cup-anemometers. The plant height (h), the leaf area index (L.A.I.) the mean leaf area density (M.L.A.D.=L.A.I./h) and the rate of heading were measured, too.Five results were obtained from these observations as follows:I. An opposite relationship between the zero-plane displacement (d) and the roughness length (z0) was found, in which a decrease in d and an increase in z0 with wind velocity were shown (see Fig. 3).II. A good correspondence was found between d versus u* (friction velocity) and M.L.A.D. with growth and development of the corn canopy (see Fig. 3).III. A good correspondence was found between z0 vs. u* and the heading of corn plants with growth and development of the corn canopy (see Fig. 3).IV. It was found that the values d and z0 moved with the particular characteristic on the rough surface diagram introduced by TAKEDA (see Fig. 4).V. It was found that the variation of H (effective plant height), obtained from equation (2), in which α is taken to be 0.087, vs, u* decreased slightly with the increase of friction velocity on 24.25 July, that H was constant, even if u* increase, on 1-3 August and that the variation of H vs. u* increased slightly with the increase of u*, furthermore the plotting points scattered under the condition of u*>80cm/sec on 5-9 August (see Fig. 5).Finally it was concluded that the theory was applicable even to the corn canopy taking structures of canopy on 24.25 July and on 1-3 August into account, and that, on the contrary, observed results on 5-9 August disagreed with the theory probably due to the oscillatory movements of plants with long periods.
PREFACE TO THE SECOND EDITION LIST OF SYMBOLS 1. SCOPE OF ENVIRONMENTAL PHYSICS 2. GAS LAWS Pressure, volume and temperature Specific heats Lapse rate Water and water vapour Other gases 3. TRANSPORT LAWS General transfer equation Molecular transfer processes Diffusion coefficients Radiation laws 4. RADIATION ENVIRONMENT Solar radiation Terrestrial radiation Net radiation 5. MICROCLIMATOLOGY OF RADIATION (i) Interception Direct solar radiation Diffuse radiation Radiation in crop canopies 6. MICROCLIMATOLOGY OF RADIATION (ii) Absorption and reflection Radiative properties of natural materials Net radiation 7. MOMENTUM TRANSFER Boundary layers Wind profiles and drag on uniform surfaces Lodging and windthrow 8. HEAT TRANSFER Convection Non-dimensional groups Measurements of convection Conduction Insulation of animals 9. MASS TRANSFER (i) Gases and water vapour Non-dimensional groups Measurement of mass transfer Ventilation Mass transfer through pores Coats and clothing 10.MASS TRANSFER (ii) Particles Steady motion 11.STEADY STATE HEAT BALANCE (i) Water surfaces and vegetation Heat balance equation Heat balance of thermometers Heat balance of surfaces Developments from the Penman Equation 12.STEADY STATE HEAT BALANCE (ii) Animals Heat balance components The thermo-neutral diagram Specification of the environment Case studies 13.TRANSIENT HEAT BALANCE Time constant General cases Heat flow in soil 14.CROP MICROMETEOROLOGY (i) Profiles and fluxes Profiles Profile equations and stability Measurement of flux above the canopy 15.CROP MICROMETEOROLOGY (ii) Interpretation of measurements Resistance analogues Case studies: Water vapour and transpiration Carbon dioxide and growth Sulphur dioxide and pollutant fluxes to crops Transport within canopies APPENDIX BIBLIOGRAPHY REFERENCES INDEX
This chapter states that the object of atmospheric diffusion as a science is the study of pollution propagation in the air. One of the most important practical problems in the development of this branch of science is that of air pollution by industry and transport, and primarily urban pollution. If atmospheric diffusion did not exist, pollution would accumulate in the lower layer of the atmosphere and the inhabitants of the cities would not be able to breathe without gas-masks. Due to atmospheric diffusion everyone is subjected to the effect of radioactivity as a result of atomic explosions. The chapter also discusses the phenomenon of atmospheric diffusion in agriculture when plants are chemically protected in the struggle against pests or when they are defended from frost by producing smoke. Sea salt and volcanic dust, bacteria and viruses, pollen and seeds of plants are distributed in the air due to atmospheric diffusion. Air masses are fed with water vapor from the sea and with dust from the deserts by the same mechanism. The study of atmospheric diffusion is of great importance for practical purposes as well as for adjacent branches of science. And at the same time, in view of the intriguing complexity of the phenomena being studied, their investigation can give an aesthetic satisfaction even to the most exacting scientists. Specialists in hydrodynamics and geophysics are mostly concerned with this study.
