The impact of parameterized subgrid-scale orographic forcing on systematic errors in a global NWP model

Abstract

The global momentum budget for December 1993, diagnosed from a series of two-time-step integrations of the U.K. Meteorological Oflice 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 lee 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 roughness length that takes account of the subgrid-scale orographic roughness elements. These new parameterizations lead to an increase in the surface stress, more low-level drag, and a better initial momentum balance. Systematic errors in the model are reduced, and an operational trial and subsequent implementation of the schemes show that many objective verification scores are improved.