Vertical Coupling: Quantifying the Influence of Lower Atmospheric Wind Variability on Mesosphere and Lower Thermosphere Gravity Wave Forcing

Abstract:

Selective filtering of upward propagating gravity waves (GWs) in the stratospheric wind field plays an important role in determining which part of the GW spectrum reaches the mesosphere and lower thermosphere (MLT), the boundary between the lower atmosphere and thermosphere-ionosphere region. GW forcing is the key driver of the MLT general circulation, highlighting the importance of the stratosphere in coupling of the lower atmosphere to the MLT. However, observations quantifying the influence of the stratosphere on the MLT are lacking. In this study, the GW spectrum reaching the MLT is quantified by new generation meteor radar observations of GW momentum flux and forcing. The wind field is derived by combining MERRA reanalysis and meteor radar winds. To study the relation between MLT GW forcing and selective filtering of upward propagating GWs by the zonal wind field in the stratosphere, an estimate of the zonal wind experienced by a symmetric distribution of upward propagating GWs is introduced. The ‘net zonal wind’, defined as the average of the most eastward and most westward wind within the column below the MLT, is subsequently related to the GW forcing to derive a quantitative relationship between the stratosphere and the MLT. This relationship is tested for a wide range of dynamical conditions over various mid-latitude sites, to study how lower atmospheric wind variability influences the GW driving of the thermosphere-ionosphere lowermost boundary.