12 August 2009 West Texas Downbursts

During the afternoon and evening of 12 August 2009, isolated convective storms developed over western Texas and produced severe downbursts in the Lubbock area. The GOES-11 imager microburst risk product indicated elevated values (brightness temperature difference, BTD) in proximity to the location of the downbursts. The first downburst wind gust (53 knots) was observed by Floydada (West Texas) mesonet station at 2135 UTC. Another severe downburst was observed at Reese Center (57 knots), west of Lubbock, at 0010 UTC 13 August. Both downbursts were produced by small multicell storms with high radar reflectivity, > 55 dBZ. Output BTD between 45 and 50K indicated convective wind gust potential of 45 to 50 knots, based on a previously derived statistical relationship. Afternoon microburst risk (1900 UTC) corresponded well with an "inverted V" RUC sounding profile that indicated the presence of a relatively deep and dry convective mixed layer with steep temperature lapse rates below 600 mb. Convective storms had a high precipitation content as indicated by radar reflectivity, signifying that loading was significant factor in downdraft intensity.


The images above are GOES-11 imager microburst products at 1900 UTC (top) and 2000 UTC 12 August 2009 (bottom), with overlying radar reflectivity imagery from Lubbock, Texas (KLBB) NEXRAD. Both images show high radar reflectivity (>50 dBZ) associated with the storms that are located in regions of elevated microburst risk (tan shading). The stronger downburst at Reese Center was associated with a rear-inflow notch (RIN) (Przybylinski 1995). The RIN indicates the channeling of dry air into the convective downdraft and subsequent precipitation evaporation, generation of negative buoyancy, and downdraft acceleration. Further negative buoyancy is generated when the precipitation descends below the cloud base into the mixed layer. The final result is a very strong downdraft that impacts the surface. The favorable environment for downbursts is best illustrated in the RUC sounding profile below.

Above is a RUC analysis sounding at 1900 UTC 12 August over Floydada. This sounding is a typical hybrid microburst sounding (Ellrod 1989): "inverted V" profile; large CAPE; deep, dry mixed layer (ML); and steep temperature lapse rate below the 600 mb level. Although sub-cloud evaporation of precipitation and subsequent negative buoyancy generation fueled strong convective downdrafts, heavy precipitation provided further downdraft forcing as a result of the contribution of the loading process.

References

Ellrod, G. P., 1989: Environmental conditions associated with the Dallas microburst storm determined from satellite soundings. Wea. Forecasting, 4, 469-484.

Przybylinski, R.W., 1995: The bow echo. Observations, numerical simulations, and severe weather detection methods. Wea. Forecasting, 10, 203-218.