Nowcasting Wet Downbursts using Polarimetric Radar

The University of Alabama in Huntsville (UAHuntsville) is collaborating with the 45th Weather Squadron (45WS) to improve the capability of nowcasting wind gusts of 35 kt and above associated with wet convective downbursts. A number of studies have shown the utility of dual-polarization radars in hydrometeor particle identification however, relatively little research has been done on using dual-polarization radar in predicting downbursts. This study will focus on the use of the UAHuntsville Advanced Radar for Meteorological and Operational Research (ARMOR) dual-polarimetric radar, which is similar to the 45WS operational weather radar (both are C-band), in combination with meteorological parameters.

The 45 WS provides weather support to America's space program at Cape Canaveral Air Force Station and NASA Kennedy Space Center on the coast of east central Florida. Central Florida is well known as ‘lightning alley' for the U.S. so thunderstorms and their associated hazards are the prime concern for 45 WS. Convective wind warnings are the second most issued warning by 45 WS with about 175 issued each year on average. Two convective warnings are used: one for ≥ 35 kt with a desired lead-time of 30 min, and one for ≥ 50 kt with a desired lead-time of 60 min. Other wind gust advisories are also used. This study focuses on improving the warnings for ≥ 35 kt since this is the most frequently issued of the convective wind warnings by 45 WS.

So far, seven downburst cases with gusts above the greater than or equal to 35 kt threshold have been identified over North-central Alabama. The high wind events were identified with a combination of ASOS stations, Storm Prediction Center Storm Reports, and Local Storm Reports as compiled by Iowa State University. These cases commonly contain moderate-to-high CAPE values, low-to-moderate environmental shear, abundant moisture, and steep low level lapse rates. A few of these high wind events (3 of 7) also contain small hail reports with a mean of 18 mm and a mode of 6 mm diameter. More downburst cases will be added to the study as they occur.

The hypothesis that will be tested is that significant concentrations of graupel and small hail are important to wet downburst development in these warm season cases due to precipitation loading, evaporation, and melting. Consequently, the presence of abundant small melting hail, which is characterized by large horizontal reflectivity (ZH > 50 dBZ) and large differential reflectivity (ZDR = 3 – 8 dB) in C-band radars, will be explored. ZH and ZDR will be used in concert with other dual-polarization variables such as specific differential phase (KDP) and correlation coefficient (HV) in a fuzzy-logic particle identification (PID) algorithm to refine hydrometeor types, including a small hail and graupel mixture, melting hail and a rain and hail mixture. The extent of loading will also be assessed using dual-polarization (e.g., ZH and ZDR) estimates of precipitation mass. Other potential radar signatures associated with downbursts considered in this study include rapidly descending reflectivity cores and convergent mid-level Doppler velocities.