Summer of 2008 was rather convectively active as documented in a report released by the Flood Control District of Maricopa County (http://www.fcd.maricopa.gov/Rainfall/publications.aspx). Strong downbursts were observed by mesonet stations in the Arizona Automated Local Evaluation in Real Time (ALERT) domain between July and September 2008 and served as validation data for the Geostationary Operational Environmental Satellite (GOES)-11 imager microburst algorithm that employs brightness temperature differences (BTD) between band 3 (upper level water vapor, 6.7μm), band 4 (longwave infrared window, 10.7μm), and split window band 5 (12μm). A strong negative correlation has been found between 6.7μm brightness temperature (Tb) and layer-averaged relative humidity (RH) between the 200 and 500-mb levels. In the middle to upper troposphere, decreases in Tb are associated with increases in RH and, thus, a large BTD between bands 3 and 5 imply a large vertical relative humidity gradient. This difference in humidity between the mid-troposphere and the surface is a condition favorable for strong convective downdraft generation due to evaporational cooling as precipitation descends in the sub-cloud layer.
The first documented microburst in the ALERT domain occurred at Magma FRS in Pinal County during the evening of 19 July 2008 as shown in the GOES-11 imager derived microburst risk product at 0100 UTC 20 July 2008 (above) with overlying radar reflectivity from Phoenix, Arizona NEXRAD (IWA) at 0343 UTC. Apparent in the microburst product image is a convective storm over Pinal County, 45 miles southeast of Phoenix. The storm produced a downburst with a measured wind gust of 39 knots at Magma FRS at 0333 UTC and occurred in close proximity to a local maximum in risk (probability), as indicated by the orange shading immediately downstream of the convective storm.
During August 2008, a strong downburst was observed at Horseshoe Lake ALERT station. Apparent in the microburst product image (above) at 2130 UTC 9 August 2008 with overlying radar reflectivity from Phoenix, Arizona NEXRAD (IWA) at 2238 UTC is a convective storm over northern MaricopaCounty that produced a downburst at HorseshoeLake at 2238 UTC. The strong downburst, with a wind gust of 46 knots, occurred in close proximity to a local maximum in risk, as indicated by the orange shading immediately downstream of the convective storm.
Finally, a strong downburst occurred at Gila Bend ALERT station during the afternoon of 11 September 2008. Theabove GOES-11 imager derived microburst risk product at 2000 UTC with overlying radar reflectivity from Phoenix, Arizona NEXRAD (IWA) at 0022 UTC 12 September displayed a convective storm over southwestern MaricopaCounty.The storm produced a downburst (42 knots) at Gila Bend at 0025 UTC and occurred in close proximity to a local maximum in risk, as indicated by the orange shading immediately downstream of the convective storm.
The ambient environments of all three downburst events were characterized by a well-mixed, convective boundary layer as exemplified by the "inverted-V" sounding profile above. This sounding, derived from Rapid Update Cycle (RUC) model analysis data at 0100 UTC 20 July 2008 over Magma FRS ALERT station, displays significant mid-level moisture overlying a dry surface layer that resulted from several hours of strong surface heating and resultant mixing. Apparent in the sounding profile is a large humidity gradient between the mid-troposphere and the surface that fostered strong convective downdraft generation due to evaporational cooling as precipitation descended in the sub-cloud layer.
Thus, based on preliminary analysis of three downburst events over central Arizona during the summer of 2008, the GOES-11 imager microburst product demonstrated effectiveness in the assessment of the potential for both dry and hybrid-type downbursts.
The first documented microburst in the ALERT domain occurred at Magma FRS in Pinal County during the evening of 19 July 2008 as shown in the GOES-11 imager derived microburst risk product at 0100 UTC 20 July 2008 (above) with overlying radar reflectivity from Phoenix, Arizona NEXRAD (IWA) at 0343 UTC. Apparent in the microburst product image is a convective storm over Pinal County, 45 miles southeast of Phoenix. The storm produced a downburst with a measured wind gust of 39 knots at Magma FRS at 0333 UTC and occurred in close proximity to a local maximum in risk (probability), as indicated by the orange shading immediately downstream of the convective storm.
During August 2008, a strong downburst was observed at Horseshoe Lake ALERT station. 
Finally, a strong downburst occurred at Gila Bend ALERT station during the afternoon of 11 September 2008. The
The ambient environments of all three downburst events were characterized by a well-mixed, convective boundary layer as exemplified by the "inverted-V" sounding profile above. This sounding, derived from Rapid Update Cycle (RUC) model analysis data at 0100 UTC 20 July 2008 over Magma FRS ALERT station, displays significant mid-level moisture overlying a dry surface layer that resulted from several hours of strong surface heating and resultant mixing. Apparent in the sounding profile is a large humidity gradient between the mid-troposphere and the surface that fostered strong convective downdraft generation due to evaporational cooling as precipitation descended in the sub-cloud layer.
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