Radar and profiler measurements of mesoscale and microscale flows within a heat burst

On 21 June 2003 a prolonged heat burst event was documented (during the BAMEX project) over western Iowa and eastern Nebraska along the northern edge of stratiform precipitation. This event persisted for about 10 h due to the continued regeneration of deep convection that maintained a deep anvil cloud over this region. Detailed measurements were obtained by the Mobile Integrated Profiling System (MIPS) which included a 915 MHz wind profiler (915), a 12-channel microwave profiling radiometer (MPR), a lidar ceilometer, an electric field mill, and surface instrumentation. In addition, high-quality Doppler radar measurements were obtained by the Omaha WSR-88D radar, located 41 km southwest of the MIPS.

The Doppler radar data are analyzed with the NCAR Variational Doppler Radar Analysis System (VDRAS) software and standard VAD techniques to examine a variety of mesoscale flows. The following mesoscale phenomena are documented within the heat burst: a) During its initial stages, the heat burst was marked by penetrative downdrafts that produced intense gravity waves within the developing nocturnal inversion over and around the MIPS site. The initial standing wave produced a 3-5 m/s updraft over a 20 minute period. WSR-88D measurements around this gravity wave showed appreciable radial convergence (25 m s-1 over several km) and a fine line, indicative of a convergent boundary zone. Less intense gravity waves persisted throughout the remainder of the event. b) The combination of mesoscale downdrafts (diagnosed using VAD analyses) and individual microbursts produced significant warming and drying within the 0.5-4.0 km layer. The evolution of this warming and drying was well documented with the MIPS 915 and MPR. c) The unstable dry layer that developed was conducive to microbursts and generation of large eddy turbulence that was diagnosed by the 88D and 915 measurements. d) Intense meso-gamma-scale circulations appeared to be generated by the collision of individual microburst outflows. One such circulation was directly sampled by the MIPS instruments.