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Conditional statistics of vertical heat fluxes in local advection conditions
Abstract
Turbulent fluctuations have been investigated in the internal boundary layer (IBL) which forms after a dry-to-wet surface transition. The IBL is defined as that part of the atmospheric surface layer where the influence of the downstream surface is noticeable. The results of the application of three different quadrant analysis techniques are presented. The three techniques, in increasing order of the amount of information supplied, provide:(1)
the diurnal variation of quadrant contribution (C
i), number fraction (T
i) and conditional average (% MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXafv3ySLgzGmvETj2BSbqefm0B1jxALjhiov2D% aebbfv3ySLgzGueE0jxyaibaiiYdd9qrFfea0dXdf9vqai-hEir8Ve% ea0de9qq-hbrpepeea0db9q8as0-LqLs-Jirpepeea0-as0Fb9pgea% 0lrP0xe9Fve9Fve9qapdbaqaaeGacaGaaiaabeqaamaabaabcaGcba% GaeyykJeUabm4DayaafaGabm4CayaafaGaeyOkJe-aaSbaaSqaaiaa% dMgaaeqaaaaa!4215!\[\langle w's'\rangle _i \], with s = T or q) of vertical sensible and latent heat fluxes,
(2)
the quadrant contribution and number of samples of different sizes depending on the relative magnitude of each sample, and
(3)
the distribution of the nondimensional probability density function.
The results show that in the IBL the vertical flux of sensible heat is maintained by (i) a small fraction of large samples with warm air carried upwards, and (ii) a larger fraction of small samples with cool air carried downwards. Both processes are almost equal in importance. In the morning and near the top of the IBL negative temperature fluctuations are limited by the near-uniform temperature conditions upstream and above the IBL. This limitation reduces, at that location, the conditional average of the sinking motions of cool air. Closer to the wet surface the negative temperature fluctuations are less susceptible to the above mentioned limitation. As a consequence contributions from all four quadrants are almost equal leading to a very small vertical heat flux. In the presence of a temperature inversion over both the upstream and the downstream terrain, shear-generated turbulence appears to be the cause of the relative abundance of sinking motions of warm air and rising motions of cool air, leading to a reversal of the sensible heat flux. The latent heat flux is positive (i.e. directed away from the surface) at all times and is maintained in almost equal amount by (i) a small number of large magnitude samples with moist air carried upwards, and (ii) small magnitude samples with sinking motions of dry air. These sinking motions of dry air are far more numerous, especially in the morning, but their conditional average is very small. The abundance of sinking motions of dry air is attributed to the fact that over the downstream terrain evaporation is greatly enhanced, leading to a skewed wq signal. This skewness is clearly visible in the wq-probability density distribution of the morning runs. In the evening the asymmetry between these two different contributions disappears. This is because evaporation is greatly reduced and large positive humidity fluctuations no longer occur.
... A good understanding of vertical profile of horizontal wind speed within and above a plant community is a prerequisite to understanding turbulent transport of water vapour as well as temperature fluctuations within a crop canopy (Arya, 2001;Figuerola and Berliner, 2006). Over time, the mean horizontal wind speed will vary rapidly with height above the ground through the roughness sub-layer, particularly within the plant canopy, due to the effects of drag exerted by the underlying surface (Raupach et al., 1991;Kroon and Bink, 1996). Water vapour and heat fluxes are some of the most important constituents of the atmosphere, which also have great biological importance (Ray et al., 2002). ...
In a cropped field, microclimate and thermal stability conditions depend on the canopy structures and the prevailing weather. The main aim of the study therefore was to characterize the vertical profiles of weather variables within and above a maize (Zea mays L.) canopy and to describe the water vapour pressure deficit (VPD) under different atmospheric and soil surface conditions for both wide and narrow runoff strips with the in-field rainwater harvesting (IRWH) system. Micrometeorological measurements of wind, temperature and relative humidity were performed at eight levels, within canopy (1.8 and 2.1 m), and just above the canopy (2.4, 2.7, 3.0, and 3.3 m) up to reference levels (3.9 and 4.5 m) when the maize reached a maximum height of 2.2 m. Under incomplete canopy cover of the IRWH system, two important factors complicated evapotranspiration estimation, namely the local advection and high temperatures of the bare soil between adjacent plant rows. Diurnal variations of water vapour related to turbulence at each locality and its position in the thermal internal boundary layers. Generally, advection was more pronounced in wide runoff strips than narrow strips. On wide runoff strips the wind was more effective in replacing the air between the rows and maintained a higher driving force for evaporation. The maximum VPD over the narrow strips was observed at reference level during a dry day, at about 2.2 kPa in the afternoon, while wet day VPD reached a maximum of 1.8 kPa. The VPD of the wide runoff strips correlated negatively with wind speed, but showed a fairly positive correlation with some scattered values on wet days after rain. Therefore, profile characteristics within and above plant canopies played a key role in determining the VPD and consequently, could help to explain transpiration rates of crops. Hence, VPD relations enhanced the understanding of the heat energy exchange processes under the heterogeneous nature of maize canopy of the IRWH tillage system.
