Numerical simulation of precipitation induced downbursts

Using the Terminal Area Simulation System (TASS), numerical simulations of downburst structure and sensitivity, based on vertical profiles of environmental temperature, humidity and wind velocity observed during the June and August 1982 JAWS project, are presented. Two-dimensional axisymmetric simulations examining downburst evolution, structure and sensitivity, assume a 40-m constant grid size on a 10-km diameter 5-km deep cylindrical domain. The three-dimensional experiment, examining the effects of vertical wind shear and other asymmetrical aspects of the downpour, assumed a 500-m constant horizontal grid size and a 35 x 35 x 18.5-km area. In the downburst primary structure depicted, outflow speeds were found to be sensitive to environmental temperature and humidity, as well as to precipitation radius and intensity. A vortex ring was found to propagate downwards, and maximum outflow winds occur when the vortex ring first reaches the surface.