Numerical Simulations of an Isolated Microburst. Part II: Sensitivity Experiments

Abstract

Isolated and stationary microburst 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 II of this series examines numerous experiments in order to evaluate the sensitivity of microbursts to the environment and other factors. The model experiments provide valuable insight into the characteristics of microbursts. Specifically, the numerical simulations indicate that microburst intensity is sensitive to 1) the vertical distribution of ambient temperature and humidity, 2) the horizontal width of either the precipitation shaft or downdraft, 3) the magnitude of the precipitation loading, 4) the type of precipitation (i.e., rain, hail, graupel, or snow), and 5) the duration of the precipitation. The environmental conditions were found to be extremely important and the horizontal scale of the precipitation shaft played a significant role in determining the strength and structure of the microburst. The presence of a ground-based stable layer weakened the ring vortex, suppressed the outflow expansion rate, and excited gravity oscillations when penetrated by a microburst. Additional experiments suggested that rotating microbursts have weaker low-level downdrafts and outflows than the nonrotating variety. Several interesting scenarios are discovered for the most elective generation of an intense microburst. In one of these, snow was found to be very effective in generating intense low-reflectivity microbursts within a typical dry-microburst environment. The structure of the snow-driven microburst was unique compared to those driven by other precipitation types, having a relatively narrow stalactite-shaped radar echo, an intense downdraft, modest cooling, and strong shear. Cooling of the air from the sublimation of snow was found to be the dominant driving process for the dry microburst with snow. Several applications of the results were investigated also. Based on the model experiments which used a variety of observed environments, an index was developed for predicting the potential for wet microbursts. The important environmental parameters included in the index are: the height of the melting level, the mean lapse rate for temperature below the melting level, and the humidity at both the melting level and 1 km above the ground. Also examined from the numerical simulations is a possible relationship between the microburst temperature drop and outflow speed.