Nowcasting Wet Downbursts using Polarimetric RadarMariana1Earth

1, Scott

Lawrence

1,2, Carey

Elise

2, Schultz

Patrick

2, White

William P.

3 Roeder

System Science Center 2Department of Atmospheric Science University of Alabama Huntsville 345th Weather Squadron, Patrick AFB

IntroductionThe University of Alabama in Huntsville (UAHuntsville) is collaborating with the 45th Weather Squadron (45 WS) to improve the capability of nowcasting wind gusts of 35 kt and above associated with wet convective downbursts.UAHuntsvilles Advanced Radar for Meteorological and Operational Research (ARMOR) dual-polarimetric radar will be used since it is similar to the 45WS operational weather radar (both are C-band).Figure 1: ARMOR radar coverage and study area.

Case SelectionSeven downburst days with gusts above the greater than or equal to 35 kt threshold have been identified over North-central Alabama (Table 1). The high wind events on these days were identified with a combination of ASOS stations, SPC Storm Reports, and Local Storm Reports as compiled by Iowa State University. These cases commonly contain moderate-tohigh CAPE values, low-to-moderate environmental shear, abundant moisture, and steep low level lapse rates (Table 1).

Table 1: Case days with report types and environmental properties. In lapse rates, 1 indicates surface to 2km while 2 indicates surface to level of free convection.

MotivationThe 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. Convective wind warnings are the second most issued warning by 45 WS. 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, along with other wind gust advisories. 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.

July 19, 2006Storm with damage reports. High ZH

High ZDRSlight positive bias that will be adjusted.

BackgroundA 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. The presence of frozen condensate with differential reflectivity (ZDR) near zero at S-band within the reflectivity core was observed in hail studies by Roberts and Wilson (1989), Wakimoto and Bringi (1988), Wilson et al. (1987), and Hjelmfelt (1987). Other observed signs of a microburst are a descending reflectivity core, mid-level inflow, misocyclone rotation, and weak echo reflectivity notches (Roberts and Wilson, 1989). Low pHV

Figure 2: Horizontal and vertical cross sections of ZH, ZDR, and pHV for a storm that produced a microburst that blew a roof off a building and caused damage to cars at a parking lot near Albertville, AL.

Figure 3: Vertical structure of Zh, Zdr, and phv for July 19, 2006 case.

HypothesisThe hypothesis that will be tested is that significant concentrations of small rain drops, graupel, and small hail are important to wet downburst development in these warm season cases due to precipitation loading, evaporation, and melting (Atlas et al. 2004 and Srivastava 2004). Consequently, the presence of abundant small melting hail, which is characterized by large horizontal reflectivity (ZH > 50 dBZ) and large ZDR (3 8 dB) in C-band radars (Vivekanandan et al. 1990, Anderson et al. 2011, Kaltenboeck and Ryzhkov 2012), will be explored.

The example case above (figure 2) shows a case that is similar to most of the cases found in the seven case days selected. High ZH cores were often co-located with high ZDR at low levels. The area of low pHV is sometimes lower than shown, indicating mixed precipitation types, and some cases show high KDP values, an indicator of the presence of high water content. Figure 3 shows evidence of potentially small melting hail is seen through increasing Zdr with decreasing height beginning near the melting level.

Future WorkUse a fuzzy-logic particle identification (PID) algorithm with dual-polarization variables such as ZH, ZDR, specific differential phase (KDP), and correlation coefficient (pHV). This would allow for a more clear separation of hydrometeor types including small hail and graupel mixture, melting hail, and a rain and hail mixture. Water loading will be assessed using dual-polarization (e.g., ZH and ZDR) estimates of precipitation mass. More downburst cases will be added to the study as they occur.