Investigating the Role of Gravity Wave Drag in Forecasting Stratospheric Sudden Warmings

Stratospheric sudden warmings (SSWs) produce dramatic reversals in the circulation of the polar winter stratosphere that can, in some cases, have an impact on tropospheric weather patterns that lasts for days or weeks after the event. SSW's can be difficult to forecast with more than a few days lead time due to (1) the highly non-linear wave-mean flow interactions that cause the phenomenon itself and (2) the limited vertical extent of operational numerical weather prediction (NWP) systems that, in the past, have lacked a fully resolved stratosphere. Recent numerical modeling studies indicate that SSW's produce vertically deep coupling between planetary waves, gravity waves, and the background zonal flow throughout the polar stratosphere and lower mesosphere. NWP models therefore need a realistic treatment of these processes in order to make accurate forecasts. This study examines the impact of parameterized gravity wave drag on SSW forecasts using a new prototype 60-level version of the Navy Global Environmental Model (NAVGEM) that extends from the surface to 0.1 hPa. Specifically, we perform a series of targeted 12-day “hindcasts” for the January 2012 SSW to examine the impact of both model vertical domain and choice of GWD parameterization on SSW forecast skill. Forecasts are compared with independent observations such as temperature measurements from the Aura Microwave Limb Sounder (MLS). Initial findings indicate that inclusion of GWD improves forecast model performance at high latitudes in the wintertime upper stratosphere.