| Atmospheric gravity wave (AGW) is one of the fundamental waves in the atmosphere, which play a very important role in influencing the dynamics in the mesosphere and lower thermosphere (MLT) region. AGWs are mostly generated in the lower atmosphere and propagate upward into the middle atmosphere. As AGWs propagate upward with the exponential growth of wave amplitude due to exponential decrease of air density or encounter the critical layer, they tend to become instable or break, and dump the momentum and energy into the mean flow causing the background temperature and wind change.The all-sky OH airglow images from2003to2009at Yucca Ridge Field Station, Colorado(40.7°N,104.9°N) and nearby MF radar wind at Platteville (40.2°N,104.7°N) are used to study the seasonal variability of characteristics of the quasi-monochromatic GW Momentum flux.The averaged occurrence frequency of AGW observed at Colorado is about60%. The maxinum of AGW occurrence frequency is up to90%at midnight in summer and while the mininum is only40%in March and Octomber.In MLT, the observed horizontal wavelengths of most gravity waves are ranged from20km to70km with peak value of～30km and averaged value of-40km. AGWs in this horizontal wavelength range are likely generated by convective activity. The observed vertical wavelengths fall in the range form10to60km clustered around10-40km. Inistric phase speeds are mainly ranged from～50to90m/s and apparent phase speeds are from30to70m/s. Most of GWs are with intrinsic wave periods below30min clustered around5-10min. For intrinsic wave period larger than5min, the number of wave events decreases fast as wave period increases. Observed wave periods are in the range of～7-20min. The differences between intrinsic phase speed and observed phase speed, intrinsic wave period and apparent wave period indicate that a large number of GWs propagate against the background wind.AGWs observed over Northern Colorado mainly tend to propagate in meridional directions and are strongly related with background wind at OH airglow altitude. AGWs propagate against (along) background wind to higher (lower) frequency and larger (smaller) vertical wavelength due to Doppler shifting by background wind. Longer vertical wavelength waves which suffer less dissipation were more easily detected by airglow imager because they suffer less cancelation effects. Thus the observed GWs tend to propagate against background wind except in October.Wave propagation directions are mainly affected by mean wind filtering, relative location to the wave sources and ducting processes and also exhibit strongly seasonal dependence, with dominant eastward and poleward propagation in summer, and preferred equatorward propagation in winter. The relative location to the wave sources and wave ducting processes determine the directionality of the wave propagation, but mean wind filtering affects the preference of wave propagation. The GWs tend to propagate poleward in summer in the mesopause region, suggesting that AGWs are likely generated by strong convection in the lower latitude troposphere, while the zonal wind between the stratosphere and the lower mesosphere will suppress the zonal propagations of GWs via mean wind filtering process. For mostly southward propagation direction in January2006, it was likely related to the stratospheric sudden warming. Recently, Doppler shifting effect is also focused on the actual distribution of the observed wave propagation direction.The magnitude of momentum fluxes was estimated to be1～15m2s-2. Momentum fluxes are strongly anti-correlated with background wind on both seasonal and nocturnal time scale. Momentum fluxes are also strongly modulated by diurnal and semi-diurnal tides. The acceleration by the observed small scale and high frequency GWs would have a potential to derive the mean flow in the mesopause region. |
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