The Impact of Parameterized Subgrid-Scale Orographic Forcing on Systematic Errors in a Global NWP Model

TLDR: In this paper, the global momentum budget for December 1993, diagnosed from a series of two-time-step integrations of the U.K. Meteorological Office global Unified Model, suggests that the parameterized mechanical dissipation in the model is underestimated.

Abstract: The global momentum budget for December 1993, diagnosed from a series of two-time-step integrations of the U.K. Meteorological Office global Unified Model, suggests that the parameterized mechanical dissipation in the model is underestimated. The initial momentum imbalance is consistent with the zonal mean systematic errors in zonal wind. Further diagnosis suggests that (i) the momentum budget residual is largest in the boundary layer and over the orography and (ii) too much gravity wave drag is applied in the model stratosphere. Two new parameterizations are proposed to alleviate this problem. The first is a new gravity wave drag scheme incorporating (i) anisotropy in the orography into the expression for surface gravity wave stress and (ii) low-level wave breaking mechanisms such as trapped Ice waves and high drag states. The second parameterization deals with the form drag due to subgrid-scale orography, a process currently ignored in the model. This is parameterized using an effective roughne...