Numerical Simulations of an Isolated Microburst. Part I: Dynamics and Structure

Abstract

Isolated and stationary microbursts are simulated using a time-dependent, high-resolution, axisymmetric numerical model. A microburst downdraft is initiated by specifying a distribution of precipitation at the top boundary of the model and allowing it to fall into the domain. Part I of this two-part series focuses on two case studies of a hazardous wet (high reflectivity) microburst. The first case assumes the environmental conditions that were observed on 30 June 1982 during the JAWS field experiment. The simulated microburst produced mean horizontal wind shears in excess of 10?2 s?1 for several minutes and was of an intensity similar to the observed microbursts on that date. The second case attempts to simulate the isolated microburst which occurred near the Dallas?Fort Worth airport on 2 August 1985. A comparison of the results to actual flight recorder data showed good agreement. Based on these case studies, the evolution and structure of the microbursts are discussed. The simulated microbursts were driven primarily by evaporative cooling of rain and, second, by cooling due to the melting of hail. Net warming within the upper regions of the precipitation shaft maintained the downdraft top near the melting level. The greatest temperature departures from ambient were at the ground. Mean horizontal wind shear was negligible 4 min prior to the time of maximum outflow winds, but reached an overall peak 2 min later. The numerical simulations produced a microburst ring vortex, cool low-level outflow with a burst-front head and nose structure, and a pressure nose beneath the microburst downdraft. The ring vortex propagated downward with the descending precipitation shaft and rapidly intensified prior to reaching the ground. The relatively cold air near the ground was apparently responsible for its rapid intensification. After the ring's circulation reached the ground, the ring vortex expanded radially outward, and lagged the leading edge of the expanding cool outflow. Surface friction was shown to have an important impact of the structure and propagation of the burst front. Negative horizontal vorticity produced in the vicinity of the ground was associated with a satellite vortex having opposite rotation to that of the primary ring vortex.