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Measurements of superadiabatic lapse rates in the middle atmosphere
Geophysical Research Letters
Abstract
A large power-aperture product Rayleigh-scatter lidar system has been successfully built and over 175 nights of middle atmosphere temperature measurements have been obtained. The high signal-to-noise ratio of these measurements allows the stability of the air in the upper stratosphere and mesosphere to be determined. A detailed methodology has been developed to attempt to differentiate between lapse rate variations due to photon counting errors and actual geophysical variations. On nights when the geophysical variations are large compared to the photon counting errors, regions of convective stability and instability can be determined at a reasonably high confidence level. Both statistics of the layers and an “image” of the layers is presented for the night of May 31, 1996. The measured percentage of unstable layers is in agreement with the predictions of Hines (1991), as is the apparently sporadic formation and distribution of the unstable regions.
... Solomon [1991, 1993] and Meriwether and Mlynczak [1995] explored the possibility of chemical heating of the mesosphere by exothermic reactions. Hauchecorne et al. [1987], Senft and Gardner [1991], Meriwether et al. [1994], Whiteway et al. [1995], Thomas et al. [1996], Sica and Thorsley [1996], Gardner and Yang [1998] and Meriwether et al. [1998] indicated that GW breaking plays an important role in the development of MIL. Various processes such as tidal disturbances, chemical heating, tides/gravity waves-mean flow interactions, semiannual oscillations and inertial instabilities may either singly or collectively be operative in the production of the MIL. ...
... The gravity waves become unstable at the height where the zonal wind velocity becomes equal to the wave phase velocity. Usually in mesosphere this condition is satisfied and breaking of gravity waves takes place [Sica and Thorsley, 1996;Thomas et al., 1996]. The turbulent heating, arising from the breaking of waves, provides a feedback mechanism that then may maintain the observed MIL. ...
1] Temperature and ozone volume mixing ratio profiles obtained from the Halogen Occultation Experiment (HALOE) aboard the Upper Atmospheric Research Satellite (UARS) over India and over the open ocean to the south during the period 1991-2001 are analyzed to study the characteristic features of the Mesospheric Inversion Layer (MIL) at 70 to 85 km altitude and its relation with the ozone mixing ratio at this altitude. We have also analyzed both the number of lightning flashes measured by the Optical Transient Detector (OTD) on board the MicroLab-1 satellite for the period April 1995 to March 2000 and ground-based thunderstorm data collected from 78 widespread Indian observatories for the same period to show that the MIL amplitude and thunderstorm activity are correlated. All the data sets examined exhibit a semiannual variation. The seasonal variation of MIL amplitude and the frequency of occurrence of the temperature inversion indicate a fairly good correlation with the seasonal variation of thunderstorms and the average ozone volume mixing ratio across the inversion layer. The observed correlation between local thunderstorm activity, MIL amplitude and mesospheric ozone volume mixing ratio are explained by the generation, upward propagation and mesospheric absorption of gravity waves produced by thunderstorms.
... When this occurs, convective instability or shear instability causes transfer of wave energy from gravity waves to the mean flow with consequent changes in the tidal wind amplitude and phase. Usually, in the mesosphere this condition is satisfied and breaking of gravity waves takes place [Sica and Thorsley, 1996;Thomas et al., 1996]. The turbulent heating, arising from the breaking of waves, provides a feedback mechanism that then may maintain the observed MIL [Fadnavis et al., 2007]. ...
1] The vertical structure of temperature observed by Sounding of Atmosphere using Broadband Emission Radiometry (SABER) aboard Thermosphere, Ionosphere, Mesosphere Energetics and Dynamics (TIMED) and sprites observations made during the Eurosprite 2003-2007 observational campaign were analyzed. Sprite observations were made at two locations in France, namely Puy de Dôme (45°46 0 19.2 00 N; 02°57 0 44.64 00 E; 1.464 km altitude) in the French Massif Central and at the Pic du Midi (42°56 0 11 00 N; 00°08 0 34 00 E; 2.877 km altitude) in the French Pyrénées. It is observed that the vertical structure of temperature shows evidence for a Mesospheric Inversion Layer (MIL) on those days on which sprites were observed. A few events are also reported in which sprites were not recorded, although there is evidence of a MIL in the vertical structure of the temperature. It is proposed that breaking gravity waves produced by convective thunderstorms facilitate the production of (1) sprites by modulating the neutral air density and (2) MILs via the deposition of energy. The same proposition has been used to explain observations of lightings as well as both MILs and lightning arising out of deep convections.
... However, the upper stratosphere, mesosphere, and thermosphere typically support waves of long to short spatial scales that, over short time and spatial scales, are clearly not in HSEQ. Examples of these differences on individual nights and in a climatological sense have been shown by Sica and colleagues [10,11]. ...
The measurement of temperature in the middle atmosphere with Rayleigh-scatter lidars is an important technique for assessing atmospheric change. Current retrieval schemes for this temperature have several shortcomings, which can be overcome by using an optimal estimation method (OEM). Forward models are presented that completely characterize the measurement and allow the simultaneous retrieval of temperature, dead time, and background. The method allows a full uncertainty budget to be obtained on a per profile basis that includes, in addition to the statistical uncertainties, the smoothing error and uncertainties due to Rayleigh extinction, ozone absorption, lidar constant, nonlinearity in the counting system, variation of the Rayleigh-scatter cross section with altitude, pressure, acceleration due to gravity, and the variation of mean molecular mass with altitude. The vertical resolution of the temperature profile is found at each height, and a quantitative determination is made of the maximum height to which the retrieval is valid. A single temperature profile can be retrieved from measurements with multiple channels that cover different height ranges, vertical resolutions, and even different detection methods. The OEM employed is shown to give robust estimates of temperature, which are consistent with previous methods, while requiring minimal computational time. This demonstrated success of lidar temperature retrievals using an OEM opens new possibilities in atmospheric science for measurement integration between active and passive remote sensing instruments.
