Initial Assessment of the GOES Imager Microburst Product over Western Texas

Analysis of a few selected downburst cases over western Texas has provided an initial assessment of the performance of the GOES imager microburst product. Four cases between June and December 2008 featuring a total of eight downbursts in the West Texas Mesonet domain have been utilized. The results presented in this study will serve as a starting point for a much larger validation effort to be conducted during the summer of 2009.

Correlation was calculated by comparing output brightness temperature difference (BTD) values derived from GOES-11 and MODIS data to wind gust observations associated with downbursts. The cases selected included observations of severe winds (> 50 knots), in which downbursts occurred one to three hours after the generation of the product image. These cases provided the opportunity to assess the imager products as predictive tools for microburst potential. It was required in the product images that skies were clear over the location of downburst occurrence to preclude cloud contamination and unrepresentatively low risk values. A correlation of .59 was found between GOES-11 microburst risk values and wind gust magnitude. By a factor of two, this correlation was stronger than that calculated between MODIS output BTD and downburst wind gust measurements. Thus, over western Texas, GOES-11 imager data would be the optimal source to calculate and display microburst risk (probability).

The summer downburst events (6 of 8) were associated with high-reflectivity convective storms (>50 dBZ), highlighting the importance of precipitation loading in the downburst generation process (Johns and Doswell 1992). In most cases the downbursts occurred during the late afternoon and evening, near the end of a day of strong solar heating that resulted in the development of a deep convective boundary layer. The imager microburst products captured the evolution of these favorable environmental conditions.



The images above exemplify a typical summer downburst event over western Texas. The West Texas Mesonet meteogram at Turkey on 27 July 2008 displays the evolution of a deep convective mixed layer as an increase in dewpoint depression (DD) , in which the maximum surface DD (45F) occurred about one hour prior to the downburst. It is apparent in the meteogram that the development and evolution of the mixed layer is tied to strong insolation throught the day. The GOES-11 imager microburst product at 2030 UTC 27 July with overlying radar reflectivity imagery at 2307 UTC shows the downburst-producing convective storm near Turkey in a region of high risk. In general, severe downbursts, with wind gusts greater than 50 knots, are likely when output BTD values are greater than 45K.

REFERENCES

Johns, R.H., and C.A. Doswell, 1992: Severe local storms forecasting. Mon. Wea. Rev., 121, 1134–1151.