Experimental GOES Sounder Products for the Assessment of Downburst Potential

Abstract

Several experimental products derived from Geostationary Operational Environmental Satellite (GOES) Sounder retrievals (vertical profiles of temperature and moisture) have been developed to assist weather forecasters in assessing the potential for convective downbursts. The product suite currently includes the wind index (WINDEX), a dry microburst index, and the maximum difference in equivalent potential temperature (e) from the surface to 300 hPa. The products are displayed as color-coded boxes or numerical values, superimposed on GOES visible, infrared, or water vapor imagery, and are available hourly, day and night, via the Internet. After two full summers of evaluation, the products have been shown to be useful in the assessment of atmospheric conditions that may lead to strong, gusty surface winds from thunderstorms. Two case studies are presented: 1) a severe downburst storm in southern Arizona that produced historic surface wind speeds and damage, and 2) multiple dry and wet downbursts in western Kansas that resulted in minor damage. Verification involved comparing the parameters with radiosonde data, numerical model first guess data, or surface wind reports from airports, mesonetworks, or storm spotters. Mean absolute WINDEX from the GOES retrievals differed from the mean surface wind gust reports by Ͻ2 kt (1 m s Ϫ1) for 82 events, but underestimated wind gusts for 7 nighttime events by 22 kt (11 m s Ϫ1). GOES WINDEX was also slightly better than that derived from the concurrent National Centers for Environmental Prediction's Eta Model first guess. There are plans to incorporate these downburst parameters into a future upgrade of the National Weather Service's Advanced Weather Interactive Processing System, with the option to derive them from either GOES Sounder data, radiosondes, or numerical model forecast data.

Climatology of severe convective wind gusts in Australia

“…The Theta-e Deficit (TeD) is defined as the maximum vertical difference in equivalent potential temperature (θe) from the surface to the middle troposphere (Ellrod et al 2000).…”

“…Wet microbursts are characterised by a low cloud base, heavy precipitation, and reduced visibility. They are mainly forced through the entrainment of midlevel dry air and precipitation loading (Fujita, 1981;Atkins and Wakimoto, 1991;Ellrod et al, 2000;Wakimoto, 2001). In contrast, dry microbursts have a high cloud bases and little or no precipitation at the surface.…”

“…Dry microburst result from the latent cooling beneath the thunderstorm cloud base through evaporation, melting, and/or sublimation (Wakimoto, 1985;Atkins and Wakimoto, 1991;Ellrod et al, 2000;Wakimoto, 2001). Hybrid microburst result from processes that occur in both wet and dry microburst (Ellrod et al, 2000). That is, they occur due to both dry air entrainment and/or precipitation loading in the mid-level as well as latent cooling in the low-level.…”

“…The Dry Microburst Index (DMI) was created by Ellrod and Nelson (1998) to address the high convective cloud bases and strong evaporational cooling in the sub-cloud layer common during dry microbursts (Ellrod et al, 2000;Wakimoto, 1985). The index looks at the 700-500hPa lapse rate, and the 700hPa and 500hPa dewpoint depressions.…”

“…The index looks at the 700-500hPa lapse rate, and the 700hPa and 500hPa dewpoint depressions. Ellrod et al (2000) found a threshold for the occurrence of dry microburst to be a value of DMI greater or equal to 6. They also suggest that DMI together with CAPE would be helpful in evaluating the potential risk of dry microburst.…”