Rayleigh Lidar Studies Of Gravity Waves In The Middle Atmosphere

This dissertation presents new studies of waves and turbulence in the Arctic middle atmosphere. The study has a primary focus on wintertime conditions when the largescale circulation of the middle atmosphere is disrupted by the breaking of planetary waves associated with sudden stratospheric warming (SSW) events. We used ongoing Rayleigh lidar measurements of density and temperature to conduct a multi-year study of gravity waves in the upper stratosphere-lower mesosphere (USLM) over Poker Flat Research Range (PFRR) at Chatanika, Alaska. We analyzed the night-to-night gravity wave activity in terms of the wind structure and the ageostrophy. We find that the weak winds during disturbed conditions block the vertical propagation of gravity waves into the mesosphere. The gravity wave activity is correlated with the altitudes where the winds are weakest. During periods of weak winds we find little correlation with ageostrophy. However, during periods of stronger winds we find the USLM gravity wave activity is correlated with the ageostrophy in the upper troposphere indicating that ageostrophy in this region is a source of the gravity waves. Inter-annually we find the wintertime gravity wave activity is correlated with the level of disturbance of the middle atmosphere, being reduced in those winters with a higher level of disturbance and weaker winds. We used rocket-borne ion gauges to measure turbulence in the wintertime middle atmosphere while documenting the larger meteorological context from Rayleigh lidar and satellites. This investigation of turbulence was called the Mesosphere-Lower Thermosphere Turbulence Experiment (MTeX). During MTeX we found a highly disturbed atmosphere associated with an SSW where winds were weak and gravity wave activity was low. We found low levels of turbulence in the upper mesosphere. The turbulence was primarily found in regions of convective instability in the topside of mesospheric inversion layers (MILs). The strongest and most persist turbulence was found in a MIL that is associated with the breaking of a monochromatic gravity wave. These MTeX observations indicate that turbulence is generated by gravity wave breaking as opposed to gravity wave saturation. These MTeX findings of low levels of turbulence are consistent with recent model studies of vertical transport during SSWs and support the view that eddy transport is not a dominant transport mechanism during SSWs.

"Rayleigh lidar measurements of the upper stratosphere and mesosphere are made on a routine basis over Poker Flat Research Range (PFRR), Chatanika, Alaska, (65°N, 147°W). Rayleigh lidar measurements have yielded high resolution temperature and density profiles in the 40-80 km altitude. These measurements are used to calculate gravity wave activity in the 40-50 km altitude. The thermal structure of the stratosphere and mesosphere is documented using an eight year data set, and the role of small scale gravity waves on the large scale meridional circulation is analyzed in terms of the synoptic structure of the Arctic stratospheric vortex, Aleutian anticyclone, and planetary wave activity. The monthly mean temperature indicates colder January temperatures that appear to be due to the increase in frequency of occurrence of stratospheric warming events from 1997-2004. The gravity wave potential energy density is analyzed during stratospheric warming events in two experimental time periods. From the first study consisting of three winters, 2002-2003, 2003-2004, and 2004-2005, the first direct measurement of suppression of gravity wave activity during the formation of an elevated stratopause following the 2003-2004 stratospheric warming event is presented. The gravity wave potential energy density at Chatanika is positively correlated with horizontal wind speeds in the stratosphere, and indicates that the wave activity in the 40-50 km altitude is partially modulated by the background flow. In the second study with more recent winters of 2007-2008 and 2008-2009, no systematic difference in the magnitude of potential energy density between the vortex displacement warming event during the 2007-2008 winter and vortex split warming event during the 2008-2009 winter is found. However, the low correlation between gravity wave potential energy and horizontal wind speed after the first warming in January 2008, and a higher correlation after the January 2009 warming suggests that while the gravity wave activity after the 2009 warming is modulated by the background flow, other wave sources modulate the gravity wave activity after the 2008 warming"--Leaf iii.

During 13 nights of Rayleigh lidar measurements at Urbana, Ill. in 1984 to 1986, thirty-six quasi-monochromatic gravity waves were observed in the 35 to 50 km altitude region of the stratosphere. The characteristics of the waves are compared with other lidar and radar measurements of gravity waves and the theoretical models of wave saturation and dissipation phenomena. The measured vertical wavelengths ranged from 2 to 11.5 km and the measured vertical phase velocities ranged from 10 to 85 cm/s. The vertical wavelengths and vertical phase velocities were used to infer observed wave periods which ranged from 100 to 1000 min and horizontal wavelengths which ranged from 70 to 2000 km. Dominant wave activity was found at vertical wavelengths between 2 to 4 km and 7 to 10 km. No significant seasonal variations were evident in the observed parameters. Vertical and horizontal wavelengths showed a clear tendency to increase with wave periods, which is consistent with recent sodium lidar studies of quasi-monochromatic waves near the mesopause. An average amplitude growth length of 20.9 km for the rms wind perturbations was estimated from the data. Kinetic energy density associated with the waves decreased with height, suggesting that waves in this altitude region were subject to dissipation or saturation effects. Gardner, C. S. and Miller, M. S. and Liu, C. H. Unspecified Center...

