Effect of a Precipitation-Driven Downdraft on a Rotating Wind Field: A Possible Trigger Mechanism for Tornadoes?

Abstract

The Physical and dynamical effects of simulated precipitation in a rotating wind field are examined by numerical experiments. The physical-dynamical model consists of the three equations of motion, a thermodynamic equation, a conservation equation for precipitation, a diagnostic 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 latent-heat exchanges are included in this 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 which, descending dry adiabatically, becomes warmer than the air outside of the downdraft because the lapse rate of potential temperature in the environmental air is assumed close to 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. 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.