... Also, we expect that K Y /K R > 1 because larger eddies will transport red scalars less effectively. This follows from the mechanism discussed by Lang et al. (1983) and Kroon and Bink (1996) whereby the larger downsweeps are Figure A1. Ratio of diffusivities for temperature and humidity plotted against the ratio of the upwind-sourced and downwind-sourced parts of the temperature gradient. ...
This paper examines the effect of non-stationarity of the wind on similarity of the eddy diffusivities for heat and vapour within a stable layer at the bottom of an internal boundary layer formed downwind of a dry-to-wet transition. First, we present some experimental data taken above a rice crop downwind of very extensive dry range lands at Warrawidgee, NSW, Australia. These data establish that periods of higher wind speed were associated with periods of higher saturation deficit in the canopy of the rice crop, and lower Bowen ratio. It is shown that Bowen ratios calculated for 30-second sub-intervals varied three-fold within a single 20-minute averaging period. Thus periods of higher wind speed corresponded to periods of higher moisture flux and smaller sensible heat flux.An idealized situation is then analysed theoretically. It is assumed that the time scale of the slow variations of the wind is long compared with the surface-layer time scale and that fetch is sufficient that the air near the ground is in continuous equilibrium with the surface. Using a two-scale Reynolds decomposition of the fluctuating wind and scalar variables into active and inactive components, it is shown that unsteadiness can lead to an eddy diffusivity for saturation deficit, calculated as the ratio of average flux to average gradient, that is larger than that for total energy calculated in a similar way. Using this ratio to calculate the ratio of diffusivities for temperature and humidity, K-T/K-q, it is found that the latter can be much larger than one if the Bowen ratio is small and negative. Despite this, assuming K-T = K-q and using the Bowen ratio method to calculate surface energy fluxes will usually incur only minor errors.
... which is more general than Equation (1) and does not suffer from the limitations and assumptions invoked by K95. Using high frequency w, T , and q measurements from the La Crau site (De Bruin et al., 1991;Kroon and Bink, 1996), BM97 concluded that predictions from Equation (4) are in good agreement with measurements. We investigated Equation (4) in Figure 1 using all measurements from K95 and found similar agreement to BM97 but the scatter in the K95 data is larger than that reported by BM97's Figure 1. ...
... In some level above the boundary layer, the stable (inversion) temperature profile above the wet surface (downwind) lost connection with the surface, and it should follow with unstable temperature profile upwind. Our results agree with Kroon and Bink (1996). They investigated the turbulent fluctuations in the internal boundary layer which forms in the wake of a dry to wet surface transition. ...
In some arid land, the irrigated fields are not contiguous and are surrounded by large patches of bare land. During the summer time and rainless season, the solar radiation flux is high and the surface temperature during daylight in the dry bare areas, is much higher than that of the air. The sensible heat generated over these areas may be advected to the irrigated fields. The crops are usually planted in rows and the irrigation systems used (trickle) do not wet the whole surface, the dry bare soil between the rows may develop high soil surface temperatures and lead to convective activity inside the canopy above the bare soil. Advection from the surrounding fields and convective activity inside the canopy affect the layer above the crop. We studied the surface layer above an irrigated tomato field planted in Israel´s Negev desert. The crop was planted in rows, trickle irrigated and the distance between the outer edges of two adjacent rows was 0.36 m at the time of measurement. The gradients in temperature and water vapor pressure were obtained at various heights above the canopy using a Bowen ratio machine. The residual in the energy balance equation was used as a criterion to determine the equilibrium layer. During the morning, unstable conditions prevail, and the equilibrium layer was between Z/h ~ 1.9 and 2.4. In some particular circumstances, in the late morning, the bare soil between the rows reached extremely high temperatures and during conditions with low wind speeds free convection was identified. During these hours the ‘‘residuals’’ of the energy budget to the heights Z/h = 1.5 and 2.4 were significantly different from zero and an extremely large variability was evident for the Z/h = 3.2 layer. Local advection took place during the afternoon resulting in an increase in the stability of the uppermost measured layer and propagated slowly downwards. The equilibrium layer was between Z/h ~ 1.5 to 2.4. The residuals were significantly different from zero for the uppermost layers Z/h = 2.7 and 3.2 during these periods. Our findings suggest that the depth and location of the internal equilibrium layer above trickle irrigated row crop fields surrounded by dry bare areas, vary in response to wind speed and the temperature of the soil in between the rows of the crop. For some time intervals, the computation of fluxes using the conventional flux-gradient approach measurements was not possible.
