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William P. Mahoney III

Abstract

The morphology, kinematic and thermodynamic characteristics of 30 gust fronts were examined with single and dual-Doppler radar and surface mesonet data collected in eastern Colorado during the summers of 1982 and 1984.

The majority of gust fronts examined exhibited the general shape of laboratory-produced gravity currents, including the elevated head, body and turbulent wake region. The average head depth was 1.3 km, only 0.1 km above the average body depth. Small-scale features in the vertical and horizontal vorticity fields were also observed. The passage of the fronts was marked, in order of event, by a pressure rise, wind direction and velocity change, and temperature drop at the surface. The average propagation speed and maximum surface wind within the outflows were 8.6 and 14.5 m s−1, respectively. The average maximum temperature drop at the surface was 3.5°C and the average hydrostatic pressure rise was 0.06 kPa.

Dual-Doppler analyses of colliding gust fronts revealed strong circulations along the frontal boundaries. Updrafts along the leading edge of individual outflows were enhanced as the fronts approached each other. In one case, vertical velocities of 16 m s−1 extended up to 3 km AGL along the convergence line shortly after the collision. Convection can be initiated or enhanced by mechanical forcing along outflow collision lines, and vertical air motions associated with such collisions can extend well above the top of outflow boundaries.

Surface divergence was often observed behind the gust fronts. These divergent regions appeared to be associated with the strong circulations that were located within the head region of the outflows.

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William P. Mahoney III and Kimberly L. Elmore

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 (θe) 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.

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