Impact of a New Thermal Roughness Length Treatment on the NCEP Global Forecast System (GFS)

Thermal roughness length (Zot) is one of the major factors to determine the surface heat exchange coefficient (Ch). The improper Zot will result in the unrealistic Ch. Hence the heat exchange between the atmosphere and the land surface cannot be simulated well. The current NCEP GFS uses the aerodynamic roughness length (Zom) as Zot. In the real world Zot should be a lot smaller than Zom particularly over non-vegetated area. Larger Zot/Ch will inflate the surface fluxes. Therefore, the cold bias of the surface skin temperature and the warm bias in the lower troposphere were found in the NCEP GFS over the arid area during the daytime.

In this study, a new thermal roughness length treatment developed by Zeng et al was tested for the summer of 2008. The new Zot scheme was found to reduce the large GFS daytime skin temperature cold bias significantly over the arid regions. Correspondingly the warm bias in the lower troposphere was reduced as well because less heat was transferred from the surface to the atmosphere. The forecast scores of the atmospheric circulations and the precipitation were improved to some degree. This new scheme will be implemented to the operational system after the further tests for the other seasons.