... In these circumstances, the statistical probability of instability-the fraction of space-time that is rendered unstable-can be represented as a function of a modified Ri (given by the unperturbed stability divided by the square of the rms wind shear) in a fashion that is illustrated by Fig. 3 of Hines (1991a). With much of the shear arising from small-scale waves, the regions of instability lie only here and there, now and then, in transient isolated patches, like whitecaps on an open ocean [as seems to be observed; see Sica and Thorsley (1996)]. This fact probably results in preferential dissipation of the small-scale waves, which alone will produce maximum contributions to instability preferentially within the boundaries that they themselves do so much to delineate. ...
... According to simulations, this ratio should be about 2 for ripples induced by convective instability. Hecht et al. [1997Hecht et al. [ , 2000 found this ratio to be between 3 and 4. Observational and numerical studies of instability are sensitive to the resolution of the studies [e.g., Sica and Thorsley, 1996;Cutler et al., 2001], so care is needed in the measurement of the instability depth in terms of N 2 and Ri. With this caveat, we believe that the accuracies of both observation and simulation presently achieved to be sufficiently accurate to provide a meaningful comparison. ...
[1] On 3 and 5 September 2002 the OH all-sky imager at Platteville, Colorado (40.2°N, 104.7°W), observed small-scale, wavelike patterns (known as ripples), with horizontal wavelengths of ∼9 km and ∼7 km and lifetimes of ∼9 min and ∼15 min, respectively. The Colorado State University sodium lidar at nearby Fort Collins, Colorado (40.6°N, 105°W), also made concurrent observations of temperature and zonal and meridional winds, which allowed us to determine the nature of the ripples observed. Our observations suggest that the 3 September ripple was induced by a convective instability located at 87.5 km and the 5 September ripple was induced by a dynamic instability at 88.5 km. The ripples clearly advected as packets with the background wind. Lidar measurements also allowed us to relate the directions of wind shear to the phase front alignments of both the ripples and the nearby short-period atmospheric gravity waves. These spatial relationships provided a meaningful comparison with previously observed ripples as well as with current theoretical models. Using the 16-hour continuous lidar data set for each case, we deduced that long-period waves created an unusually large temperature perturbation at the ripple times on 3 September and an unusually large wind shear perturbation on 5 September. These perturbations prepared the background atmosphere to be near the verge of local instability, but, as revealed again by lidar observation, it was the superposition of smaller-scale perturbations at the time of the ripples that helped to actually reach the conditions required for instability and generation of the ripples.
... The methodology for determining layers of stability and potential instability is, by necessity, tedious (Sica and Thorsley 1996a). In principle one could simply take a temperature profile, differentiate it and be done. ...
Measurements of the gravity wave spectrum in the upper stratosphere and mesosphere are scarce compared to those in the troposphere and lower thermosphere, which are easier to probe via in situ and remote sensing techniques. This paucity of measurements impacts the parame- terization of gravity waves in general circulation models of the middle atmosphere, since the effects of gravity waves are not adequately con- strained by measurements. The development of a larger power-aperture product lidar at The University of Western Ontario has allowed the measurement of the gravity wave spectrum at high temporal-spatial resolution. These measurements have been used to impact three key areas relevant to parameterizations: the thermodynamic perturbations caused by gravity waves, the variability of the spatial and temporal spectra and the estimation of the eddy diffusion coefficient. After a brief introduction to the instrumentation (Section 2), Section 3 describes an extension of the initial measurements of superadiabatic lapse rates by Sica and Thorsley (1996a). A taste of the spatial and temporal spectral retrievals possible with the lidar measurements is given in Section 4. In
... Strictly speaking small positive Brunt-Väisälä frequencies are still assumed to represent stable conditions. However, convective instabilities are expected only for few minutes [e.g., Sica and Thorsley, 1996; Sherman and She, 2006]. Because of our longer integration time of 1 hour convective instabilities are often smoothed out to small, but positives values of N 2 . ...
1] Since August 2002, temperature measurements from 1 to 105 km are performed at the Leibniz Institute of Atmospheric Physics in Kühlungsborn (54°N) with a combination of different lidars. Results from 14 nights in winter and 31 in summer are presented. Nightly mean profiles and fluctuations with a temporal and vertical resolution of 15 min and 1 km, respectively, are derived. In both seasons, wave energy propagates upward (phase propagates downward) with vertical phase speeds of À0.25 to À1.9 m/s. Phase speeds are generally larger in the mesosphere compared to the stratosphere because of decreasing static stability. Wave periods are found in the entire range of 1–8 hours (given by experimental constraints) with no preference for particular periods. The observed vertical wavelengths cover the entire instrumental range of 5–50 km, but the majority lies below 22 km in both seasons. In single nights, a few waves (up to three) dominate the wave spectrum and represent 45–65% of the entire variability. Wave amplitudes generally increase with altitude with a scale height of $18–22 km, i.e., less than expected (14 km) for propagation conserving momentum flux density. The gravity wave energy loss changes with altitude and is different in both seasons. Local fluctuation minima (''nodes'') are often observed, frequently colocated with convective instabilities. Temperature fluctuations are generally smaller in summer compared to winter (maximum values are 10 and 25 K, respectively). Applying gravity wave polarization relations to the mean winter and summer lidar temperature profiles, the difference of fluctuations is basically determined by the background conditions, especially at $60–80 km.
... It is necessary to resolve the important waves, which can have periods as small as a few minutes, and vertical wavelengths as small as a few kilometers. Observational studies of dynamic and static instabilities are mostly limited to the stratosphere and lower mesosphere using balloon and Rayleigh lidar observations (Sica and Thorsley, 1996;Allen and Vincent, 1995;Pfenninger et al., 1999). During the past 10 years, several campaigns have been conducted to characterize shear and convective instabilities in the mesopause region in order to understand the turbulence structure (e.g., MAC/EPSILON conducted in 1987, Blix et al., 1990;DYANA conducted in 1990DYANA conducted in , O ermann, 1994L ubken et al., 1994). ...
