Indirect and Direct Parameterizations of Subgrid-scale Turbulent Orographic Form Drag and Implementations in CESM

Abstract:

Turbulent drag due to the form drag exerted by subgrid obstacles has significant effects on atmosphere. Subgrid-scale turbulent orographic form drag is represented by using parameterization methods in large-scale numerical prediction models. An indirect parameterization scheme, which is originally used in CESM, called the effective roughness length scheme, and a direct scheme called BBW04, which has only been tested in short-term weather forecast models and regional models, were used in the climate prediction model CESM1.0.4. The results show that both indirect and direct scheme have improved the model’s performance in middle and high latitude areas. The two schemes were compared and analyzed based on three aspects: terrain height standard deviation data, scheme equations and differences between indirect and direct parameterization methods. The Analysis indicates that the direct parameterization scheme BBW04 is more reasonable and theoretically convincing than the indirect scheme.