Oklahoma Spearhead Echoes and Downbursts

During the afternoon of 9 September 2009, scattered convective storms developed along a weak cold front as it was tracking southeastward over Oklahoma. Although there was a weak temperature contrast across the front, the front acted as a convergence zone and a trigger for deep, moist convection. The pre-convective environment downstream of the cold front over western Oklahoma was dominated by vertical mixing that fostered the development and evolution of a convective boundary layer. Elevated Geostationary Operational Environmental Satellite (GOES) imager brightness temperature difference (BTD) values in the vicinity of downburst occurrence over western Oklahoma served as evidence of the presence of a well-developed mixed layer. Strong downbursts that were recorded by Oklahoma Mesonet stations in southwestern Oklahoma between 2145 and 2250 UTC resulted from a combination of precipitation loading and sub-cloud evaporation of precipitation. These downbursts occurred in proximity to high microburst risk values as indicated in the 2000 UTC imager microburst product.


The images above are a Geostationary Operational Environmental Satellite (GOES) imager microburst product with overlying radar reflectivity from Frederick, Oklahoma NEXRAD (KFDR) at 2145 UTC (top) and 2249 UTC (bottom), times of downburst occurrence at Medicine Park and Apache Oklahoma Mesonet stations, respectively. The product image displayed scattered convective storms developing along the cold front over western Oklahoma. Downburst wind gusts of 54 and 41 knots were recorded at Medicine Park and Apache stations at 2145 and 2250 UTC, respectively. Apparent in both images is the spearhead echo associated with both downbursts, occurring in a region of high microburst risk as indicated by the orange shading. Output BTD near 50K corresponded to wind gust potential near 50 knots. Fujita and Byers (1977) related the spearhead echo signature in radar reflectivity imagery to the occurrence of downbursts as illustrated below.


The Rapid Update Cycle model (RUC) analysis sounding above at 2000 UTC at Medicine Park shows an "inverted v" profile, and indicates the presence of a deep and dry mixed layer that favored the development of intense downdrafts due to the evaporation of precipitation in the sub-cloud layer. This profile corresponded well with high output BTD associated with downbursts and resultant spearhead echoes over southwestern Oklahoma.

References

Fujita, T. T., and H.R. Byers, 1977: Spearhead echo and downbursts in the crash of an airliner. Mon. Wea. Rev., 105, 1292-146.