The structure and seasonal variations of static (convective) and dynamic (shear) instabilities in the mesopause region (80–) are examined using high-resolution wind and temperature data obtained with a Na lidar at the Starfire Optical Range, NM. The probabilities of static and dynamic instability are sensitive functions of , where N is the buoyancy frequency and S is the total vertical shear in the horizontal winds. The mesopause region is most stable in summer when the mesopause is low, N is large and S is small. Monthly mean varies from a maximum value of about 1.06 in mid-summer to a minimum of 0.68 in January. The annual mean values of N and S are, respectively, and . The probabilities of static and dynamic instabilities are maximum in mid-winter when they average about 10% and 12%, respectively, and are minimum in summer when they average about 7% and 5%, respectively. The observations are generally consistent with theoretical predictions based on Gaussian models for the temperature and wind fluctuations induced by gravity waves. They also show that statically unstable conditions are generally preceded by dynamically unstable conditions. The instability probabilities vary considerably from night to night and the structure of the unstable regions are significantly influenced by atmospheric tides. Tides alone are usually not strong enough to induce instability but they can establish the environment for instabilities to develop. As the tidal temperature perturbations propagate downward, they reduce the stability on the topside of the positive temperature perturbation. Instabilities are then induced as gravity waves propagate through this layer of reduced static stability.
... When this occurs, convective instability or shear 279 instability causes transfer of wave energy from gravity waves to the mean flow with 280 consequent changes in the tidal wind amplitude and phase. Usually in the mesosphere this 281 condition is satisfied and breaking of gravity waves takes place [Sica and Thorsley, 1996: 282 Thomas et al., 1996]. The turbulent heating, arising from the breaking of waves, provides a 283 feedback mechanism that then may maintain the observed MIL [Fadnavis et al., 2007]. ...
The vertical structure of temperature observed by SABER (Sounding of Atmosphere using Broadband Emission Radiometry) aboard TIMED (Thermosphere, Ionosphere, Mesosphere Energetics and Dynamics) and sprites observations made during the Eurosprite 2003 to 2007 observational campaign were analyzed. Sprite observations were made at two locations in France, namely Puy de Dome in the French Massif Central and at the Pic du Midi in the French Pyrenees. It is observed that the vertical structure of temperature shows evidence for a Mesospheric Inversion Layer (MIL) on those days on which sprites were observed. A few events are also reported in which sprites were not recorded, although there is evidence of a MIL in the vertical structure of the temperature. It is proposed that breaking gravity waves produced by convective thunderstorms facilitate the production of (a) sprites by modulating the neutral air-density and (b) MILs via the deposition of energy. The same proposition has been used to explain observations of lightings as well as both MILs and lightning arising out of deep convections. Comment: 34 pages, 5figures. Accepted in Journal of Geophysical Research, USA
... The gravity waves become 14 unstable at the height where the zonal wind velocity becomes equal to the wave phase 15 velocity. Usually in mesosphere this condition is satisfied and breaking of gravity waves 16 takes place (Sica and Thorsley, 1996: Thomas et al., 1996). The turbulent heating, arising 17 from the breaking of waves, provides a feedback mechanism that then may maintain the 18 observed MIL. ...
Temperature and ozone volume mixing ratio profiles obtained from the Halogen Occultation Experiment (HALOE) aboard the Upper Atmospheric Research Satellite (UARS) over India and over the open ocean to the south during the period 1991-2001 are analyzed to study the characteristic features of the Mesospheric Inversion Layer (MIL) at 70 to 85 km altitude and its relation with the ozone mixing ratio at this altitude. We have also analyzed both the number of lightning flashes measured by the Optical Transient Detector (OTD) onboard the MicroLab-1 satellite for the period April 1995 - March 2000 and ground-based thunderstorm data collected from 78 widespread Indian observatories for the same period to show that the MIL amplitude and thunderstorm activity are correlated. All the data sets examined exhibit a semiannual variation. The seasonal variation of MIL amplitude and the frequency of occurrence of the temperature inversion indicate a fairly good correlation with the seasonal variation of thunderstorms and the average ozone volume mixing ratio across the inversion layer. The observed correlation between local thunderstorm activity, MIL amplitude and mesospheric ozone volume mixing ratio are explained by the generation, upward propagation and mesospheric absorption of gravity waves produced by thunderstorms. Comment: 45 pages, 10 figures, 2 tables, PDF format, version published in Journal of Geophysical Research-Atmosphere
... MCS's ability to map the thermal structure of the middle atmosphere globally also may allow it to detect and map dry convective instabilities within the middle atmosphere: a phenomenon of interest for martian middle atmospheric dynamics and comparative planetology with the Earth. Since the 1960s (e.g., Knudsen and Sharp, 1965; Hodges, 1967; Lindzen, 1981; Whiteway and Carswell, 1994; Sica and Thorsley, 1996; Williams et al., 2002), dry convective instabilities have been observed throughout the Earth's stratosphere and mesosphere in association with wave-like perturbations. Recent studies in the terrestrial extratropics have observed convective instabilities in thermal profiles and/or convective roll structures near the mesopause (Collins and Smith, 2004; Liu et al., 2004; Williams et al., 2006), which they interpret to result from superposition of internal gravity waves with the thermal tides. ...
Dry convective instabilities in Mars’s middle atmosphere are detected and mapped using temperature retrievals from Mars Climate Sounder observations spanning 1.5 martian years. The instabilities are moderately frequent in the winter extratropics. The frequency and strength of middle atmospheric convective instability in the northern extratropics is significantly higher in MY 28 than in MY 29. This may have coupled with changes to the northern hemisphere mid-latitude and tropical middle atmospheric temperatures and contributed to the development of the 2007 global dust storm. We interpret these instabilities to be the result of gravity waves saturating within regions of low stability created by the thermal tides. Gravity wave saturation in the winter extratropics has been proposed to provide the momentum lacking in general circulation models to produce the strong dynamically-maintained temperature maximum at 1–2 Pa over the winter pole, so these observations could be a partial control on modeling experiments.
... For the two earlier nights, the individual temperature pro®les show evidence of dry adiabatic lapse rates chie¯y above 65 km on 13 February and above 60 km on 19 February, and over very limited height ranges between 45 and 60 km. It seems likely that with a height resolution better than 300 m, other regions of dry adiabatic lapse rates might be identi®ed (Sica and Thorsley, 1996). However, the present results suggest that the primary eects of convective instabilities occur at heights above 50 km. ...