Reported failures of a generalized wind-profile equation to predict correctly the shearing stress under certain conditions are found to be associated with low wind speeds close to the surface. The increase of drag coefficient of the surface-roughness elements (e.g., grass blades) at low Reynolds numbers is the source of these discrepancies.
Some characteristics of turbulence near the ground can begin to be
interpreted There is yet little information about others (even near the ground)
such as spectra measured along the vertical lines, or cross spectra between two
variables. Further, results of turbulence characteristics over water are few and
confusing, and no information exists about the effects of cities and deserts. As
we go aloft, our information about spectra becomes more scanty, although the
general areas of large turbulence intensity are fairly well known (auth)
The displacement height appears in the logarithmic velocity profile for rough-wall boundary layers as a reference height for the vertical co-ordinate. It is shown that this height should be regarded as the level at which the mean drag on the surface appears to act. The equations of motion then show that this also coincides with the average displacement thickness for the shear stress. A simple analytical model, experimental results and dimensional analysis are all used to indicate how the displacement height depends upon the detailed geometry of the roughness elements. - Author
From over 1000 half‐hour observations of near‐surface wind profiles at Hay, NSW, more than 500 high quality sets of data are selected. In unstable conditions, these closely confirm previous analyses and suggest near equality between z/L and Ri . The data are well described by either the KEYPS relationship (Panofsky) or that of Businger: ϕ M = (1–16 z/L ) −1/4 . In stable conditions, a log‐linear formulation (ϕ M = 1+ az/L ) is found to give an adequate description of the wind profile up to z/L ≃ 0.5, with some evidence for a slight variation in α between the values 4.0 at neutral and about 6 when z/L ≃ 0.2. The average value of α between these limits is found to be 5.0±0.2.
In conditions of very high stablity, a linear profile ( du/dz = cu * /( kL )) is suggested above the height z ≃ 10 L . In the transition region between the log‐linear and linear profile regimes, the log‐linear formulation appears to tend towards a purely logarithmic law as stability increases. There is no evidence for any sudden change in behaviour, nor is there any suggestion that a purely logarithmic relationship is ever attained as an average situation. The value of c in the purely linear relationship is found to be between 0.4 and 0.9. The data also indicate that in extremely stable conditions ( L ≃ 50cm) the dimensionless gradients of heat and of momentum may differ by about a factor of two, with ϕ H being the larger.
The roughness length of the site used is found to be 1.2±0.1 mm, considerably less than the values appropriate to earlier experiments performed in the same general area. There is some evidence for an increase in z 0 with decreasing wind speed (reaching about 3 mm when the wind at 1 m is about 1 ms ⁻¹ ), in accord with Deacon's hypothesis concerning the form drag of roughness elements. From the point of view of applying a low‐level drag coefficient in order to estimate friction velocities, the errors arising from the change in roughness length are sometimes comparable to those resulting from stability effects.
Not surprisingly, the data show that in slightly stable conditions dewfall was low, whereas in extremely stable situations dewfall accounted for most of the heat loss from the air.
The data used here form part of a much larger body of information obtained during 1967 and known as the ‘Wangara’ experiment.
An analysis is made of the Monin-Obukhov function ΦM in the familiar wind profile equation, using data from two recent expeditions to Gurley (New South Wales) and Hay (New South Wales). In one, the friction velocity u* is determined directly by the eddy correlation method, and in the other, conducted during mid-winter when small heat-fluxes were experienced, by the use of a friction coefficient applied to a low-level wind.
By collating with a similar earlier analysis for heat and water vapour transfer, the variations of ΦM, ΦH and ΦW with stability are presented in tabular form in the z/L range − 0.01 to − 1.0. Within this range the empirical relationships ΦM = (1 − 16 z/L)−1/4 and ΦH, W = (1 − 16 z/L)−1/2, and the implied equality between Ri and z/L, are found to approximate the data to within a few per cent.
Values of the total vertical flux of sensible and latent heat over a level forested region, obtained from aerodynamic (profile‐gradient) formulae appropriate to airflow over relatively smooth surfaces, are found to fall consistently short of independent energy‐balance estimates by a factor of 2 to 3 in unstable and near‐neutral conditions (Richardson number, Ri , in the range −0·4 to +0·01), whereas for Ri > + 0·02 no similar discrepancy is detected. These results, based on tangents drawn to (semi‐logarithmic) profiles at a height of about nine aerodynamic roughness parameters ( z 0 ) above the zero plane displacement level ( d ) of the forest, rely on the basic assumption that the value of d established in very nearly neutral conditions (| Ri | < 0·003), namely 0·76 mean tree heights, holds under all conditions of thermal stability.