... Our results agree with Kroon and Bink (1996). They investigated the turbulent fluctuations in the internal boundary layer which forms in the wake of a dry to wet surface transition. ...
Arid and semi-arid regions are heterogeneous landscapes in which irrigated fields are surrounded by arid areas. The advection of sensible heat flux from dry surfaces is a significant source of energy that has to be taken into consideration when evaluating the evaporation from crops growing in these areas. The basic requirement of most of the common methods for estimating evapotranspiration [Bowen ratio, aerodynamic and Penman-Monteith (PM) equation] is that the horizontal fluxes of sensible and latent heat are negligible when compared to the corresponding vertical fluxes. We carried out measurements above an irrigated tomato field in a desert area. Latent and sensible heat fluxes were measured using a four-level Bowen machine with aspirated psychrometers. Our results indicate that under advective conditions only measurements carried out in the lowest layer are satisfactory for the estimation of latent heat fluxes and that the use of the PM equation with an appropriately parameterized canopy resistance may be preferable.
A comprehensive study of the turbulent structure of the atmospheric boundary layer in unstable conditions has been carried out using turbulence data obtained from the Gobi desert, grassland, suburban and urban sites based on the same instrumentation, data acquisition and data processing systems. The normalized standard deviations of u and v over the suburban and urban sites are systematically smaller than those over the Gobi desert and grassland sites. However, the normalized standard deviations of w, temperature and humidity q over the suburban and urban sites are quite close to those over the Gobi desert and grassland sites. The normalized humidity standard deviations are quite similar to those of temperature over the grassland and suburban sites. The temperature and humidity spectra are found to be independentof atmospheric stability in all frequency ranges, but spectra obtained from the Gobi desert and grassland sites reveal flatter peaks than those of the suburban and urban sites in the lower frequency range. The normalized spectral curves of temperature and humidity are independent of atmospheric stability both at high and low frequencies. The spectral characteristics of humidity over the grassland and suburban sites aresimilar to those of temperature, but the humidity spectra over the Gobi desert site are quite different from temperature spectra due to evaporation and/or large eddies in the boundary layer.
On June 28, 1996, RASS (radio-acoustic sounding system) and aircraft observations were taken of the evolving boundary layer as part of the Montreal experiment on regional mixing and ozone (MERMOZ). The vertical air velocity measurements of the RASS were exceptionally free of clutter or interference. The RASS observation period spans from 1045 to 1400 EST. The aircraft observations were taken between 1155 and 1335 EST, along 35 km long east-west runs, at a distance of 30 km north-east of the RASS site. Time-height plots of the RASS clear-air signal-to-noise ratio, virtual potential temperature, and vertical air velocity reveal a boundary layer that is very convective. The inversion height starts at 600 m AGL and grows to about 1 km AGL, and well defined vertical structures of rising and sinking air are evident. Derived quantities from RASS data include virtual temperature fluctuations, heat flux traces, and heat flux quadrants. Corresponding time-height plots are consistent with the descriptions of a convective boundary layer given by SCHMIDT and SCHUMANN (1989), among others. The convective boundary layer is seen to be very nonstationary. Heat flux profiles of RASS and aircraft data are comparable up to 500 m AGL, but heat flux cospectra of RASS and aircraft data are shown to have different forms.