Observations made with the co-located
Rayleigh lidar and MST radar systems at Aberystwyth (52.4°N, 4.1°W) in Wales
and radiosondes from Valentia (51.9°N, 10.2°W) in Eire are used to investigate
the changes in the vertical propagation of gravity waves during periods of 4
days in June 1995 and February 1993. In each month, the lidar observations show
that the wave activity in the upper stratosphere and lower mesosphere changes
between two pairs of days. The radar and radiosonde measurements indicate that
mountain waves make no contribution to the changes in intensity. Instead, the
changes seem to arise largely from the presence or absence of long-period waves
with vertical wavelengths near 8 and 10 km in June and February, respectively.
The influence of such waves on the vertical wavenumber spectra is examined and
related to the evidence for convective instabilities provided by the temperature
profiles.Key words. Rayleigh lidar · MST radar systems ·
Radiosondes · Gravity waves
Characteristics of mesospheric temperature inversions (MTIs) remotely sensed in the CRISTA-1 experiment (November 1994) are analyzed. The original dataset consisted of about 900 vertical temperature profiles in the range of altitudes from 40 to 120 km. MTIs were observed in 84% of cases. One and two inversions were mainly observed (in 40% and 37% of cases, respectively). Profiles with three inversions occurred in about 7% of cases. Profiles with four inversions accounted for less than 1% of cases. Multiple inversions were observed mainly in the equatorial region and in the Northern Hemisphere. The mean amplitude of inversions for the whole dataset is 24.3 K, with the most probable value of 7 K. When only MTIs with a maximum amplitude are taken into account in the case of multiple inversions, the mean amplitude is 30.6 K and the maximum of the frequency distribution of amplitudes lies in the range from 20 to 30 K. Inversions with an amplitude of 80-90 K were also detected. The maximum amplitude was 104 K. Zonal mean amplitudes in 5° latitudinal zones have maxima in the equatorial region and in the midlatitudes (at 30°-40° S and around 50° N). The maximum of the frequency distribution of the vertical extent of inversions is 5 to 7.5 km. No inversions with a vertical extent greater than 15 km were observed. Zonal mean vertical extents of inversions have minima south of 45° S. Zonal mean altitudes of inversions (altitudes of a temperature maximum) decrease sharply from 87.5 km in the southern latitudes to 80-82.5 km at the equator. In the northern latitudes, the inversion altitudes increase only slightly.
Brunt-Vaisala frequency squared (N2) measures the static stability of the atmosphere, and reflects the general structure of the atmosphere in term of vertical temperature gradient. For middle atmosphere the response of the middle atmospheric structure to the global warming still lacks investigation currently. The historical data from rocket sounding network in 1962-1991 are used to investigate the long-term trend of N2 in the middle atmosphere. For six stations spanning from the tropical latitudes to the northern mid-latitudes, our estimates show that, in the upper stratosphere and middle mesosphere, i.e., 48-60 km high, the significant decreasing of static stability is observed in an N2 anomalies averaged over 48-60 km range. For two tropical stations, long-term trend in N2 exhibits a similar magnitude, i.e., -0.11×10-4 s-2/decade; it is also observed that the trend increases with latitude, with trend estimates from -0.16×10-4 s-2/decade at 22°N (Barking Sand station) to -0.22×10-4 s-2/decade at 38°N (Wallops Island station).
Rayleigh lidar observations of mesosphere temperature profiles obtained from 40 to ∼100 km from Logan, Utah (41.7, 111.8 W, altitude, 1.9 km) over 10 nights in late February, 1995, revealed an interesting development between 60 to 75 km of a winter mesosphere inversion layer with an amplitude of ∼20–30 K and a downward phase progression of ∼1 km/hr. The data also showed two altitude regions exhibiting significant cooling of 10–30 K in extent. These were located below and above the peak of the inversion layer, respectively, at altitudes of ∼50–55 km and ∼70–80 km. When these results were compared with the predictions of a global wave scale model (GSWM), the observed thermal mesosphere structure is similar to the computed composite tidal structure based upon the semi-diurnal and diurnal tides with the exception that observed amplitudes of heating and cooling are ∼10x larger than predicted GSWM values. We suggest that these events over Utah are caused through a localized mechanism involving the coupling of gravity waves to the mesopause tidal structure.
Rayleigh lidar measurements of the stratosphere and mesosphere have been made on an ongoing basis over a three-year period at Poker Flat, Alaska (65° N, 147° W). These observations have yielded 27 nightly measurements of the middle atmosphere temperature profile (~40-80 km). These nighttime measurements are distributed between August and April. Mesospheric inversion layers have been observed on five occasions. The average altitude of the inversion layer peak is 60 km, with average amplitude of 18 K. The temperature gradients on the topside of the inversion layers approach the adiabatic lapse rate. The inversion layers do not exhibit the apparent downward phase velocities that are commonly observed at lower latitudes. Furthermore, the inversion layers appear significantly less frequently than at lower latitudes. The observations are discussed in terms of current models and observations at other sites.
Studies of the Mesosphere, Stratosphere and Troposphere by radar have shown great growth in the last 10–15 years. The growth has occurred in both diversity and depth in all areas of technology, fundamental research, and application. New technology and faster computers have permitted real-time analysis routines that were previously impossible, and allowed better spatial and temporal resolution. Interferometric and imaging techniques, and spectral processing, have been a particular focus of new technological advances, and systems with multiple receivers and multiple radar frequencies have been developed. Better computer-processing capability, coupled with multiple frequency capability, has allowed procedures like Capon's method to be applied, for example. With regard to fundamental processes, new diffusive mechanisms have been detected, such as the slow diffusion associated with “dressed aerosols” in the Polar Mesosphere, leading to Polar Mesosphere Summer Echoes. Higher resolution studies of turbulent breakdown have allowed better examination of wave breakdown processes. Some old techniques have had a substantial rebirth, such as applications of meteor radars, which can now measure both winds and temperatures in the mesopause region. New techniques have also allowed smaller and less powerful instruments to make significant contributions in important areas. An example is development of new momentum flux measurement techniques, which have allowed small compact meteor radars to make measurements of this parameter, thereby increasing the global coverage of radars that can do this. In regard to application, windprofilers (a significant offshoot of MST radars) have become key components in many meteorological networks, and provide important input to numerical forecast models in many countries. These developments, and many more, are discussed within this review. It is of course impossible to give due discussion to all new areas, and some selectivity is necessary within the review, but hopefully we will at least touch on most of the key new areas.
