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- Author or Editor: Fred H. Proctor x
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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.
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.
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.
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.
