Representation of Subgrid-Scale Orographic Effects in a General Circulation Model. Part I: Impact on the Dynamics of Simulated January Climate

Abstract

Effects of subgrid-scale orography are represented in most large-scale models of the atmosphere by means of parameterizing subgrid-scale orographic gravity wave drag and/or enhancing grid-scale orography, such as ?envelope orography,? with the use of subgrid-scale orographic variance. A new gravity wave parameterization scheme and an envelope orography have been implemented in the UCLA general circulation model. The impact of gravity wave drag and envelope orography on simulations using the tropospheric-stratospheric 15-layer version of the model are briefly discussed and compared. The gravity wave parameterization scheme and the envelope orography have a qualitatively similar and beneficial impact on ensemble means of simulated January climate. The midlatitude westerlies are weakened at all levels and the polar atmosphere is warmed in the Northern Hemisphere. A combination of the two produces the best results. Sensitivity experiments with the parameterization scheme indicate the importance of the selective enhancement of low-level drag. Although the overall impact of gravity wave drag on the mean fields is similar when using the standard version of orography or the envelope orography, the magnitudes of gravity wave drag are systematically different in simulations using the two representations of orography in the midlatitude Northern Hemisphere. The modification in the magnitudes of simulated meridional eddy momentum fluxes by gravity wave drag with the standard orography is as in earlier studies. This is, however, not the case with the envelope orography. Whereas the impact of gravity wave drag and the envelope orography on the mean fields is similar, it is not necessarily true in terms of the individual components of simulated angular momentum budget.