Impact of orographic gravity wave drag on extended—range forecasts with the COLA—GCM

BENJAMÍN KIRTMAN, ANANDU VERNEKAR, D. DEWITT, JIAYU ZHOU

Abstract

The impact of gravity wave drag (GWD) on the COLA (Center for Ocean-Land-Atmosphere Interactions) –GCM is studied by simulating two pairs of 30-day extended range forecasts. One is initialized on January 8, 1990, and the other on July 15, 1989. With each initial condition forecasts were made both with and without the GWD effects. The results show that GWD effects improve the forecast for both January and July particularly in the winter hemisphere. The GWD reduces the westerly bias in the zonal wind and the cold bias in the temperature. As a consequence of the changes in the momentum and temperature there are changes in the meridional mass transport such that monthly mean sea level pressures are improved particularly in the polar regions. Furthermore, as the westerly bias in the zonal momentum is reduced the momentum of the large scale flow is brought down to the Earth's surface where it is dissipated. This dissipation process increases the effective surface drag and enhances the mean meridional circulations. The results indicate that the mean meridional circulations are enhanced by 15 and the divergent kinetic energy is enhanced by 30% after day-15. Associated with the enhanced secondary circulations is increased zonally averaged precipitation in the summer hemisphere and decreased zonally averaged precipitation in the winter hemisphere in July. In January the precipitation increases in both hemispheres.

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