A new mobile lidar was used to obtain data on the variations that occur in the high latitude atmosphere between 20 and 85 km. Data were obtained on 26 nights during the program. Standard meteorological balloon and rocket payloads also measured the density, for comparison with the lidar data. More than a thousand profiles of atmospheric density were obtained with the lidar and 20 meteorological rockets were launched. Comparisons between the lidar data and the meteorological rocket data showed generally good agreement. However, the lidar data from the February period is offset 12-14 percent from the rocket data. The March and April data, including 14 sets of overlapping data, generally agree to within 3 percent. Keywords: Lidar; Atmospheric density; Middle atmosphere; Atmospheric variations; High latitude atmosphere.

PAGEOPH, stratosphere, these differences provide us with new evidence, interpretation of which can materially help to advance our understanding of stratospheric dynamics in general. It is now weil established that smaller-scale motions-in particular gravity waves and turbulence-are of fundamental importance in the general circulation of the mesosphere; they seem to be similarly, if less spectacularly, significant in the troposphere, and probably also in the stratosphere. Our understanding of these motions, their effects on the mean circulation and their mutual interactions is progressing rapidly, as is weil illustrated by the papers in this issue; there are reports of observational studies, especially with new instruments such as the Japanese MV radar, reviews of the state of theory, a laboratory study and an analysis of gravity waves and their effects in the high resolution "SKYHI" general circulation model. There are good reasons to suspect that gravity waves may be of crucial significance in making the stratospheric circulation the way it is (modeling experience being one suggestive piece of evidence for this). Direct observational proof has thus far been prevented by the difficulty of making observations of such scales of motion in this region; in one study reported here, falling sphere observations are used to obtain information on the structure and intensity of waves in the upper stratosphere.

During the 13 night of Rayleigh lidar measurements thirty-six quasi- monochromatic gravity waves were observed in the 35 to 50 km altitude region of the stratosphere. The characteristics of the waves are compared with other lidar and radar measurements of gravity waves and with theoretical models of wave saturation and dissipation phenomena. The measured vertical wavelengths (lambda from 2 to 11.5 km and the measured vertical phase velocities (c sub z) ranged from 10 85 cm/s. The vertical wavelengths and vertical phase velocities were used to infer observed wave periods (T sub ob) which ranged from 100 to 1000 min and horizontal wavelengths (lambda sub x) which ranged from 70 to 2000 km. However, there may be errors in the inferred values of the horizontal wavelengths because they were calculated by assuming that the observed period equals the intrinsic period. Dominant wave activity was found at vertical wavelengths between 2-4 km and 7-10 km. No significant seasonal variations were evident in the observed parameters. Vertical and horizontal wavelengths showed a clear tendency to increase with T sub ob, which is consistent with recent sodium lidar studies of quasi-monochromatic waves near the mesopause. An average amplitude growth length of 20.9 km for rms wind perturbations was estimated from the data. Kinetic energy density associated with the waves decreased with height, suggesting that waves in this altitude region were subject to dissipation or saturation effects. (jhd).

We present lidar observations of the middle atmosphere at the South Pole (90$spcirc$S), Syowa (69$spcirc$S, 39$spcirc$E), and Urbana (88$spcirc$W, 40$spcirc$N). The South Pole stratospheric observations yield a high resolution data set of stratospheric temperature and polar stratospheric cloud (PSC) backscatter ratio profiles during the austral winter and spring of 1990. The observations show that the seasonal development of the clouds is primarily determined by the behavior of the temperature field. The PSCs are composed predominantly of nitric acid trihydrate particles. Correlation with frost-point measurements shows that nitric acid mixing ratios are depressed in the spring. The small-scale structure of the clouds appears to be controlled by gravity waves propagating upward through the clouds. Lidar measurements of the mesospheric Na layer at the South Pole in 1990 and Syowa in 1985 are used to characterize mesopause region gravity wave activity over Antarctica. The structure of the Na layer reflects the general circulation of the high-latitude mesopause. The monochromatic waves observed over Antarctica show the same general characteristics as those reported from other sites. The mean density variance of the gravity wave perturbations at the South Pole is similar to that observed at a variety of lower latitude sites. A distinct feature of the South Pole observations is the presence of strong coherent oscillations in the bottomside density contours of the Na layer close to the inertial frequency. Na Doppler/temperature lidar measurements of Na density and temperature at Urbana yield a high resolution seasonal data set of gravity wave activity. The direct measurement of the Brunt-Vaisala period allows accurate calculation of the horizontal velocity and vertical displacement from the density measurements. The horizontal velocity and vertical displacement m-spectrum magnitudes and indices show considerable seasonal and nightly variability, behaviors which contradict the predictions of Linear Instability Theory and Scale-Dependent Diffusion Theory. We present a detailed comparison of the observations with the predictions of the Scale-Independent Diffusive Filtering Theory. The magnitudes of the m-spectrum, the form of the joint (m,$omega$) spectrum, the systematic relationships between the monochromatic gravity wave periods, wavelengths, and amplitudes agree remarkably well with those predicted for Scale-Independent Diffusive Filtering Theory. This observational study suggests that the complex nonlinear interactions of the gravity wave field can be modeled successfully as a diffusion process, where the diffusivity is a function of the total wave variance.