Hail nowcasting over the South African Highveld

Abstract

Hailstorm is one of the main meteorological phenomena that signify sources of damage to vehicles, property, infrastructure and agriculture over the Highveld. This event poses a strain to societies and is one of the costliest insured natural hazards in South Africa. This dissertation aim is to evaluate the current state of a hail nowcasting system that provides early hail warning to end-users over the Gauteng Highveld. The objectives of this dissertation are three folded: First, to review the current nowcasting procedures of the South African Weather Service (SAWS). Secondly, to evaluate the objective nowcasting algorithm used by SAWS and thirdly, to assess the perceived benefits of hail nowcasts. Radar data was obtained from SAWS in Pretoria, Irene for the period November 2013-February 2015 using the S-band weather radar. The Thunderstorm Identification Tracking Analysis Nowcasting (TITAN) algorithm was programmed to run for 3 years. During this time period, 6 hail cases were reported. The nowcast products of TITAN were used to nowcast hailstorms for periods of 0-2h and verification of nowcasts was undertaken. Media and hail reports were used to subjectively identify hail events and results were correlated to the verification scores on a storm to storm basis; identifying how well the algorithm performed. Lastly, open-ended interviews were undertaken with individuals residing within the Gauteng Highveld. The aim of the interview was to explore the perceived benefits of nowcasts and what would be a successful nowcast to enhance or maximise those benefits. It was found that SAWS do not forecast hailstorms or tornadoes, however, severe thunderstorms are forecasted. The criteria used in issuing warnings for severe thunderstorms was found to be similar to that of National Severe Storms Laboratory (NSSL). The forecast process was similar to the ones for the Meteorology Office UK and the Indian Meteorology Department (IMD). When forecasting severe thunderstorms, the main tools that were used were weather models and radiosonde observations for identifying storms. Along with instability, wind shear was the biggest role player when it came to identifying hail events. TITAN was used with its default settings and was not customised to identify big damaging events. The algorithm indicated a heavy overestimation of hail events. The hit rate performed extremely well and had a POD score of 0.85. The FAR was exceedingly high and had a score of 0.93. TITAN performed poorly in terms of the Critical Success Index (CSI) and scored 0.05 which showed no skill of the forecasts. The verification scores indicated a poor performance with low CSI and high FA scores, although some events are warned during these occurrences. TITAN displayed a low Heidke Skill Score (HSS) with a forecast skill of 0.01 which indicated no skill due to the great amount of FA. It was found that South African radars have severe limitations and algorithms cannot be used alone, specialists are needed to interpret these products. The weather service needs better and customised algorithms for radars using TITAN. Nowcasts can help individuals in ensuring the safety of their loved ones. Interestingly, it was found that hail nowcasts could help city officials in making sure the drainage systems are well organised thus reducing floods caused by severe storms. Nowcasting could make a difference at the weather service, where there is currently a gap in this space.