There is a paucity of high-frequency temporal gravity wave spectra in the middle upper stratosphere (30–40 km) as most instruments that are capable of accessing this region do not have the capability to make measurements for any extended period of time. The spectral analysis of density fluctuations obtained with the Purple Crow Rayleigh scatter lidar has shown that middle upper stratospheric temporal spectra follow the expected −1.5 to −2.0 power law relationship for frequencies less than 1×10−3 s−1. At frequencies greater than 1×10−3 s−1, the shape of the temporal spectrum is much more variable, with strong quasi-monochromatic features appearing intermittently on certain nights. These higher-frequency features have been attributed to nonlinear wave interactions and suggest that estimates of gravity wave activity in the middle upper stratosphere may be larger and much more variable than previously assumed. This variability also suggests that middle upper stratospheric energy dissipation and eddy diffusion values may be much larger and more variable than previously assumed. Current parameterizations of subgrid dynamical processes in general circulation models that resolve this region may need to be reexamined in this light.
Turbulence is thought to be produced by gravity waves that become
unstable in the upper atmosphere. Owing to a relatively short vertical
wavelength of the solar diurnal tide, turbulent viscous-type mixing is
expected to be particularly important for tidal dissipation in the
mesosphere and lower thermosphere (MLT). Within the Doppler-spread
parameterization (DSP) of gravity waves, the vertical eddy exchange
coefficients may be calculated from the GW energy deposition rate. The
DSP, as implemented in the Spectral Mesosphere/Lower Thermosphere Model,
is shown to provide realistic simulations of both the diurnal tide and
turbulent mixing in the MLT. In particular, the vertical structure and
seasonal variations of eddy mixing agree well with in situ observations
and independent estimates from global thermal balance. The seasonal
variations of turbulence are found to be primarily responsible for a
strong semiannual variation of the diurnal tide in agreement with recent
satellite observations.
An active topic of current research in aeronomy is the study of the dynamics of the mesosphere and lower thermosphere (MLT) from 60 to 130 km, especially in regard to the influences that govern variability. The physical processes of this region are diverse and complex with strong coupling between the MLT and the adjacent atmospheric regions brought about largely by the propagation and dissipation of atmospheric gravity waves (GWs) from sources above and below. The measurements of MLT winds and temperatures required for such studies represent daunting technical challenges. At low and midlatitudes the mesosphere inversion layer (MIL) phenomenon, a ∼10 km wide region of enhanced temperatures (ΔT ∼ 15–50K), is observed with great regularity in both the upper mesosphere (60–70 km) and the mesopause (90–100 km). Observations are largely based upon Rayleigh and Na temperature lidar systems but coherent radar observations have shown that the MIL phenomenon is linked to layers of turbulence occurring in both the topside and the bottomside regions. GW activity is believed to play an important role in the development of a linkage between the MIL and the tidal structure through GW coupling that results in an amplification of the tidal thermal structure. This linkage is readily evident for the upper MIL but is seen only occasionally for the lower MIL. Further study of MIL properties should emphasize continual 24 hour temperature observations, especially for the lower MIL region, to confirm the linkage of the development of the MIL to the MLT tidal structure.
High-resolution temperature profile data collected at the Urbana Atmospheric Observatory (40°N, 88°W) and Starfire Optical Range, NM (35°N, 106.5°W) with a Na lidar are used to assess the stability of the mesopause region between 80 and 105km. The mean diurnal and annual temperature profiles demonstrate that in the absence of gravity wave and tidal perturbations, the background atmosphere is statically stable throughout the day and year. Thin layers of instability can be generated only when the combined perturbations associated with tides and gravity waves induce large vertical shears in the horizontal wind and temperature profiles. There is a region of reduced stability below the mesopause between 80 and 90km where the temperature lapse rate is large and the buoyancy parameter N2 is low. The vertical heat flux is maximum in this region which suggests that this is also a region of significant wave dissipation. There is also a region of enhanced stability above 95km in the lower thermosphere where N2 is large. There appears to be little wave dissipation above 95km because the temperature variance increases rapidly with increasing altitude in this region and the vertical heat flux is zero.
Part I of this series demonstrated the advantages of parametric models in estimating the gravity wave spectrum from density fluctuation measurements using a large power-aperture-product Rayleigh-scatter lidar. The spectra calculated using the parametric models are now used to estimate energy dissipation due to gravity waves. Energy dissipation for an individual wave in the spectrum is also estimated using Prony's method, which allows the frequency, amplitude, damping, and phase of individual waves to be estimated. These two independent estimates of energy dissipation highlight the variability of the energy dissipation on short timescales due to gravity waves and turbulence. A combination of the information obtained from the parametric models of the spatial and temporal spectra with the theoretical work of M. E. McIntyre is used to estimate profiles of the eddy diffusion coefficient. This estimate attempts to include the degree of saturation of the vertical wavenumber spectrum, which determines the constant used in the calculation of the eddy diffusion coefficient. The height profile of the eddy diffusion coefficient thus obtained in the upper stratosphere and mesosphere is in good agreement with previous estimates. The degree of saturation of the vertical wavenumber spectrum is shown to increase proportionally to the Hines parameter, a measure of the transition wavenumber from a linear to a nonlinear tail spectrum. It is speculated that this fact can be interpreted as a change in the atmosphere from an `amplifier' state where the tail spectrum is highly nonlinear, but weak when the spectral `gain' is high, to a state of saturation where the high wavenumber tail spectrum is more linear, but has lower gain and more energy available to dissipate at smaller spatial scales.
