Analysis of UARS MLS radiance variances and their relationship with stratospheric wind

Recently, the 63 GHz radiance variance data obtained from the Upper Atmospheric Research Satellite (UARS) Microwave Limb Sounder (MLS) has shown important spatial and seasonal patterns that may link to strong gravity wave activities at mid- to high-latitudes of winter hemispheres and at sub-tropical regions of summer hemispheres. It has been found that MLS data depends strongly on viewing geometries associated with various instrument operation modes. The viewing angle dependence is caused by the spatial distribution and orientation of the MLS temperature weighting function. Because of the spatial asymmetry of the weighting function, the MLS radiance response to atmospheric temperature perturbations depends strongly on wave propagating direction. The observed radiance variance is larger when the instrument line-of-sight (LOS) direction is aligned with wavephase lines. When the gravity wave energy propagates upward into the stratosphere, the local airflow perceives it at a Doppler-shifted frequency, lower if the wave phase travels in the same direction and higher if the two velocities are opposite. Therefore, for a gravity wave to grow stronger and go through into the upper stratosphere, the horizontal direction of the wave vector needs to be in opposite of the background wind velocity. Otherwise, if the Doppler shift is so severe that the phase of the wave becomes stationary in the wind frame, the wave becomes part of the flow and is absorbed. Such effect is called the directional filter. The background wind absorbs waves in one direction but allows them to pass in the opposite