Wake diffusion and thermal seeding effects are discussed as possible additional transfer mechanisms acting to reduce profile gradients immediately over aerodynamically rough surfaces. In terms of the former mechanism (assumed to operate below d + 25 z 0 , or so), approximate empirical formulae are derived which attempt to quantify the observed discrepancy in terms of Ri and the proximity of the surface.
It is concluded that aerodynamic equations ought not to be used to give independent flux estimates close to aerodynamically rough surfaces.
The diabatic mean profile forms in the surface layer are studied, by applying analysis methods having high resolving power to data from O'Neill, U.S.A. (heights up to 6.4 m) and from Kerang and Hay, Australia (heights mostly up to 16 m).
It is found, concordantly from the O'Neill and Australian data, that the log‐linear law is valid for z/L values between — 0.03 and + 1, which includes a small range of unstable and a surprisingly wide range of stable conditions. For all quantities studied (wind, potential temperature, and specific humidity), it is concluded that the Monin‐Obukhov coefficient α is near 4.5 in unstable and 5.2 in stable conditions, within a standard error of about 10 per cent. The ratios K H /K M and K W /K M evidently remain constant, equal to unity, over the whole of the log‐linear range (and somewhat beyond).
In stable conditions, the log‐linear law implies that Ri approaches a critical value α ⁻¹ , approximately 0.2, as z/L → ∞. However, at z/L a second régime sets in, in which the profiles are only quasi‐determinate, approximating, on the average, a simple logarithmic form (gradients proportional to z ⁻¹ ); this régime covers the range approximately 1 < z/L < (α + 1), i.e. (α + 1) ⁻¹ < Ri < 1. A third range of extreme stability, Ri > 1, is practically unrepresented in the data examined.
The major part of the unstable range, for z/L < − 0.03, will be discussed in a later paper.
Measurements were made in a wind-tunnel of the drag on elements of a simply-structured artificial crop, and of the wind profiles above and within the crop. Analysis demonstrates
The relation z0 = 0.36 (h – d) is suggested for the roughness parameter of vegetation of height h.
Calculated values of the drag force, f, on unit column of a real stand of beans in the field, using individual-element drag coefficients (Cd) and measured wind speeds, give f = 3.5 τ0 where τ0 is the downward momentum flux derived from the shape of the wind profile above. On the evidence of conclusion (i) and the dense and complex nature of the bean canopy, the factor 3.5 is attributed to mutual sheltering of neighbouring canopy elements rather than as evidence that the Cd – values are modified by turbulent shear flow.
For the artificial crop, and for the real crop, recognition of the wind-speed dependence of the individual-element drag coefficients gives values of eddy viscosity, KM, almost constant in the height range h/3 < z ⩽ h and significantly larger than those found when constant drag coefficients are assumed. Constant KM within a crop canopy is consistent with the wind profile u(z)/u(h) = {1 + α(1 – z/h)}−2: an explicit expression is given for the parameter α.
Observations from two towers situated in flat, tree-covered terrain (z0 lying between 0.4 and 0.9 m) have been used to investigate the flux-profile relations in the height range z/z0 from 5 to 85, where z is the height above the zero-plane displacement. The analysis confirms a lower height limit (at z = z*) to the validity of the Monin and Obukhov functions ϕM, H(z/L) in unstable conditions and, by implication, of the logarithmic wind law in neutral conditions. We find z*/z0 ≃ 35 and 150 for wind at the denser and less dense (lower z0) surfaces, whilst for temperature z*/z0 ≃ 100.
The level z* corresponds with the top of the transition layer, within which it is assumed the profiles depend additionally upon a length scale zs, related to surface wake generation. On the assumption that z* α zs, modification of the profiles in the transition layer is then described through a function ø(z/z*) whose explicit form is derived from length-scale considerations in a region of wake-shear interaction. The observed non-dimensional profiles ϕ° are well represented by
ϕ° ≃ 0.5ϕ(z/L)exp(0.7z/z*)
Both for wind and temperature. For wind at both surfaces, the depth z* is approximately constant in unstable conditions and equal to 3δ, δ being the tree spacing. We tentatively conclude that δ is the relevant surface length scale zs characterizing the wake field and depth of penetration z*.