Four hundred and twenty-two nights of stratospheric gravity wave observations were obtained with a Rayleigh lidar in the High Arctic at Eureka (808N, 868W) during six wintertime measurement campaigns between 1992/ 93 and 1997/98. The measurements are grouped in positions relative to the arctic stratospheric vortex for comparison. Low gravity wave activity is found in the vortex core, outside of the vortex altogether, and in the vortex jet before mid-December. High gravity wave activity is only found in the vortex jet after late December, and is related to strengthening of the jet and decreased critical-level filtering. Calculations suggest that the drag induced by the late-December gravity wave energy increases drives a warming already observed in the vortex core, thereby reducing vortex-jet wind speeds. The gravity waves provide a feedback mechanism that regulates the strength of the arctic stratospheric vortex.
Parametric models of spectral analysis offer several distinct advantages over statistical methods such as the correlogram analysis. These advantages include higher spectral resolution and the ability, in principle, to separate correlated (i.e., wave) behavior from noise-driven (i.e., turbulent) behavior in the measurements. Here parametric models are used to highlight the spatial and temporal intermittency of the gravity wave spectrum. In Part II of this series the spatial and temporal spectrum are used to calculate energy dissipation and the eddy diffusion coefficient. The spectra are computed from measurements of density fluctuations obtained using a large power- aperture product lidar during a 6-h period on 30 August 1994. It is shown that parametric models provide an excellent representation of the temporal and spatial data series. One difficulty of parametric models is selecting the model order, an analogous situation to determining the proper lag in the correlogram procedure or the window length in the periodogram method. Extensive experimentation has shown that the ratio of the data matrix eigenvalues to the photon noise eigenvalues is an excellent indicator for the selection of the model order. The underlying spectral form found using the parametric models is similar to the standard correlogram method, that is, nominal underlying spatial and temporal spectral slopes between 22 and 24 and 21.25 and 22, respectively, with variability outside this range. The spatial-temporal behavior of the spectra is highly variable with numerous intermittent and intense features rising well above the photon noise floor. The vertical wavenumber spectra on this night may show a variation of spectral slope with height; however, the slope is both extremely sensitive to the noise level of the data, steepening as the signal-to-noise level increases, and highly variable in time. The temporal spectra also show considerable variation with height, both in magnitude and slope.
Light detection and ranging (lidar) is a technique in which a beam of light is used to make range‐resolved remote measurements. A lidar emits a beam of light, that interacts with the medium or object under study. Some of this light is scattered back toward the lidar. The backscattered light captured by the lidar's receiver is used to determine some property or properties of the medium in which the beam propagated or the object that caused the scattering.
The lidar technique operates on the same principle as radar; in fact, it is sometimes called laser radar. The principal difference between lidar and radar is the wavelength of the radiation used. Radar uses wavelengths in the radio band whereas lidar uses light, that is usually generated by lasers in modern lidar systems. The wavelength or wavelengths of the light used by a lidar depend on the type of measurements being made and may be anywhere from the infrared through the visible and into the ultraviolet. The different wavelengths used by radar and lidar lead to the very different forms that the actual instruments take.
The major scientific use of lidar is for measuring properties of the earth's atmosphere, and the major commercial use of lidar is in aerial surveying and bathymetry (water depth measurement). Lidar is also used extensively in ocean research and has several military applications, including chemical and biological agent detection.
Atmospheric lidar relies on the interactions, scattering, and absorption, of a beam of light with the constituents of the atmosphere. Depending on the design of the lidar, a variety of atmospheric parameters may be measured, including aerosol and cloud properties, temperature, wind velocity, and species concentration.
This article covers most aspects of lidar as it relates to atmospheric monitoring. Particular emphasis is placed on lidar system design and on the Rayleigh lidar technique.
The stability of the middle atmosphere ranging from 30 km to 60 km over Wuhan is elementarily studied, according to the observational
data obtained in about one year through the WIPM Rayleigh lidar. The stability parameter (buoyancy frequency squared), which
was obtained through the temperature field, shows that the middle atmosphere over Wuhan is stable all the year round, but
the stability appears variable because the stability of summer and winter is lower, and that of spring and autumn is higher.
Gardner (1996, henceforth G96), examines a number of theories that seek to account for the vertical-wavenumber (m) spectrum of horizontal wind perturbations (u′) induced by gravity waves in the middle atmosphere. He notes that all the theories predict much the same spectra, and so he seeks to “identify the experimental data required to test the fundamental physics upon which these theories are based …”While I applaud Gardner's goal and will at a later stage contribute to it, I must first comment adversely on some aspects of his preliminary summaries and of his observational inferences. My remarks are limited almost exclusively to his discussion of the linear instability theory of Dewan and Good (1986, henceforth DG86) and Smith et al. (1987, henceforth SFV87), called by him the LIT, the diffusive filtering theory (DFT) of Gardner (1994, henceforth G94), and the Doppler spread theory (DST) initiated by Hines (1991b).In all of the theories under discussion, the u′ power spectral density (PSD) is taken in G96 to exhibit an increase with m at m values less than some characteristic value and a decrease, more-or-less proportional to m−3, in a ‘tail’ region at greater m values up to some upper limit mM, whereafter turbulence is said to set in. The transition at may be thought of, for convenience of discussion, as being abrupt, though each theory in fact either assumes or derives a smooth transition. The various theories differ from one another in the physical processes that are assumed to establish the tail portion of the spectrum, the tail's range in m, and the intensity of the PSD of the tail. That intensity has been said observationally to be invariant under changes of circumstance and height, a characteristic that is widely attributed to a ‘saturation’ of the spectrum or of the corresponding waves. The tail portion of the spectrum is therefore said by many to be saturated, though the theory of G94 purports to disagree with this concept and in fact provides possible alternatives to the m−3 form (as, indeed, does DG86). The LIT, DFT and DST will be discussed below in sequence.
