Impacts of Orographic Gravity Wave Drag Parameterization on Typhoon Intensity and Path Forecasting

The scheme of gravity wave drag from sub-grid scale orography (GWDO) is implemented into typhoon models for the South China Sea and southeastern Asia based on the Global and Regional Assimilation and Prediction System for the Tropical Mesoscale Model (GRAPES-TMM). Its performance is evaluated against non-GWDO parameterization of nine typhoons that made landfall in 2012 in addition to the sensitivity of various criteria of the Richardson number (Ri_c) for typhoon path and intensity forecasting. The results reveal that the KA95 scheme improves the overall performance of GRAPES for typhoon forecasting when the GWDO parameterization is implemented, particularly with longer forecasts. Double typhoons Damrey and Saola are used as case studies to show that GWDO parameterization is highly effective for forecasting typhoons in the middle and lower troposphere. Moreover, the calculation of gravity wave drag indicates that significant and negligible low-level wave drag is predicted with large and small Richardson numbers (Ri_c=1.0, Ri_c=0.25), respectively. By defining a moderate Richardson number (Ri_c=0.75), improvements in predicting the paths and intensities of typhoons are achieved.