The Evolution and Finc-Scale Structure of a Microburst-Producing Cell

Abstract

The structure and evolution of a microburst-producing cell were studied using dual-Doppler data collected in eastern Colorado during the summer of 1987. Eight volumes of multiple-Doppler data with a temporal resolution of 2.5 min were analyzed. The radar data were interpolated onto a Cartesian grid with horizontal and vertical spacing of 250 m and 200 m, respectively. The analysis of this dataset revealed that the 56 dBZ, storm produced two adjacent microbursts with different kinematic structures. The first microburst, which de-veloped a maximum velocity differential of 16 m s?1 over 2.5 km, was associated with a strong horizontal vortex (rotor) that developed new the surface at the precipitation edge. The second stronger micreburst obtained a velocity dilterential of 22 m s?1 over 3.2 km and was associated with a strengthening downdraft and collapse of the cell. Both microbursts developed ?14 min after precipitation reached the surface. Trajectory and equivalent potential temperature (?e) analyses were used to determine the history of the microburst-producing cell. These analyses indicate that the source region of air for the rotor-associated microburst was below cloud base and upwind of the precipitation shaft. Air entered the cell from the west at low levels, ascended over the horizontal rotor, and descended rapidly to the ground on the east side of the rotor. The source height of the air within the second microburst was well above cloud base. As the cell collapsed and the microburst developed, air accelerated into the downdraft at midlevels and descended to the surface. Features associated with this microburst included a descending reflectivity echo, convergence above cloud base, and the development and descent of strong vertical vorticity.