Role of gravity waves in the spatial and temporal variability of stratospheric temperature measured by COSMIC/FORMOSAT-3 and Rayleigh lidar observations

This study utilizes COSMIC satellite and lidar observations to examine the spatial and temporal variability of stratospheric temperature at a number of scales. The geographic variation of the RMS temperature difference between pairs of COSMIC profiles shows a strong correspondence to previous climatologies of gravity wave activity. In addition, the second-order structure functions we form can be directly related to the horizontal wave number power spectrum. These structure functions for different seasons and altitudes display a close correspondence to previous studies which examined the form of the horizontal wave number power spectra. Our analysis suggests that the wavefield may be particularly affected by changes in the zonal wind between 15 and 25 km and that the wind reversal between tropospheric westerlies and stratospheric easterlies in summer strongly contributes to critical-level filtering. Inspection also shows that longer horizontal wavelength waves are preferentially removed in this region. At low altitudes, the variability related to gravity waves shows a remarkably similar pattern as a function of horizontal separation in both hemispheres but is quite different at higher altitudes. Such contrast implies that seasonal variability at higher altitudes may be dominated by changes in propagation conditions in the lower stratosphere. Examination of temperature variability as a function of spatial and temporal separation indicates that gravity wave activity dominates stratospheric temperature variability, and this has impacts on validation study site selection. For example, validation exercises in the summer hemisphere stratosphere are likely to be less affected by geophysical variability than those in the winter hemisphere.