Observations of a mesoscale convective vortex, downbursts, and small-scale vortices associated with a heatburst event

A heat burst event that occurred over western Kansas is analyzed using data from the WSR-88D radar at Goodland, KS, high temporal resolution surface measurements, and video photography. The heatburst was confined to a relatively small area (~1000 km2) within the stratiform precipitation region of a weakening curved squall line, oriented WSW to ENE. The onset of the heatburst event near 0200 UTC was spatially correlated with a mesoscale convective vortex that originated as a cyclonic "bookend" vortex two hours earlier on the northeastern end of the linear convective system. This vortex intensified as it translated to the more benign anvil region on the upshear side of the convective line. Heatburst events were coincident with a rapid spin-up of the MCV, suggesting that the two phenomena were closely related. We hypothesize that very dry and unstable layer below 4 km provided enhanced stretching (strong downdrafts below, mesoscale updraft above) during this rapid intensification.

The onset of the heat burst activity was also very rapid. Radar observations show the emergence of 11 descending pockets of reflectivity beneath the anvil base, close to the convergent flow region of the MCV. Individual cells about 4 km in diameter exhibited a relatively uniform spacing of 5-8 km within a 30 x 30 km domain. Corresponding surface-based videos at this location and time recorded multiple microbursts associated within the reflectivity cells (15-20 dBZ) that descended to low levels in the form of dry microbursts. The large number of microbursts over such a small region resulted in numerous inferred collisions of microburst outflows, and the spin-up of relatively intense vortices, two of which were sampled in the surface time series. The picture that emerges during the height of this activity is an extremely turbulent, deep, and chaotic boundary layer.

The surface instruments recorded very warm (T = 36 „aC) and dry (Td = -2 „aC; relative humidity of 8%) air. A maximum wind gust of 32 m/s was associated with a microburst, while a secondary wind gust of 24 m/s was measured near the outer periphery of a vortex that passed just south of the observation location.