Lower-tropospheric enhancement of gravity wave drag in a global spectral atmospheric forecast model

The impacts of enhanced lower-tropospheric gravity wave drag induced by subgrid-scale orography on short- and medium-range forecasts as well as seasonal simulations are examined. This study reports on the enhanced performance of the scheme proposed by Kim and Arakawa, which has been used in the National Centers for Environmental Prediction ( NCEP) Global Spectral Model since 1997. The performance is evaluated against a traditional upper-level drag scheme that is also available in the model. The experiment results reveal that the Kim-Arakawa scheme improves the movement and intensity of an extratropical cyclone and a continental high pressure system that was accompanied by heavy snowfall over Korea on 14-15 February 2001. The monthly verification for medium-range forecasts in December 2006, which are initialized by the NCEP operational analysis, demonstrates overall improvements in the forecasts of large-scale fields in the Northern Hemisphere. Moderate improvements are also found in the seasonal simulation of December-February for the years 1996/ 97, 1997/98, and 1999/2000. This study concludes that the enhanced lower-level drag should be properly parameterized in global atmospheric models for numerical weather prediction and seasonal prediction.