Arizona Downbursts: Recap and Look Ahead

As noted in a previous entry, "Arizona Downbursts 2008", analysis of three downburst events over central Arizona during the summer of 2008 revealed that the GOES-11 imager microburst product demonstrated effectiveness in the assessment of the potential for both dry and hybrid-type downbursts. In addition, it was also found these downburst events, observed by mesonet stations in the Arizona Automated Local Evaluation in Real Time (ALERT) domain between July and September 2008, served as effective validation data for the GOES-11 microburst product. This entry expands on the previous entry and looks ahead to the 2009 convective season validation effort over Arizona.


The images above are examples of southwestern U.S. GOES-11 imager microburst risk products that were generated prior to significant downburst events over central and southern Arizona during August and September 2008. Both images display convective storm activity developing over central and western Arizona during the afternoon that would produce downbursts during the following one to four hours. The product image at 2130 UTC 9 August 2008 (top) displays a favorable microburst environment near Horseshoe Lake ALERT station (AHL), where a strong convective wind gust of 46 knots would occur about one hour later at 2238 UTC. In a similar manner, the product image at 2000 UTC 11 September 2008 (bottom), indicates a moderate to high risk of microbursts near Gila Bend (AGL), where a downburst wind gust of 42 knots would be observed about four hours later at 0025 UTC 12 September.

Validation during the upcoming 2009 convective season will entail comparing surface observations of downburst wind gusts by Arizona ALERT stations to GOES-11 imager microburst and GOES-11 sounder Microburst Windspeed Potential Index (MWPI) product output generated one to three hours prior to each event. Wind gust magnitude for each downburst event will be compared directly to the closest representative microburst product output (BTD for imager product, MWPI for sounder product). In addition, radar reflectivity imagery will be utilized to verify that observed wind gusts are in fact associated with convective storms. Surface wind histograms from respective ALERT stations will be analyzed to identify the time and intensity of observed downbursts as demonstrated in the image below:

The above wind histogram at Horseshoe Lake ALERT station represents downburst occurrence as a sharp peak in wind gust speed near 1540 LST 9 August. This information, in conjunction with high radar reflectivity (>55 dBZ) associated with the parent convective storm, as displayed in the previous entry, confirmed that this wind event was clearly associated with a downburst. Thus, product validation will follow the methodology as outlined in Pryor (2009). It is expected that such a procedure should yield a statistically significant sample size, from which product performance should be effectively evaluated using classical statistical analysis. Validation involving three downburst events during the 2008 convective season did yield favorable results: a correlation of .50 that was statistically significant at the 84% confidence level. It is promising that such a small sample size indicated a significant correlation between GOES-11 microburst algorithm output and observed downburst wind gust speeds. More detail pertaining to this study can be reviewed in the following paper published in Arxiv.org: http://arxiv.org/abs/0904.0446.

Regional climatology has identified that the typical warm-season microburst environment over central Arizona can be best described as "hybrid" with an "inverted V" vertical profile characterized by significant CAPE, a mid-tropospheric moist layer, and a deep, dry convective boundary layer. These attributes are most effectively captured by both the GOES imager and sounder-derived MWPI products.

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.