Simplified expressions describing the frequency response of eddy correlation systems due to sensor response, path-length averaging, sensor separation and signal processing are presented. A routine procedure for estimating and correcting for the frequency response loss in flux and variance measurements is discussed and illustrated by application to the Institute of Hydrology's Hydra eddy correlation system.The results show that flux loss from such a system is typically 5 to 10% for sensible and latent heat flux, but can be much larger for momentum flux and variance measurements in certain conditions.A microcomputer program is included which, with little modification, can be used for estimating flux loss from other eddy correlation systems with different or additional sensors.
Recent observations of flux-gradient anomalies in atmospheric flow close to forests, and similar rough surfaces, prompted a wind-tunnel investigation in which cross-wire anemometry was used to study the vertical development and horizontal variability of adiabatic flow over five regularly arrayed rough surfaces, encompassing a 32-fold range of roughness concentration . The roughness elements were cylinders, 6 mm in both height and diameter.Below a layer in which the velocity profile is semi-logarithmic, two surface influences upon the mean velocity field can be distinguished: wake diffusion and horizontal inhomogeneity. The wake diffusion effect causes non-dimensional vertical velocity gradients to be smaller than in the semi-logarithmic region; at least for elements with aspect ratios l/h 1, it is governed by the transverse dimension l of the roughness elements, and is observed when z > h + 1.5l (where z is height above the underlying surface, and h is the height of the roughness elements). A simple diffusivity model successfully describes the horizontally averaged velocity profiles in the region of wake influence, despite conceptual disadvantages. The horizontal inhomogeneity of the flow is negligible when z > h + D (D being the inter-element spacing), and does not entirely mask the wake diffusion effect except over very sparsely roughened surfaces ( 0.02). A criterion for negligibility of both effects, and hence for applicability of conventional turbulent diffusivity theory for momentum, is z > h + 1.5D. These results are compared with atmospheric data, and indicate that wake diffusion may well cause some underestimation of the zero-plane displacement d over typical vegetated surfaces.
Vertical flux densities of momentum and sensible heat, obtained from simultaneous wind speed and air temperature profiles in the surface layer, depend on the displacement height of the profile system and the surface roughness. A criterion for selecting the displacement height and the surface roughness is introduced, which requires a minimum value for the error squares between the observed and a calculated wind speed profile as determined by diabatic surface layer theory. Values of displacement height and surface roughness, which provide a minimum error squares fit within a desired tolerance, are selected by the rule of false position. The method is programmed for digital computer solution and applied to a total number of 628 profiles obtained during a 7-day period at a micrometeorological test site near Davis, California, using five measurement levels to 160 cm height.
From half-hourly averaged observations of net radiation, latent and soil heat fluxes over wheat, the sensible heat flux is calculated as the residual component of the surface energy balance. Then, the aerodynamic surface temperature is obtained by solving iteratively the aerodynamic equation for sensible heat flux, taking into consideration the bluffbody (differences in roughness height for heat and momentum exchange) and the stability (differences in surface and air temperatures) corrections to the aerodynamic resistance. The aerodynamic temperatures are found to be lower (higher) than the infrared thermometric observations under stable (unstable) atmospheric conditions. However, when the infrared temperatures were used in a resistance-energy balance equation to estimate the latent heat flux, then the estimated fluxes showed a high linear correlation (r = 0.96) and a moderate standard error (47 W m−2) under regression analysis with the observed fluxes.
Evapotranspiration was evaluated by combining remotely sensed reflected solar radiation and surface temperatures with ground station meteorological data (incoming solar radiation, air temperature, windspeed, and vapor pressure) to calculate net radiation and sensible heat flux. Soil heat flux was estimated as a fraction of the net radiation. Instantaneous values of ET were calculated for 18 wheat plots for 44 cloudless days over a growing season. Three of the 18 plots contained lysimeters which provided data to compare against the instantaneous values. For the remaining plots, daily ET was estimated from the instantaneous data and compared with values calculated from soil water contents measured with a neutron moisture meter. For generally clear sky conditions, the comparisons indicated that ET could be adequately evaluated using a combination of remotely sensed and ground based meteorological data. The results suggest that ET maps of relatively large areas could be made using this method with data from airborne sensors. The extent of the area covered appears to be limited by the distance that air temperature and windspeed data can be extrapolated.
