Ground-Truth Model Evaluation of Subgrid Orographic Base-Flux Parameterization

Abstract

High-resolution simulation can be a powerful means of evaluating and tuning orographic drag schemes, but connecting the parameterized drag, which is a local forcing, with the model drag, which is fundamentally global, is not entirely straightforward. The simplest idea is to filter the velocity down to its divergent component and exploit Bernoulli’s law to define a local form drag. Using regional simulations over the Rockies, the Andes, and Greenland, we investigate the validity of this approach, which assumes that both the included nonorographic divergence and the missing orographic deformation will not significantly alter the diagnostic. The local drag is checked for consistency with the nonlocal drag at scales containing most of the gravity wave drag and blocking drag. The agreement is found to be satisfactory unless the drag is weak and nonlinear. In that case, we find it necessary to remove a steady pattern from the nonlocal drag in order to uncover a correlation. We test a specific mountain drag scheme using the proposed diagnostic and describe procedures for tuning the scheme’s drag coefficients and treatment of anisotropy.