A NUMERICAL STUDY OF THE EFFECTS OF LONGWAVE RADIATION AND SURFACE FRICTION ON CYCLONE DEVELOPMENT

Abstract

An eight-level primitive equation model has been developed incorporating orography, large-scale release of latent heat, longwave radiation, and surface and internal friction. The clouds and moisture patterns used in the radiation calculations are predicted (i.e., change with time). Drag coefficients vary spatially. Thirty-six-hr predictions are performed over North America for an intense midlatitude winter cyclone. The inclusion of longwave radiation lowers 300-mb heights by as much as 190 m after 36 hr and significantly improves the forecasts at that level. However, there is little influence at lower levels or on predicted precipitation amounts. Less intense Highs and Lows result when surface friction is included. In the cyclone area, 1000-mb heights are raised by as much as 110 m after 36 hr. Maximum Ekman layer wind speeds are reduced from about 50 to 25 m/s. However, precipitation amounts are not significantly affected.