Available experimental results indicate that as the density of roughness elements over a horizontally homogeneous surface is varied, the roughness length, z
0, varies in a manner that exhibits a maximum at intermediate density values. In an attempt to explain this behaviour, the available analytical solutions for the wind profile inside dense homogeneous canopies were reviewed. The review indicated that the variation of z
0 with density depends on the interrelationship between the leaf density, a, and the mixing length, l. In view of this finding, a numerical model was devised based on a simple rule for constructing mixing-length profiles in the canopy. The rule states that the actual value of l is the maximum possible under the two constraints: l ⩽ l
i and ¦dl/dz¦ ⩽ k, where k is the von Karman constant and the intrinsic mixing length, l
i, is a function of the local internal structure of the canopy. The model which ensures a smooth transition from dense to thin canopy, was used to reproduce the observed maximum of z
0. The model is also capable of handling vertically non-homogeneous canopies.
In this state-of-the-art review of turbulence in and above plant canopies, the authors stress the fact that local diffusion models of transport are seriously deficient in the canopy environment. They also emphasise that organised structures in the overlying boundary layer are the principal agents determining the nature of canopy turbulence. (from paper)
Surface temperatures, T(s), were estimated for a natural vegetative surface in Owens Valley, California, with infrared thermometric observations collected from an aircraft. The region is quite arid and is composed primarily of bushes (approximately 30%) and bare soil (approximately 70%). Application of the bulk transfer equation for the estimation of sensible heat, H, gave unsatisfactory values when compared to Bowen ratio and eddy correlation methods over a particular site. This was attributed to the inability with existing data to properly evaluate the resistance to heat transfer, r(ah). To obtain appropriate r(ah)-values the added resistance to heat transfer, kB−1, was allowed to vary although there is both theoretical and experimental evidence that kB−1 for vegetative surfaces can be treated as constant. The present data indicate that for partial canopy cover under arid conditions kB−1 may be a function of T(s) measured radiometrically. The equation determining kB−1 was simplified and tested over another arid site with good results; however, this had a limited data set (i.e., 6 data points). The dimensionless kB−1 equation is simplified for use over full canopy cover and is shown to give satisfactory estimates of H over a fully-grown wheat crop.
The surface roughness parameters, zo, can be estimated with different techniques. These techniques are analyzing the mean wind profile, estimating the surface drag coefficient and using the universal functions according to the Monin-Obukhov similarity theory. The first two techniques mentioned have been applied over more or less homogeneous terrain. As a result, it appears that the drag method is very sensitive to shear stress measuring erros and to local changes and irregularities in terrain. -Authors
The estimation of evapotranspiration on a regional scale may be possible using remotely sensed inputs to surface energy balance models. Energy balance considerations lead to a relation that includes net radiation, surface and air temperatures, and an aerodynamic resistance, as inputs. The resistance term was examined as to its behavior under both stable and unstable temperature conditions, several surface roughness conditions, and at various windspeeds. The model shows that the evapotranspiration is higher than net radiation when the surface is cooler than the air and lower when the surface is warmer than the air. The aerodynamic resistance changes due to surface-air temperature differences play a substantial role in determining evapotranspiration.To test the model, evapotranspiration was calculated using remotely sensed temperatures, with the remaining inputs conventionally assessed. The calculations were made for a one-time-of-day period near midday, as would be required for a remote sensing technique, and were compared to lysimetrically determined evapotranspiration. The measured data were obtained at several locations in the Western United States, and were for a variety of crops. The good agreement between calculated and measured values indicates that the goal of developing techniques that produce accurate evapotranspiration estimates over large areas is attainable.
Combining remotely sensed data with ground-based meteorological data allows the evaluation of evapotranspiration (the evaporation of water from soil and plant surfaces) at local and regional scales. Remote sensors can provide information on reflected solar radiation and surface temperatures. The remaining variables in the energy balance equations must be measured at ground level, estimated, modeled, or ignored. It is how these variables are evaluated that distinguish the several approaches to estimating evapotranspiration. In general, regional scale methods would apply to part or all of a satellite image, and use meteorological data from local weather stations. Local scale techniques would rely largely on airborne remote sensors and on-site measurements of the pertinent meteorological factors at the time of remote-data collection. In this paper, methods for estimating evapotranspiration on both local and regional scales are reviewed, and some factors that complicate its measurement are discussed.
Wind profile relationships from the Surface roughness parameter estimated with a drag technique
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A wind tunnel study of turbulent flow close to regularly arrayed rough surfaces Evapotranspiration calculated from re-mote multispectral and ground station meteorological data
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Problems in the estimation of sensible heat flux over incomplete canopy cover with thermal infrared data
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