The S2K Severe Weather Detection Algorithms and Their Performance

Abstract

One of the main goals of the Sydney 2000 Forecast Demonstration Project was to demonstrate the efficacy and utility of automated severe weather detection radar algorithms. As a contribution to this goal, this paper describes the radar-based severe weather algorithms used in the project, their performance, and related radar issues. Participants in this part of the project included the National Severe Storm Laboratory (NSSL) Warning Decision Support System (WDSS), the Meteorological Service of Canada Canadian Radar Decision Support (CARDS) system, the National Center for Atmospheric Research Thunderstorm Initiation, Tracking, Analysis, and Nowcasting (TITAN) system, and a precipitation-typing algorithm from the Bureau of Meteorology Research Centre polarized C-band polarimetric (C-Pol) radar. Three radars were available: the S-band reflectivity-only operational radar, the C-band Doppler Kurnell radar, and the C-band Doppler polarization C-Pol radar. The radar algorithms attempt to diagnose the presence of storm cells; provide storm tracks; identify mesocyclone circulations, downbursts and/or microbursts, and hail; and provide storm ranking. The tracking and identification of cells was undertaken using TITAN and WDSS. Three versions of TITAN were employed to track weak and strong cells. Results show TITAN cell detection thresholds influence the ability of the algorithm to clearly identify storm cells and also the ability to correctly track the storms. WDSS algorithms are set up with lower-volume thresholds and provided many more tracks. WDSS and CARDS circulation algorithms were adapted to the Southern Hemisphere. CARDS had lower detection thresholds and, hence, detected more circulations than WDSS. Radial-velocity-based and reflectivity-based downburst algorithms were available from CARDS. Since the reflectivity-based algorithm was based on features aloft, it provided an earlier indication of strong surface winds. Three different hail algorithms from WDSS, CARDS, and C-Pol provided output on the presence, the probability, and the size of hail. Although the algorithms differed considerably they provided similar results. Size distributions were similar to observations. The WDSS provided a ranking algorithm to identify the most severe storm. Many of the algorithms had been adapted and altered to account for differences in radar technology, configuration, and meteorological regime. The various combinations of different algorithms and different radars provided an unprecedented opportunity to study the impact of radar technology on the performance of the severe weather algorithms. The algorithms were able to operate on both single- and dual-pulse repetition frequency Doppler radars and on C- and S-band radars with minimal changes. The biggest influence on the algorithms was data quality. Beamwidth smoothing limited the effective range of the algorithms and ground clutter and ground clutter filtering affected the quality of the low-level radial velocities and the detection of low-level downbursts. Cycle time of the volume scans significantly affected the tracking results.