Effect of a transient density anomaly on a rotating-wind field

Abstract

The physical and dynamical effects of simulated precipitation in a rotating-wind field are examined by numerical experiment. The numerical model consists of the three equations of motion, a thermodynamic equation, a conservation equation for precipitation, a derived pressure equation, and appropriate boundary conditions, that are solved numerically by use of central space and time differences in a 1.84 km by 1.82 km grid. While no moisture and heat exchanges are included in the model, the effect of rain and hail is simulated through differing terminal velocities. The results of two experiments show that vorticity is concentrated by the precipitation-induced, accelerating downdraft. The downdraft, which descends dry adiabatically, becomes warmer than the air outside of the downdraft because the lapse rate of potential temperature in the environmental air is only slightly less than moist-adiabatic. Near the surface, the air in the downdraft attains sufficient positive buoyancy to overcome the negative buoyancy of the precipitation and begins to be accelerated upward. In fact, two updrafts form near the surface: one on the axis of symmetry and the other approximately 250 m from the axis. Because of mass continuity, the accelerating updraft is accompanied by horizontal inflow near the surface that acts to concentrate vorticity in the lower part of the region near the axis.