The paper describes a convective boundary layer experiment conducted in
April 1978 at the Boulder Atmospheric Observatory, and examines the
spectral behavior of wind velocity and temperature from the
Observatory's 300 m tower, from aircraft flights alongside the tower and
from a surface network of anemometers, for evidence of terrain influence
on turbulence structure. The gently rolling terrain at the site does not
seem to affect the turbulence spectra from the tower in any perceptible
manner, except for minor shifts in the vertical velocity and temperature
spectral peaks. The aircraft vertical velocity spectra showed different
shapes for alongwind and crosswind sampling directions, as in earlier
measurements over ocean surfaces, and their peaks are displaced to
higher wavenumbers compared with the tower spectra. Long-term spectra of
horizontal wind components from surface stations around the tower
exhibit no particular sensitivity to site selection. Under
near-stationary conditions the peak of the spectrum of the streamwise
component tends to reflect more closely the predominant boundary layer.
convective scales than does the peak of the lateral wind component. The
problem of identifying those scales in the presence of large shifts in
wind direction is discussed.
In this study a moderate-sized alfalfa field was downwind of a large dry region. Measured vertical profiles of temperature, humidity and wind at upwind and downwind sides of the field were used to calculate the short-term evaporation rate, as well as the contribution of horizontally transported or advected heat energy to the evaporation. The vertical profiles must be measured at least to the height at which air is modified by the new surface. In this case that height was as large as 18 m over a several hundred meter long traverse.
Evaporation rates calculated by such an approach were in very close agreement to surface fluxes measured by an eddy correlation system near the surface. The difference between calculated and measured values averaged 9.5%. The reduction in sensible heat content of the air of the control volume was substantial during passage over the field. If all this energy was assumed to have been used in evaporation, then the advection of heat contributed from 35 to 86% of the total evaporation rate.
It appears that for an inhomogeneous surface, knowledge of the distortion of some properties of local boundary layers can yield reliable estimates of local evaporation. The approach has little empiricism as it is based on simple conservation laws.
The Reynolds stress field of the urban roughness sublayer is studied experimentally at a site in the centre of Zrich (Switzerland). Turbulence observations at various heights within and above a street canyon are used together with profiles of mean variables to determine an average profile of Reynolds stress for the lowest few tens of meters of an urban roughness sublayer.The spatially averaged net vertical transport of momentum or Reynolds stress is found to be essentially zero at a height close to the average zero plane displacement and increases higher up in the urban roughness sublayer. A parameterisation for the height dependence is provided based on the height above zero plane displacement and a reference friction velocity. Resultes of a quadrant analysis for Reynolds stress indicate that eddies of the organized shear flow are broken up in the vicinity of the zero plane displacement into smaller, less correlated (random) flow patterns. Although not constant with height, turbulent flux of momentum is shown to be the relevant process for the description of the profile of mean wind speed even in the urban roughness sublayer.
The surface fluxes of water vapor, sensible heat, and momentum are important in many meteorological, agricultural, and hydrological problems. In most applications it is (tacitly) assumed that the Earth’s surface is, on the scale of interest, horizontally homogeneous. An example is the way in which the surface fluxes are parameterized in models for the prediction of weather and climate. Also, most measuring techniques used for the determination of the surface fluxes are based on the assumption of horizontal homogeneity.
Sensible heat flux measurements were carried out to examine the characteristics related to frequency and duration of extreme flux events above a desert surface. First, the flux bursts were identified by a modified hyperbolic hole ``quadrant analysis'' in which the threshold criterion depends on the mean and standard deviation of the flux time statistics. Second, the frequency distribution of the extreme flux burst length was examined. It was found that the frequency distribution of the burst length is well approximated by two-parameter power laws which account for two distinct types of bursting phenomena: one is organized, coherent, and has a relatively long duration, while the other is short lived, less organized, and has a relatively short duration. Linear regression analysis was used to determine the two power exponents of the burst length probability density function. Both exponents compare favorably with other reported measurements on concentration bursting in dispersing plumes from an elevated source. The cutoff burst length from one power law to the next also matches other studies. A strong correlation between bursting frequency and flux integral time scales was observed under a variety of atmospheric stability and surface wetness conditions. This strong correlation suggested a relationship between atmospheric stability and the mean time interval between bursts. It was found that the variation in the interval between bursts for the dynamic and dynamic convective sublayers is strongly related to the variation in stability. This was not the case for near-convective and stable atmospheric conditions.
