Gust Front Characteristics and the Kinematics Associated with Interacting Thunderstorm Outflows

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.