KINEMATIC, THERMODYNAMIC, AND VISUAL STRUCTURE OF LOW-REFLECTIVITY MICROBURSTS

On 9 July 1987, a series of low-reflectivity microbursts were studied over Colorado using dual-Doppler analyses, cloud photogrammetry, and i n situ measurements collected by aircraft. These types of wind-shear e vents are particulars hazardous to the aviation community since the pa rent cloud and pendant virga shafts appear innocuous. The microburst d owndrafts are shown to develop at the location where the virga shafts are, visually, the lowest and opaque. As the downdraft intensifies, su blimation and evaporation (to a smaller extent) rapidly deplete the hy drometeors and result in a shift of the axis of maximum negative verti cal velocities into a relatively low reflectivity and transparent regi on of the virga shafts. Comparisons with weak downdrafts or null cases reveal that the maximum radar reflectivities within the parent clouds for the two cases are comparable; however, the microburst storm consi stently exhibits a larger horizontal area encompassed by the 10-dBZ co ntour at midlevels prior to downdraft formation.