We present a theory for probe-induced flow distortion which is applicable in the atmosphere at heights greater than about ten times the obstacle size. We use the theory to calculate the behavior of Reynolds shear stress and velocity variances ahead of a cylinder and a sphere. The stress is found to be most seriously distorted, the extent depending on the nature of the trailing wake. -from Author
The conditional sampling method is a means of distinguishing and providing quantitative information of shear, periodic, and coherent structure flows in turbulent conditions. The technique produces data ranging from zone and point averages obtained in turbulent regions bounded by turbulent-nonturbulent interfaces to point averages relative to the occurrence of structures. The conditional sampling method can be applied to the study of the effect of the Reynolds number on the large structure, the spatial geometry of the Reynolds-number-dependent fine structure and its interaction with large structure, and the connection and interaction between different events at different times
Quadrant analysis has been used to investigate the events contributing to the Reynolds shear stress in zero-pressure-gradient turbulent boundary layers over regularly arrayed rough surfaces of several different densities, and over a smooth surface. By partitioning the stress into ejections, sweeps, and inward and outward interactions, it is shown that sweeps account for most of the stress close to rough surfaces, and that the relative magnitude of the sweep component increases both with surface roughness and with proximity to the surface. The sweep-dominated region delineates a ‘roughness sublayer’ with a depth of up to several roughness element heights, in which the turbulence characteristics depend explicitly on the roughness. In the remainder of the inner (or constant-stress) layer, and in the outer layer, the flow obeys familiar similarity laws with respect to surface roughness.
The difference Δ S 0 between the fractional contributions of sweeps and ejections to the stress is shown to be well related everywhere to the third moments of the streamwise and normal velocity fluctuations. Experimental proportionalities are established between the third moments and δ S 0 , and are shown to agree with predictions made from cumulant-discard theory.
The time scale for the passage of large coherent structures past a fixed point, T , is assumed proportional to the mean time between occurrences in a specified quadrant of an instantaneous stress u'w’ at least H times the local mean stress u'w’ , where H is a threshold level. For both the ejection and sweep quadrants and for any choice of H , it is found that T scales with the friction velocity u * and the boundary-layer thickness δ, such that Tu * /δ is invariant with change of surface roughness.
I prove that in a steady velocity field the frequency spectrum of a conservative scalar is unaffected by flow distortion. This is a good approximation in turbulent flow if (qa1/3/U1l1/3)<< 1, where q and l are velocity and length scales of the energy-containing turbulence, a is the crossstream dimension of the body, and U1 is the mean flow speed. Criteria seem easily met for scalar mixing ratio measurements in typical aircraft applications but are more difficult to satisfy on towers. At aircraft speeds, crosstalk from air density fluctuations can seriously contaminate species density signals measured in regions of strong flow distortion. These errors can be very important in aircraft measurements of the vertical fluxes of CO2 and water vapor, whose sensors typically measure species density rather than mixing ratio. These errors can be minimized through boom design. Temperature measurements from aircraft can also be seriously affected by flow distortion. -from Author
A comprehensive study of atmospheric turbulence over an urban surface has been carried out. In this Part I of the study the spectral characteristics are analysed. Results involving all the important atmospheric variables, Measured at two heights (z'/z0=21 and 37) under unstable conditions, are presented, normalized by the respective variances/covariances, and within the Monin-Obukhov similarity framework. Although the shapes and location of the peaks of the urban spectra/cospectra are in good agreement with homogeneous surface layer data, the analysis of the normalized dissipation rates and spectral correlation coefficients reveals differences which can be attributed to the rough surface. For example, the dissipation of turbulent kinetic energy is relatively small, perhaps owing to an increase in the transport and export of locally produced turbulent energy; there is a very efficient transfer of Momentum which it is suggested may be a result of wake production associated with bluff bodies, and it is found that the transfer of heat and moisture are dissimilar.
The convective mass flux parameterization often used in meteorological modeling expresses the vertical flux of a transported scalar as proportional to the product of the difference in mean values of the scalar in updrafts and downdrafts and their characteristic velocity. The proportionality factor is a constant to be specified. We show that this proportionality factor also appears in the relaxed eddy accumulation technique of Businger and Oncley. That associates the surface-layer flux of a scalar with the product of the standard deviation of vertical velocity and the mean concentration difference between updrafts and downdrafts.We show that this constant (b) is determined uniquely by the joint probability density (jpd) of vertical velocity and the scalar. Using large-eddy simulation, we generate this jpd for a conservative scalar diffusing through a convective boundary layer. It has quite different forms in top-down and bottom-up diffusion geometries. The bottom-up jpd is fairly well represented by a jointly Gaussian form and implies b ~ 0.6, in good agreement with the surface-layer value reported by Businger and Oncley. The top-down jpd is strikingly non-Gaussian and gives b ~ 0.47. Updrafts carry the bulk of the scalar flux - 70% in the bottom-up case, 60% in the top-down case.
