Atmospheric gravity wave effects on polar mesospheric clouds: A comparison of numerical simulations from CARMA 2D with AIM observations



[1] The effects of atmospheric gravity waves (AGWs) on Polar Mesospheric Cloud (PMC) evolution and brightness are studied using a two dimensional version of the Community Aerosol and Radiation Model for Atmospheres (CARMA 2D). The primary objectives for doing CARMA modeling of AGW effects on PMCs are to address the question of whether AGWs can account for the rapid, orbit by orbit changes in cloud structure and brightness seen in overlapping regions from images of the Cloud Imaging and Particle Size (CIPS) experiment on board the Aeronomy of Ice in the Mesosphere (AIM) spacecraft. We present comparisons of PMC brightness changes between our numerical simulations and observations from the CIPS experiment. Previous modeling studies have indicated a much longer life-time for PMC than the 90 min between CIPS orbits. We present CARMA 2D results showing dependence of ice particle growth and PMC brightness on AGW perturbation of background temperatures and water vapor concentrations. The model shows differences in brightness of PMCs due to differences in number of large ice particles depending on the scale and periods of the AGWs and also indicates that overall cloud brightness is a function of the wave period. While the maximum rate of change in PMC brightness from the model is still almost a factor of two less than the CIPS observed maximum rate of change in brightness, our study indicates that the variation in PMC brightness is in part due to the upward transport of water vapor into water depleted region by AGWs and the growth of ice particles from sub visual to visual and to larger sizes than they normally would have without AGWs. The presence of short period AGW cause periodic oscillations in cloud brightness about the no-AGW brightness while long-period AGW can temporarily increase the brightness of PMCs compared to the PMC brightness under no-AGW case. However, both the short-period and long-period AGW ultimately reduce the domain averaged PMC brightness in the long-term. This agrees with CIPS observations of generally dimmer PMCs in regions of high AGW activity. The seasonal variation in PMC albedos and the day to day variations seen in CIPS can be reproduced using a spectrum of short and long period AGW.