Using Dual Polarimetric Radar Products as Forecast Guidance for Lightning Cessation

The U.S. Air Force's 45th Weather Squadron (45WS) seeks to improve their forecasts of lightning cessation at the Kennedy Space Center (KSC) and Cape Canaveral Air Force Station (CCAFS) in Florida. The 45WS currently keeps lightning advisories active longer than necessary to ensure the safety of personnel and facilities. This paper describes the use of lightning and dual-polarimetric radar data to develop lightning cessation guidance for east central Florida.

Since the dual-polarimetric radar at Patrick Air Force Base in Florida was delayed in becoming operational, we chose central Oklahoma as the domain for the first phase of study because dual-polarimetric data already were available. Pulse thunderstorms in Oklahoma during the warm seasons of 2010 and 2011were chosen to represent the types of warm season (April-July) thunderstorms commonly observed at KSC/CCAFS. Total lightning data were obtained by combining data from the Oklahoma Lightning Mapping Array and the National Lightning Detection Network (NLDN). Storms were tracked using the Warning Decision Support System – Integrated Information software (WDSS-II). The tracking produced time series of radar-derived and lightning parameters. Trends in conventional reflectivity (ZH), as well as various dual-polarimetric parameters at three temperatures crucial to storm electrification (i.e., 0oC, -10oC, -20oC), were analyzed for 30 storms in Oklahoma near the end of their lightning activity. Results from the Oklahoma storms were used to develop numerous potential cessation algorithms.

The second phase of study focused on 30 independent storms in Florida during the early 2012 warm season when dual-polarimetric data became available at NWS Melbourne and NWS Tampa. The algorithms developed from the storms in Oklahoma were tested on storms in Florida. Total lightning data for Florida storms were obtained by combining data from the Lightning Detection and Ranging network at CCAFS and the NLDN. WDSS-II again was used extensively.

We created a new evaluation methodology that considers the average time of successful cancellations, the average time of premature cancellations, the number of hits, and the average timing of each event. Premature cancellations are weighed most heavily (negatively) because they are the most dangerous situation. An overall score was calculated for each of the 39 cessation algorithms.

Results reveal that the best performing cessation algorithm in Florida utilizes ZH > 40 dBZ and the presence of graupel (PID = 12 from WDSS-II) at -10oC. One waits 10 min after these thresholds are no longer met before ending an advisory. This procedure did not prematurely end a lightning advisory for any of the 30 Florida storms. The average cancellation time after lightning actually occurred was 12.5 min. This is a considerable time savings over the 45WS' most conservative wait-time of 30 min, providing substantial economic benefits without sacrificing safety.

This study shows the importance and utility of using graupel to improve lightning cessation guidance for storms near KSC/CCAFS. However, the results are tentative, and the procedure must be tested and refined on a much larger dataset.