Mesosphere and Lower Thermosphere
Presiding: E R Talaat, Applied Physics Laboratory, Johns Hopkins University Applied Physics Laboratory; H Liu, National Center for Atmospheric Research
SA53A-01 13:30h
Comparative Study of Short Term Tidal Variability
The wind and temperature measurements from an unusually long period operation of the sodium lidar at Colorado State University (41N, 105W) around September equinox 2003 shows significant short term tidal variability (She et al, 2004). Coincident with the large tidal changes, a strong temperature inversion layer was also observed above 90 km. Examination of the SABER temperature at the same time not only confirms the existence of the inversion layer but also reveals the global nature of the inversion. It thus suggests the presence of a planetary wave in the mesosphere, according to Sassi et al (2002). Because the strong inversion was only observed for a few days, the planetary wave was probably a transient wave, consistent with previous studies of planetary waves around equinox (Taylor et al., 2001, Liu et al., 2001). The large tidal variability, therefore, is probably a consequence of the interaction between the transient planetary wave and the tides. This possibility is investigated using the NCAR thermosphere-ionosphere-mesosphere-electrodynamics general circulation model (TIME-GCM) and comparing model results with the lidar, SABER, and TIDI measurements. With a large transient planetary wave specified at the model lower boundary, the model is able to produce strong tidal variability comparable to that from the lidar observation. Temperature inversion due to planetary wave decaying is similar to that from the SABER measurement. Model results also show that non-migrating tides are generated by the planetary wave/tidal interaction, as noted in Hagan and Roble (2001). The latitudinal structure of the migrating diurnal tide also changes quite significantly as a result of the interaction, suggesting excitation of higher gravitational modes and/or rotational modes.
SA53A-02 13:45h
Climatology of Small-Scale Mesospheric Gravity Waves Observed over Antarctica
As part of a collaborative research program between the British Antarctic Survey and Utah State University an all-sky CCD imager was deployed in Antarctica to investigate the occurrence and properties of short-period (<1 hour) mesospheric gravity waves at high latitudes. The measurements were made from two sites: Halley Station (75.5° S, 26.7° W), and Rothera Research Station (67.3° S, 68.1° W) to help investigate differences in wave propagation characteristics associated with mountain wave generation over the Antarctic peninsula (as observed from Rothera). Observations were made over five consecutive winter seasons from 2000-2004 (two seasons from Halley Station and three from Rothera). In this study we have analyzed the spatial and temporal properties of over 300 spatially extensive, short-period gravity wave events observed in the NIR OH emission (altitude ~87 km). Our results show significant anisotropy in the gravity wave propagation headings observed from both sites suggesting a dominant poleward component for the wave propagation headings (> 80% events) towards the Antarctic Continent, and marked east-west variability that alters with the evolving winter seasons.
SA53A-03 14:00h
Observational Study of the 4-Day Wave in the Mesosphere Preceding the Sudden Stratospheric Warming Event
The winter polar stratosphere is characterized by an extensive vortex region that dynamically interacts with the winter mean flow. Under certain atmospheric conditions, when the mean winds in the high latitude winter stratosphere reverse from westerly to easterly, the polar vortex breaks down causing sudden stratospheric warmings (SSW). Earlier studies of the connection of SSW and mesosphere suggest mesospheric cooling trend preceding the stratospheric warming. It is expected that such a drastic change in the atmosphere will significantly alter the dynamical state of the mesosphere. In this study, we examine OH airglow temperature data from Eureka (80.0° N, 85.9° W), Resolute Bay (74.68° N, 94.90° W), and South Pole (90° S) for evidence of planetary wave activity during SSW events. Wavelet analyses of the temperature data show wave activity with a period of 4-day preceding the cooling in the mesosphere, which is followed by the stratosphere warming. We will also address the possible sourcing mechanisms for the generation of the 4-day wave that occurs before the onset of SSW.
SA53A-04 14:15h
Analysis of wind speed and wind shear statistics and small scale structure using Doppler lidar data from New Mexico and Hawaii
We present an analysis of the statistical distribution of the wind speeds and vertical wind shears in the upper mesosphere and lower thermosphere by using Doppler sodium lidar data from the Starfire lidar in New Mexico and from the Maui/MALT lidar in Hawaii. Here, we focus on the distribution of wind speed and shear magnitude as a function of height in the altitude range from 75 km to 105 km. A similar analysis was carried out by Larsen (JGR, 2002) for an extensive set of midlatitude rocket chemical release wind measurements in the altitude range from 90 to 140 km. The overlap in the height range coverage by the lidar and rocket measurements allows us to compare the results directly, at least in the region where the height coverage overlaps. In addition, the lidar data set extends the altitude coverage for the analysis below 90 or 95 km where the available chemical release data is much more limited. The results indicate a general decrease in the maximum wind speeds with height below the altitude of the maximum found in the earlier study, but the wind speeds are still generally larger and have larger shears than would be expected from tidal theory or general circulation model predictions.
SA53A-05 14:30h
Zonal-mean temperature variations inferred from SABER measurements on TIMED compared with UARS observations
In the Numerical Spectral Model, small-scale gravity waves propagating in the north/south direction can generate zonal mean (m = 0) meridional wind oscillations with periods between 2 and 4 months (Mayr et al., JASTP, 2003). These oscillations tend to be confined to low latitudes and have been interpreted to be the meridional counterpart of the wave-driven Quasi Biennial Oscillation in the zonal circulation. Intra-annual scillations with similar periods have been seen in the meridional winds based on UARS data (Huang and Reber, JGR, 2003). Wave driven meridional winds across the equator should generate, due to dynamical heating and cooling, temperature oscillations with opposite phase in the two hemispheres. We have analyzed SABER temperature measurements in the altitude range between 55 and 95 km to investigate the existence of such variations. Because there are also strong tidal signatures (up to about 20K) in the data, our algorithm estimates both mean values and tides together. Based on SABER temperature data, the intra-annual variations with periods between 2 and 4 months can have amplitudes up to 5 K or more, depending on the altitude. Their amplitudes are in qualitative agreement with those inferred from UARS data (from different years). The SABER and UARS temperature variations reveal pronounced hemispherical asymmetries, which are qualitatively consistent with wave driven meridional wind oscillations across the equator.
SA53A-06 14:45h
Atmospheric tides observed from the stratosphere to the lower thermosphere as observed by TIMED
Satellite studies have obtained long-term, global-scale depictions of diurnal tides in the middle atmosphere. However, the nature of satellite sampling is that the dynamic features are shifted Doppler-shifted by the satellite motion. As a result, the migrating tide structure is aliased into the zonal mean field for sun-synchronous satellites. Precessing satellites allow a composite picture of the dynamic field to be collected in local time that can be analyzed for diurnal or semidiurnal signals. However, these composites typically take 1-2 months to collect with two node (i.e., the ascending/descending portions of the satellite orbit) sampling. In this paper, we introduce a new method to extract the migrating tides from observations taken by a slowly precessing satellite. Specifically, we present observations of migrating tides, nonmigrating tides, and planetary waves from the lower stratosphere to the lower thermosphere as observed by TIMED/SABER. The zonal mean temperatures are then subtracted from each day of data and the perturbation fields are fitted to extract the nonmigrating tides and planetary waves. We describe the temporal variability of the migrating tide and evaluate possible nonlinear sourcing mechanisms for the nonmigrating tidal components using both observations and simulations from a mechanistic fully nonlinear dynamical model.