A review is given of relevant work on the internal boundary layer (IBL) associated with:(i)
Small-scale flow in neutral conditions across an abrupt change in surface roughness,
(ii)
Small-scale flow in non-neutral conditions across an abrupt change in surface roughness, temperature or heat/moisture flux,
(iii)
Mesoscale flow, with emphasis on flow across the coastline for both convective and stably stratified conditions.
The major theme in all cases is on the downstream, modified profile form (wind and temperature), and on the growth relations for IBL depth.
This paper reports on measurements of sensible and latent heat and CO2 fluxes made over an irrigated potato field, growing next to a patch of desert. The study was conducted using two eddy correlation systems. One measurement system was located within the equilibrium boundary layer 800 m downwind from the edge of the potato field. The other measurement system was mobile and was placed at various downwind positions to probe the horizontal transition of vertical scalar fluxes. Latent (LE) and sensible (H) heat fluxes, measured at 4 m above the surface, exhibited marked variations with downwind distance over the field. Only after the fetch to height ratio exceeded 75 to 1 didLE andH become invariant with downwind distance. When latent and sensible heat fluxes were measured upwind of this threshold, significant advection of humidity-deficit occurred, causing a vertical flux divergence ofH andLE.The measured fluxes of momentum, heat, and moisture were compared with predictions from a second-order closure two-dimensional atmospheric boundary layer model. There is good agreement between measurements and model predictions. A soil-plant-atmosphere model was used to examine nonlinear feedbacks between humidity-deficits, stomatal conductance and evaporation. Data interpretation with this model revealed that the advection of hot dry air did not enhance surface evaporation rates near the upwind edge of the potato field, because of negative feedbacks among stomatal conductance, humidity-deficits, andLE. This finding is consistent with results from several recent studies.
This is the first of two papers reporting the results of a study of the turbulence regimes and exchange processes within and above an extensive Douglas-fir stand. The experiment was conducted on Vancouver Island during a two-week rainless period in July and August 1990. The experimental site was located on a 5o slope. The stand, which was planted in 1962, and thinned and pruned uniformly in 1988, had a (projected) leaf area index of 5.4 and a heighth=16.7 m. Two eddy correlation units were operated in the daytime to measure the fluctuations in the three velocity components, air temperature and water vapour density, with one mounted permanently at a height of 23.0m (z/h=1.38) and the other at various heights in the stand with two to three 8-hour periods of measurement at each level. Humidity and radiation regimes both above and beneath the overstory and profiles of wind speed and air temperature were also measured. The most important findings are:(1)
A marked secondary maximum in the wind speed profile occurred in the middle of the trunk space (aroundz/h=0.12). The turbulence intensities for the longitudinal and lateral velocity components increased with decreasing height, but the intensity for the vertical velocity component had a maximum atz/h=0.60 (middle of the canopy layer). Magnitudes of the higher order moments (skewness and kurtosis) for the three velocity components were higher in the canopy layer than in the trunk space and above the stand.
(2)
There was a 20% reduction in Reynolds stress fromz/h=1.00 to 1.38. Negative Reynolds stress or upward momentum flux perisistently occurred atz/h=0.12 and 0.42 (base of the canopy), and was correlated with negative wind speed gradients at the two heights. The longitudinal pressure gradient due to the land-sea/upslope-downslope circulations was believed to be the main factor responsible for the negative Reynolds stress.
(3)
Momentum transfer was highly intermittent. Sweep and ejection events dominated the transfer atz/h=0.60, 1.00 and 1.38, with sweeps playing the more important role of the two atz/h=0.60 and 1.00 and the less important role atz/h=1.38. But interaction events were of greater magnitude than sweep and ejection events atz/h=0.12 and 0.42.
Boundary-layer flow over very rough surfaces is poorly understood so the applicability of standard micrometeorological theory is uncertain. This study presents observations of the turbulent fluctuations of meteorological parameters over a suburban area. Even though the height of measurement is considered to be close to the junction between the inertial and roughness sub-layers, the wind and temperature spectra and the momentum and sensible heat flux cospectra are in good agreement with reference data from smoother surfaces. Recommendations are made concerning site requirements, height of measurement and averaging times for the study of turbulence and turbulent fluxes over suburban terrain.
