Studies of gravity wave and thermal tide and their interactions in the middle atmosphere
Gravity waves (GWs) and tides are two strongest and most persistent waves in the middle atmosphere of the Earth. They are usually generated in the lower atmosphere and propagate upward to the middle and upper atmosphere, where they play important roles in the atmospheric composition, chemistry, dynamics and energetics. This dissertation focuses on a case-study of the propagation and dissipation characteristics of an inertial GW, the seasonal variation of the diurnal tide based on both the observations and models, and also the interactions between GWs and tides. One-night (October 28, 2003) temperature and horizontal wind measurements by a resonance sodium (Na) wind/temperature lidar in Maui (20.7 N, 156.3 W) and temperature measurement by a Rayleigh lidar at Mauna Loa Observatory (MLO, 19.5 N, 155.6 W), HI, are used as a case study of the GW propagation from the lower stratosphere to the lower thermosphere (35-103 km). A dominant wave mode is identi ed from the simultaneous temperature observations by both lidars. The wave is partially dissipated and propagates upward with an e-fold height of temperature amplitude at 14 km. A damping layer is present around the stratopause where the wave amplitude is relatively smaller, corresponding to a low static stability layer. The vertical wavelengths are larger in the mesosphere (12-13 km) than in the stratosphere (6-7 km), consistent with the decreasing static stability with altitude. The wave is propagating northward and the horizontal wavelength is 2140 km and intrinsic period is 15 hrs in the region of 84-103 km. The apparent period is 6 hrs and consistent with Doppler shift of the background wind. It is suggested that the convective zone over the equator to the south of Hawaii provides a constant GW source that is responsible for the observed GW throughout the night. The seasonal variability of the diurnal tide in the mesosphere and lower thermosphere (MLT) over Maui, HI is investigated using the meteor radar horizontal wind measurement from years 2002 to 2007. The semiannual oscillation (SAO) of tidal amplitudes is dominant above 88 km, with amplitudes at the equinoxes 2-3 times larger than at the solstices. Below 88 km, the annual oscillation (AO) dominates and its magnitude is smaller than the SAO. The AO dominates in the phase variation of the diurnal tide, which advances in winter and lags in summer as compared with the equinoxes. The vertical wavelength also has a noticeable seasonal variation with shorter vertical wavelengths found at the equinoxes. The reconstruction of the diurnal tide by superposing the migrating and nonmigrating tides derived from Thermosphere Ionosphere Mesosphere Energetics and Dynamics/Doppler Interferometer (TIMED/TIDI) and TIMED/Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) temperature is compared with the meteor radar observation, and a consistency is found in the seasonal variation of the tidal amplitude. Based on the TIDI and SABER measurements, the migrating diurnal tide (DW1) is the dominant tidal component while three other nonmigrating tides, DW2, DS0 and DE3, are non-negligible. The seasonal variation of the diurnal tide is well captured by the Global Scale Wave Model (GSWM) and the Whole Atmosphere Community Climate Model (WACCM) while discrepancies are also presented and discussed. Since the WACCM is capable of reproducing the tidal seasonality, it is used to examine the physical mechanisms. First, the e ects of GW forcing and advection on the momentum balance of DW1 are investigated, because they are the two most dominant terms in the momentum equation that account for the discrepancies between classical tidal theory and the calculations based on the full primitive equations. In the WACCM, GW forcing in the wave breaking region always damps DW1 and advances its phase, thus shortening the vertical wavelength of the tide locally. The linear advection largely determined by the latitudinal shear of the zonal mean wind mostly contributes to the phase change in the zonal wind. For the meridional wind, however, nonlinear advection is more important than GW drag and linear advection for the amplitude and phase changes. The DW1 amplitudes are smaller than TIMED observations, suggesting that the GW forcing is overestimated in the WACCM and result in an unrealistic large damping on DW1. Second, the seasonal variations of GW forcing, tidal heating and mean wind e ects are examined using the WACCM. Similar to the tidal amplitude, stronger GW forcing is also found at the equinox, which can not account for the tidal seasonality because GW forcing always damps DW1. Instead, the radiative tidal heating due to the water vapor absorption of infrared solar radiation largely determines the SAO of DW1. The e ffect of mean winds leads to a 1-month time shift of the maximum amplitude. The AO in the tidal phase is due to the seasonal change of mean winds. At the solstice, a stronger antisymmetric (1,2) Hough mode is generated which signi cantly distorts the tidal structure. Because the phase of the (1,2) mode changes by 12-hrs every half a year, it causes a phase advance in winter and a lag in summer, thus leading to an AO of the phase. As GWs and tides reach the MLT region, they can maintain large amplitudes thus strong interactions between them are expected. High-frequency GW variances are calculated as the residual horizontal wind variances based on the meteor radar measurements in Maui, HI and Urbana, IL (40 N, 88 W). Monte-Carlo simulations are performed in order to evaluate the sensitivity of the GW variance calculation on the meteor rate. It is indicated that the residual horizontal wind variance can be used as a good proxy of GW activities. The diurnal and semidiurnal variations of the GW variances are most dominant, while periods of 2-day, 5-day and 10-day are also observed. The vertical phase structures of the GW variances and tidal winds are consistent with each other, implying the GW variances are modulated by tides. In most cases, the GW variances increase with altitude and the growth rates are slower than freely propagating waves. A further study on the physical mechanisms resulting in the tidal modulations is needed in the future.
Gravity Wave, tide, atmospheric dynamics, atmospheric waves, middle atmosphere, lidar, meteor radar
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign