3 August 2009 Downbursts and Heatburst

During the evening of 3 August 2009, convective storms developed along the dryline over the Oklahoma Panhandle and produced both dry and hybrid downbursts. The pre-convective environment in the vicinity of the locations of downburst occurrence was very unstable with a deep, well-mixed convective boundary layer and a steep temperature lapse rate below the 500mb level. High Geostationary Operational Environmental Satellite (GOES) imager brightness temperature difference (BTD) values in the vicinity of downburst occurrence over the panhandle served as evidence of the presence of a deep, sub-cloud dry adiabatic lapse rate layer. Strong downburst wind gusts that were recorded by Boise City, Oklahoma Mesonet station (41 knots) at 2340 UTC 3 August and by Hooker mesonet station (55 knots) at 0330 UTC 4 August 2009, respectively, resulted from sub-cloud evaporation of precipitation. Although these downbursts occurred in similar environments, the Boise City downburst was associated with a higher reflectivity storm (>50 dBZ) and could be classified as a "hybrid". The Hooker downburst was associated with a heat burst. A heat burst is defined as a "localized, sudden increase in surface temperature associated with a convective storm often accompanied by extreme drying"(courtesy AMS Glossary of Meteorology).

The images above are GOES-11 imager microburst products at 2200 UTC 3 August (top) and 0000 UTC 4 August 2009 (bottom), with overlying radar reflectivity imagery from Amarillo, Texas (KAMA) NEXRAD.The Boise City and Hooker downbursts occurred in proximity to high microburst risk values as indicated by the orange shading in the respective GOES-imager microburst products. Radar imagery shows that the Boise City downburst (top) was associated a classic spearhead echo signature, typically associated with wet microbursts. Interestingly, the stronger downburst observed at Hooker (bottom) was associated with a storm that appeared more innocuous on radar imagery with reflectivity below 30 dBZ. Also noteworthy was the occurrence of a heat burst at Hooker between 0300 and 0400 UTC. The Hooker meteogram below clearly shows this heat burst as a 10F temperature increase and 15F dewpoint decrease near the time of downburst occurrence.


This favorable environment for a heat burst was best illustrated in the above RUC analysis sounding at 0000 UTC 4 August over Hooker. This sounding is similar to the heat burst sounding identified in Conder et al. (2006): "inverted V" profile; deep, dry mixed layer (ML); and steep temperature lapse rate below the 500 mb level. Although sub-cloud evaporation of precipitation and resulting negative bouyancy fueled strong convective downdrafts, subsidence warming associated the the dry downburst at Hooker was primarily responsible for the anomalous heat burst.

References

Conder, M.R., S. R. Cobb, and G. Skwira, 2006: West Texas Mesonet observations of wake lows and heat bursts across northwest Texas. Preprints, 23rd Conf. on Severe Local Storms, St. Louis, MO, Amer. Meteor. Soc.