IMPROVED FORECASTING OF COLD AIR OUTBREAKS OVER SOUTHERN CHINA THROUGH OROGRAPHIC GRAVITY WAVE DRAG PARAMETERIZATION

Abstract:

The parameterization of gravity wave drag induced by sub-grid scale orography (GWDO), which has been used in the regional model based on the Global/Regional Assimilation and Prediction System for Tropical Mesoscale Model (GRAPES_TMM), is extended to include the effect of mountain flow blocking drag (MBD). The extended scheme is evaluated against non-GWDO parameterization, including a cold air outbreak over southern China and a monthly verification in February 2012. The experiment results show that the GWDO and MBD parameterization both improves the forecasting of the cold air outbreaks over southern China, as well as alleviations of system bias of GRAPES_TMM. (1) The extended scheme alleviates the strong southerly wind and high surface temperature simulation during the cold air outbreak, especially over northern Guangxi and Guangdong (NGG) province, where local high surface temperature simulation reduces nearly 5 degree. (2) The MBD parameterization improves southerly wind simulations over NGG, as well as surface temperature forecasts improvement over Guangxi, Guizhou province and southern Yunnan-Guizhou plateau (YUP), and low level southerly wind simulation improvement over intertidal zone over south China. (3) The formation of MBD is mainly locates the mountain area (Wuyi, Daba mountain, east of YUP) and coastal area. The MBD over plateau, which is mainly formed at the west of 105°E, is stronger and thicker than that over Nanling mountain. (4) The improvement of GWDO and MBD parameterization is stable in model physics. MBD parameterization demonstrates more overall improvements in the forecasts than GWDO, and the larger of the model forecast error is, the greater improvements of MBD contribute to. Overall, the extended GWDO scheme successfully improves the simulations of meteorological elements forecasting during cold air outbreaks.