Hail events across the Greater Metropolitan Severe Thunderstorm Warning Area

Abstract. This study addresses the recent climatology of hail occurrence in the Greater Metropolitan Severe Thunderstorm Warning Area (GMSTWA) of New South Wales (NSW). The study area is a sprawling suburban area with a population of nearly 4.7 million and one of Australia's largest metropoles. The main objective is to highlight the recent temporal–spatial fluctuations of hail event frequencies and magnitudes (sizes) for each of recognized and vastly inhabited local government areas (LGAs). The relevant hail event data from 1989 to 2013 were initially derived from the severe storm archive of the Australian Bureau of Meteorology. A climatologically oriented GIS technique was then applied in the examining and mapping procedure of all hail events and hail days reported throughout the study area. By applying a specific criterion, severe hail (defined as 2 cm or more in diameter) was cautiously selected for relevant analysis. The database includes 357 hail events with sizes 2–11 cm which occurred in 169 hail days (a day in which a hail event at least more than 2 cm reported) across the region during the past 25 years.

The hail distribution patterns are neither temporally nor spatially uniform in magnitude throughout the study area. Temporal analysis indicated that most of hail events occur predominately in the afternoons with peak time of 1–5 p.m. Australian eastern standard time (EST). They are particularly common in spring and summer, reaching maximum frequency in November and December. There is an average of 14.3 events per year, but a significant decreasing trend in hail frequency and associated magnitude in the recent years has been identified. In turn, spatial analyses also established three main distribution patterns over the study area which include the Sydney metropolitan, the coastal and the most pronounced topographic effects. Based on the understanding of the favorable factors for thunderstorm development in the GMSTWA, the potential impacts from climate variability and future climate change have been briefly discussed.