Wind fluctuations in the middle atmosphere exhibit an Eulerian spectral tail approximating to the form m−3 at large vertical wavenumbers m. The tail is taken here to result from a characteristically Eulerian nonlinearity in the governing fluid dynamic equations, a nonlinearity that is absent from a Lagrangian description. It is modeled here via a previously described analytic transformation from assumed model Lagrangian input spectra. The present work gives numerical expression to the earlier analytic results for the intensity of the tail. The resultant tail spectra are found to represent well the relevant data at reasonable values of Richardson number. Under an assumption of constant Richardson number, they exhibit saturation as height increases, as do the observations. Previously obtained approximate results, including those of the Doppler-spread theory of saturation, are found to represent well the new numerical results. Implications for that theory and for the associated Doppler-spread parameterization are discussed in a companion paper.
A Rayleigh-scatter lidar operated from Utah State University (41.7°N, 111.8°W) for a period spanning 11 years ― 1993 through 2004. Of the 900 nights observed, data on 150 extended to 90 km or above. They were the ones used in these studies related to atmospheric gravity waves (AGWs) between 45 and 90 km. This is the first study of AGWs with an extensive data set that spans the whole mesosphere. Using the temperature and temperature gradient profiles, we produced a climatology of the Brunt-Väisälä (buoyancy) angular frequency squared, N2 (rad/s)2. The minimum and maximum values of N2 vary between 2.2×10-4 (rad/s)2 and 9.0×10-4 (rad/s)2. The corresponding buoyancy periods vary between 7.0 and 3.5 minutes. While for long averages the atmosphere above Logan, Utah, is convectively stable, all-night and hourly profiles showed periods of convective instability (i.e., negative N2). The N2 values were often significantly different from values derived from the NRL-MSISe00 model atmosphere because of the effects of inversion layers and semiannual variability in the lidar data.Relative density fluctuation profiles with 3-km altitude resolution and 1-hour temporal resolution showed the presence of monochromatic gravity waves on almost every night throughout the mesosphere. The prevalent values of vertical wavelength and vertical phase velocity were 12-16 km and 0.5-0.6 m/s, respectively. However, the latter has the significant seasonal variation. Using these two observed parameters, buoyancy periods, and the AGW dispersion relation, we derived the ranges of horizontal wavelength, phase velocity, and source distance. The prevalent values were 550-950 km, 32-35 m/s, and 2500-3500 km, respectively. The potential energy per unit mass Ep showed great night-to-night variability, up to a factor of 20, at all heights. Ep grew at approximately the adiabatic rate below 55-65 km and above 75-80 km. Step function decreases in Ep imply that the AGWs in between gave up considerable energy to the background atmosphere. In addition, Ep varies seasonally. Below 70 km, it has a semiannual variation with a maximum in winter and minima in the equinoxes. At the highest altitudes it has an annual variation with a maximum in winter and a minimum in summer.
Sodium resonance-fluorescence lidar is an established technique for measuring atmospheric composition and dynamics in the mesopause region. A large-power-aperture product (6.6-W m(2)) sodium resonance-fluorescence lidar has been built as a part of the Purple Crow Lidar (PCL) at The University of Western Ontario. This sodium resonance-fluorescence lidar measures, with high optical efficiency, both sodium density and temperature profiles in the 83-100-km region. The sodium lidar operates simultaneously with a powerful Rayleigh- and Raman-scatter lidar (66 W m(2)). The PCL is thus capable of simultaneous measurement of temperature from the tropopause to the lower thermosphere. The sodium resonance-fluorescence lidar is shown to be able to measure temperature to an absolute precision of 1.5 K and a statistical accuracy of 1 K with a spatial-temporal resolution of 72 (km s) at an altitude of 92 km. We present results from three nights of measurements taken with the sodium lidar and compare these with coincident Rayleigh-scatter lidar measurements. These measurements show significant differences between the temperature profiles derived by the two techniques, which we attribute to variations in the ratio of molecular nitrogen to molecular oxygen that are not accounted for in the standard Rayleigh-scatter temperature analysis.
Rayleigh lidar observations of mesospheric temperature over southern France have shown that a temperature inversion is often present which persists for several days at the same altitude. The statistical characteristics of this inversion were determined from more than 500 nightly mean profiles. The characteristics of the inversion are compared with estimates of the amplitude growth with height of a gravity wave. It is concluded that these two phenomena have the same origin, i.e., the turbulent field generated by the breaking of gravity waves.
A lidar system has been built to measure atmospheric-density fluctuations and the temperature in the upper stratosphere, the mesosphere, and the lower thermosphere, measurements that are important for an understanding of climate and weather phenomena. This lidar system, the Purple Crow Lidar, uses two transmitter beams to obtain atmospheric returns resulting from Rayleigh scattering and sodium-resonance fluorescence. The Rayleigh-scatter transmitter is a Nd:YAG laser that generates 600 mJ/pulse at the second-harmonic frequency, with a 20-Hz pulse-repetition rate. The sodium-resonance-fluorescence transmitter is a Nd:YAG-pumped ring dye laser with a sufficiently narrow bandwidth to measure the line shape of the sodium D(2) line. The receiver is a 2.65-m-diameter liquid-mercury mirror. A container holding the mercury is spun at 10 rpm to produce a parabolic surface of high quality and reflectivity. Test results are presented which demonstrate that the mirror behaves like a conventional glass mirror of the same size. With this mirror, the lidar system's performance is within 10% of theoretical expectations. Furthermore, the liquid mirror has proved itself reliable over a wide range of environmental conditions. The use of such a large mirror presented several engineering challenges involving the passage of light through the system and detector linearity, both of which are critical for accurate retrieval of atmospheric temperatures. These issues and their associated uncertainties are documented in detail. It is shown that the Rayleigh-scatter lidar system can reliably and routinely measure atmospheric-density fluctuations and temperatures at high temporal and spatial resolutions.
The slope and power spectral density of atmospheric velocity fluctuations versus vertical wavenumber at large wavenumbers are observed to be nearly independent of altitude. It is suggested that such a universality is due to saturation of short vertical-scale fluctuations. A brief review of linear gravity wave saturation theory indicates a physical basis for such spectra. It is demonstrated that observed saturation spectra are not solely due to individually saturated waves but most likely result from amplitude limiting instabilities arising from wave superposition. It is also shown that, while the spectrum is saturated at large wavenumbers, the total kinetic energy per unit mass and the characteristic vertical wavelength increase with altitude. Both of these predictions are consistent with observations.