Turbulent fluctuations of atmospheric properties near the soil-vegetation-atmosphere interface have been investigated in a situation of strong local advection. To this end, an analysis has been made of data from the 1987 field experiment in ‘la Crau’ (France), where an extensive dry terrain with sparse vegetation was situated upwind from an irrigated field completely covered with grass. The change in surface roughness was negligible, the main effect being a change in surface heat flux. In the experiment, turbulence measurements were made of horizontal and vertical wind speed, temperature and humidity at various locations over both terrains. In this study, 12 runs of 1 h are analysed, roughly half of which were made when an advective inversion was present over the downstream terrain. Both vertical and horizontal velocity fluctuations are almost unaffected by the change in terrain. Temperature fluctuations are influenced by the fact that over the downstream terrain production of variance almost ceases and advection of variance becomes important. This results in a deviation of the relation found for homogeneous situations. Also, the temperature spectra of the unstable runs are affected, showing a different inertial subrange roll-off rate. Humidity fluctuations adjust rapidly to the new surface and, for all but the lowest frequencies, behave as if measured over homogeneous terrain.
Data from two 100-min runs on the turbulent atmospheric wind, temperature and humidity fields above a pine forest have been analysed using conditional sampling techniques. With the aid of the temperature time series, ramp events were identified and all fields were averaged in order to remove smaller scale turbulence and random low-frequency turbulence, and to map the organized structures revealed in this way. It is shown that the turbulent fluxes of momentum, heat and humidity, determined from these ramp-events, in fact constitute a large part of the total fluxes, about 90% during the actual events.
A comprehensive experiment for turbulence and profiles has been carried out over a very rough natural mallee bushland. In Part I, the large inherent structures of the turbulence are presented. Short-period space-time correlations of wind and temperature indicate that periodic fluctuations pervade the lower surface layer above the canopy. These are associated with the appearance of large turbulent structures. Prominent peaks in the spectra are identified with the arrival of these structures, which show that they have a relatively large length scale over rough surfaces. These findings suggest a different mechanism for the enhancement of turbulent diffusivity over rough surfaces from that by the simple generation of turbulence by plant wakes.
Analysis via conditional sampling of the turbulent components has revealed that ‘bursts’ make almost the same contributions as ‘gusts’ (at the measurement height of 8.4 m above ground) for either momentum or temperature under neutral conditions; for unstable conditions and high ‘hole’ size H (see Equation (3)), gusts dominate for momentum while bursts dominate for heat transfer. This implies that the mechanisms for momentum and heat transfer are different.
"Stellingen" inserted. Thesis (doctoral)--Landbouwuniversiteit te Wageningen, 1996. Includes bibliographical references (p. 173-182).
Thesis (Ph. D.)--Utah State University. Dept. of Plants, Soils and Biometeorology, 1994. Includes bibliographical references.
The observed effects of sharp changes in sea surface temperature (SST) on the air-sea fluxes, surface roughness, and the turbulence structure in the surface layer and the marine atmospheric boundary layer are discussed. In situ flux and turbulence observations were carried out from three aircraft and two ships within the FASINEX framework. Three other aircraft used remote sensors to measure waves, microwave backscatter, and lidar signatures of cloud tops. Descriptions of the techniques, intercomparison of aircraft and ship flux data, and use of different methods for analyzing the fluxes from the aircraft data are described. Changing synoptic weather on three successive days yielded cases of wind direction both approximately parallel and perpendicular to a surface temperature front. For the wind perpendicular to the front, wind over both cold-to-warm and warm-to-cold surface temperatures occurred. Model results consistent with the observations suggest that an internal boundary layer forms at the SST.
Intra-Field Variability of Scalar Flux Densities across a Transition between a Desert and an Irrigated Potato Field', Boundary-Layer MeteorolTurbulent Exchange above a Pine Forest. 11. Organized Structures', Boundary-Layer Meteoml
- D Baldocci
- K S And Rao
Baldocci, D. and Rao, K. S.: 1995, 'Intra-Field Variability of Scalar Flux Densities across a Transition between a Desert and an Irrigated Potato Field', Boundary-Layer Meteorol. 76, 109-l 36. Bergstrom, H. and Hogstrom, U.: 1989, 'Turbulent Exchange above a Pine Forest. 11. Organized Structures', Boundary-Layer Meteoml. 49,23 l-263.