An Anomalous Nighttime Downburst Event

During the early morning of 10 March 2009, a strong downburst occurred over western Texas near Lubbock. The downburst, with an associated wind gust of 39 knots, was recorded at Anton West Texas Mesonet station, 20 miles northwest of Lubbock, at 0830 UTC. This event was unusual due the fact that the downburst originated from a cluster of showers with low reflectivity during the nighttime hours. A 100-mb (3000 feet) deep residual mixed layer (Stull 1988) was in place over the greater Lubbock area and fostered downdraft instability as precipitation descended in the sub-cloud layer. The GOES-11 imager microburst risk product effectively captured this favorable environment for downburst winds. This case may serve as an example of the effectiveness of the GOES imager product over the GOES-sounder Microburst Windspeed Potential Index (MWPI) in a region of widespread middle and high-level clouds.




The images above are a GOES sounder-derived MWPI product and corresponding imager microburst risk product at 0700 UTC 10 March 2009. Apparent in the GOES imager product is a cluster of showers over Hockley County, west of Lubbock. The location of downburst occurrence is marked with an "X". The closest clear-sky output brightness temperature differences (BTD) were indicated in excess of 40K (orange shading) over Lubbock. Previous validation (Pryor 2009) has established that output BTD greater than 40K was associated with wind gust potential of 40 knots or greater. However, since no sounding retrievals were available in the Lubbock area (REE) at 0700 UTC due to the presence of widespread mid-level clouds, no corresponding MWPI values were plotted. MWPI values of 11 to 14 (wind gust potential less than 35 knots) were plotted east of Plainview (PVS) and Ralls (RLS). Thus, the GOES imager product was able to indicate wind gust potential in the greater Lubbock area while sounder retrieval data over western Texas was very sparse due to the presence of middle and high clouds. In addition, the output BTD values, near 40K, more accurately indicated wind gust potential than was indicated by the MWPI product.

The above RUC model analysis sounding at 0700 UTC over Anton displayed the presence of the residual mixed layer that had developed and evolved during previous day. The layer, about 100 mb deep, provided sufficient downdraft instability that resulted from a steep, near dry-adiabatic lapse rate and large vertical relative humidity gradient. These conditions favored evaporative cooling and negative buoyancy generation as precipitation descended below the cloud base that was at a height near 10,000 feet. Thus, the cluster of relatively weak showers was capable of producing intense downdrafts due to a sub-cloud thermodynamic structure that provided very little inhibition. The residual mixed layer was overlying a conditionally unstable layer from the 800-mb level to the near the surface and a shallow stable boundary layer based at the surface. Surface observations at Anton (not shown) indicated a slight temperature increase at the time of downburst occurrence (0830 UTC), likely resulting from the mixing of warmer and drier air in the residual layer to the surface. Overall, the GOES-11 imager microburst risk product effectively indicated downburst potential in this nighttime, elevated mixed layer environment.

References

Pryor, K.L., 2009: Microburst windspeed potential assessment: progress and developments. Preprints, 16th Conf. on Satellite Meteorology and Oceanography, Phoenix, AZ, Amer. Meteor. Soc.

Stull, R.B., 1988: An introduction to boundary layer meteorology. Kluwer Academic Publishers, Boston, 649 pp.