This paper questions arguments used in favor of the current linear-instability theory for saturation of gravity waves in the middle atmosphere, which assumes that saturation results from linear instability, showing that the arguments are not valid. A new theory is then developed, based on the Doppler shifting of the smaller-scale members of the spectrum, that accounts for the saturated portion of the gravity spectrum in the middle and the upper atmospheres and for the formation of turbopause. It is shown that the Doppler-spread theory explains both the formation of the turbopause and the formation of layered turbulence beneath it. Both were found to be consistent with observations.
The Rayleigh lidar technique has been applied to observe temperature fluctuations induced by gravity waves within the upper stratosphere. Observations were carried out on a routine basis for 1 year (130 clear nights) at the campus of York University near Toronto (44°N, 80°W). The waves were on occasion observed to induce marginal convective instability while exhibiting no substantial vertical amplitude growth. In general, the vertical variation in the amplitude of fractional temperature perturbations and associated available potential energy density implied the waves were strongly dissipated. Dramatic changes in the distribution of spectral energy with respect to vertical wave number were observed over the course of a few hours. The total resolved available potential energy in the gravity wave field varied considerably from day to day and seasonally with a winter maximum and summer minimum.
There has recently been a great deal of interest in the possibility that vertically propagating internal gravity waves may be dissipated by small-scale convective or shear instabilities in the upper stratosphere and mesosphere. In the present study, a very simple analysis of about 3000 rocket soundings of temperature and wind at several stations between 8°N and 59°N was conducted in order to obtain quantitative estimates of the frequency of occurrence of dynamically unstable conditions as a function of height, latitude, and season. It was found that in about one-third of the profiles, the local Richardson number dropped below 0.25 at some level near the stratopause. From the results, it appears that gravity wave "breaking" generally occurs at considerably higher altitudes in the tropics than in midlatitudes. There is also a fairly clear indication of higher wave breaking levels in summer than in winter, at least at high latitudes.
The irregular winds of the middle atmosphere are commonly attributed to an upwardly propagating system of atmospheric gravity waves. Their one-dimensional power spectrum has been reported to exhibit a nearly universal behavior in its [open quotes]tail[close quotes] region of large m: both the form ([approximately]m[sup [minus]3]) and the intensity of the tail are approximately invariant with meteorological conditions, time, place and height. This universality is often described as resulting from [open quotes]saturation[close quotes] of the system, with the physical cause of saturation being left for separate identification and analysis. Of current theories as to physical cause, the most fully developed and widely employed assumes that saturation results from linear instability: that the waves of the tail grow in amplitude with height until the system as a whole, or each portion of its tail, is rendered unstable and prevented from growing further. Initially the form and then the intensity of the tail are said to result from this process. The arguments in favor of this view are questioned in the present paper and found wanting (though the claim of instability remains unchallenged and is even reinforced). The waves of the tail are then recognized as being subject to a strong wave-wave interaction arising from the Eulerian advective nonlinearity -- from the Doppler shifts that can be imposed upon them by the larger-scale winds of the wave system -- a fact recognized in the corresponding oceanographic literature for about a decade now. In a companion paper, a rudimentary analytic approximation to the advective nonlinearity is introduced, and its consequences are shown to yield a spectral form and intensity quite similar to those obtained observationally. The linear instabilities (and some formulas) of the present paper are then invoked to establish the length, rather than the form and intensity, of the tail, at least below the turbopause. 36 refs., 4 figs.
A theory is presented which explains the universal nature of one-dimensional vertical wave number, k, power spectral densities (PSDs) of horizontal winds as measured in the atmosphere and predicted by VanZandt. The theory is that the PSD amplitude at any given wave number (greater than a certain minimum, k*) is determined by its saturation value due either to shear instability (i.e., critical Richardson Number) or, more likely, to convective instability. This explains why the PSD amplitudes observed do not grow exponentially with increasing altitude. This saturation theory assumption plus other considerations leads to a PSD of the form N2/kn, where n is in the range of about 2.5 to 3 and N is the Brunt frequency. A simplified model involving superimposed narrow bands of gravity waves as well as a model based merely on dimensional arguments both lead to n = 3. The full model not only explains the observed spectral slopes but also predicts the PSD amplitude in the troposphere to be 3.5 times smaller than in the stratosphere. The derivation of the model is based on the saturation condition that ∫ k2PSD(k) dk = N2. The model may also apply to the ocean and explain the Garrett-Munk vertical wave number spectrum.
Mesospheric temperature inversion and gravity wave breaking The saturation of gravity waves ia the middle atmosphere. Part I: Critique of linear-instability theory
- A Hauchecorne
- M L Chanin
- R Wilson Hines
- C O Oppenheim
- R W Schafer
Hauchecorne, A., M. L. Chanin, and R. Wilson, Mesospheric temperature inversion and gravity wave breaking, Geophys. Res. Lett., 14, 933-936, 1987. Hines, C. O., The saturation of gravity waves ia the middle atmosphere. Part I: Critique of linear-instability theory. J. Atmos. $ci., 48, 1348-1359, 1991. Oppenheim, A. V. and R. W. Schafer, Discrete-lYme Signal Processing. Prentice-Hall, Inc., 256-266, 1989. Press, W. H., S. A Teukolsky, W. T. Vetterling, and B. P. Flannery., Numer-ical Recipes in FORTRAN, the Art of Scientific Computing, 2nd edi-tion. Cambridge University Press, 613-622, 1992.
Lidar studies of temperature and den-sity using Rayleigh scauering, in Handbook for MAP: Ground-Based Techniques, 13, ICSU Scientific Committee on Solar Terrestrial Phys-ics
- M L Chanin
- A Hauchecorne
Chanin, M. L. and A. Hauchecorne, Lidar studies of temperature and den-sity using Rayleigh scauering, in Handbook for MAP: Ground-Based Techniques, 13, ICSU Scientific Committee on Solar Terrestrial Phys-ics, Urbana, Illinois, paper 7, 1984.