Detailed observations of severe heat bursts: microbursts, intense microscale vortices, and high amplitude gravity waves

Several intense heat bursts (HB) events producing very warm temperatures and severe wind gusts have been documented within the last two decades. This paper provides a review of the common features of severe HB's and provides an analysis of two contrasting HB events, directly sampled by the UAHuntsville Mobile Integrated Profiling System, that add considerable insights on the HB dynamics and thermodynamics. These two HB events displayed similarities and differences. On 20 June 2002 a severe heat HB essentially formed on top of the MIPS over western Kansas. This HB was quite warm (35 °C), dry (9% relative humidity), and produced peak wind gusts of 33 and 24 m s-1 over a 30 min period. Direct visual observations indicate that this HB was composed of multiple dry microbursts that formed nearly simultaneously over a region of about 103 km2. Collisions of microburst outflows produced intense vortices, two of which were directly observed in close proximity to the MIPS. In this case, the emergence of the heat burst coincided with the arrival and intensification of a mesoscale convective vortex over the heat burst region. The second, long-lived severe HB event was sampled over extreme western Iowa on 23 June 2003. In this case, a deeper layer of cool stable air existed at the surface, and the HB generated a maximum surface temperature of about 29 °C. Strong winds were associated with high amplitude gravity waves, produced by dry microbursts impinging on the surface-based stable layer. The passage of microscale vortices was also sampled. In this case, the MIPS microwave profiling radiometer documented the dramatic changes in thermodynamic profiles associated with the HB onset. When both cases are considered, strong winds within HBs may be generated by three sources: (i) microburst outflows, (ii) microscale vortices produced by collisions of microburst outflows, and (iii) high amplitude gravity waves also produced by microbursts pounding the low level stable layer. The presence of such diverse phenomena implies that a very chaotic boundary layer is present during HB events.