Improvements to Convective Wind Nowcasting at Cape Canaveral, Florida Using Subjective Dual-Polarization Radar Analyses

The United States Air Force’s 45th Weather Squadron (45WS) is responsible for providing weather support to America’s space program at Cape Canaveral Air Force Station and NASA Kennedy Space Center (CCAFS/KSC). The 45WS issues wind warnings for 35 knots or greater (Threshold-1), with 30 minutes of desired lead time, and 50 knots or greater (Threshold-2), with 60 minutes of desired lead time. Given the challenges associated with nowcasting convection in Florida during the warm season, the 45WS desires improvements to their wind warnings in the form of increased lead times and reduced false alarms, especially for Threshold-1 winds.

In an effort to improve 45WS warnings for convective wind events, data from the 45WS’s C-band dual-polarization radar were subjectively (i.e., manually) analyzed in 32 Threshold-1 and 32 below-threshold downburst events from the 2015 CCAFS/KSC warm season. Common radar signatures were identified and analyzed using vertical cross sections of radar variables in combination with ground truth winds obtained from the weather tower network at CCAFS/KSC and environmental conditions obtained from CCAFS/KSC soundings to better understand the physical processes occurring within. Signatures with the strongest statistical skill scores were selected as potential real-time nowcasting tools for 45WS forecasters.

Five dual-polarization radar signatures were identified herein or tested from past studies: peak height of 1 dB differential reflectivity (Z_dr) column, peak height of co-located radar reflectivity (Z_h) ≥ 30 dBZ and Z_dr around 0 dB (“Precipitation Ice Signature”), peak Z_h value, height below 0 °C level where Z_dr increases to 3 dB during melting within a descending reflectivity core (DRC), and vertical Z_dr gradient within a DRC. Multiple heights or magnitudes were tested for each signature within the single updraft-downdraft cycle that produced the recorded Threshold-1 or below-threshold winds in each case. The potential to increase lead times via identification of these features in earlier cells within multicell storms was also evaluated. The results suggest that the Precipitation Ice Signature, especially upon reaching a height of 4.5 km above the environmental 0 °C level, is arguably the most powerful standalone signature tested for nowcasting convective winds at CCAFS/KSC. Within the single updraft-downdraft cycle that produced recorded Threshold-1 winds, mean lead times ranged from 20.0 – 28.2 minutes for cumulus and mature stage signatures and from 12.8 – 14.9 minutes for dissipating stage signatures. Skill scores and lead times were improved by considering these signatures in earlier cells within multicell storms that eventually produced Threshold-1 winds at CCAFS/KSC.

A similar analysis was conducted to evaluate the performance of these signatures in differentiating Threshold-2 winds from winds that remained below Threshold-2 status. A total of nine Threshold-2 downbursts from the 2015 and 2016 CCAFS/KSC warm seasons were analyzed and compared to the 61 downburst cases from 2015 that were below Threshold-2 status. Skill scores and lead times were calculated for the Threshold-2 analysis and compared with the Threshold-1 results.