NUMERICAL STUDIES OF EFFECTS OF SURFACE FRICTION ON LARGE-SCALE ATMOSPHERIC MOTIONS

Abstract

A simple relationship is obtained between pressure changes associated with friction and the geostrophic drag coefficient. From this, the imbalance between frictionally induced mass inflow and outflow is shown to be one or two orders of magnitude smaller than either the inflow or outflow. Numerical integrations using the primitive equations are performed for an axially symmetric autobarotropic low-pressure system. The velocity components and pressure tendencies are found to depend critically on the drag coefficient. Two actual synoptic cases are studied using a quasi-geostrophic numerical model incorporating release of latent heat. Computations are performed with and without surface friction. When friction is excluded, the 1000-mb Highs and Lows are more intense. Two methods of computing the surface stress are compared. One is based on variations in terrain height and the other on the nature of the vegetation. Differences are large, especially over the western part of North America.