The Microbursts of 22 June 1982 in JAWS

Abstract

On 22 June 1982 a line of storms developed in the Joint Airport Weather Studies (JAWS) network near Denver, Colorado, in response to interaction of boundary-layer convergence lines and wind-parallel lines of convection. These storms produced six microbursts and one microburst line. Single and multiple Doppler radar data and surface network data were used to describe the structure and lift cycle of the microburst outflows and to examine potential forcing mechanisms. The microburst outflows can be classified into three types: isolated, generally symmetric outflows; outflows embedded in strong low-level environmental flow which reveal their symmetric diverging outflow structure only when the mean flow is eliminated and the microburst line. Some of the microburst decayed by weakening; others grew to larger-scale, less intense outflows. All of the microbursts were associated with descending high-reflectivity cores, many of which showed evidence of convergence and associated reflectivity decreases below cloud base. Most of the microbursts exhibited some rotation aloft. However, the relationship between the location of rotation and the downdraft was variable. Well-developed rotation also tended to be short lived, and timing of the appearance of rotation during the microburst life cycle varied from case to case. Application of the downdraft model of Srivastava indicated that sufficient negative buoyancy could be produced by evaporation and precipitation loading to account for the magnitude of the observed microburst downdrafts. It is concluded that the microbursts on this day were predominantly driven by microphysical and related thermodynamic effects due to processes of precipitation loading and water phase change.