Climatological aspects of south-eastern Australian hailstorms and applications using Radar data

Climatology

The first in-depth hail climatology for New South Wales (NSW), Australia, based on reports of hailstones from 1791 to 2003 is described. The data comprise the Australian Bureau of Meteorology’s severe weather database as well as a detailed compilation of scientific and newspaper accounts of hailfall for the greater Sydney area for the period 1805-1996. A survey conducted in the aftermath of the April 1999 Sydney hailstorm, Australia’s costliest insured natural disaster, was also included. The focus of the database is on the greater Sydney area with its high exposure to thunderstorm and hail by virtue of its population and building density. Comparisons are drawn between metropolitan Sydney and the rest of NSW and between coastal and inland areas of the state.

Over the study period, a total of 1570 thunderstorms produced hail. On average 10 (38) hailstorms per year were recorded in the Sydney area (rural NSW) in the last 50 (14) years. The magnitude of hailstorms, as measured by hailstone size, is slightly higher in rural and coastal areas compared with metropolitan Sydney.

Distributions of hailstorms show marked seasonal, diurnal and geographical preferences. Hailstorms occur most frequently between October and February (Australian spring and summer) in NSW, with peak activity in November and December. The hail season in metropolitan Sydney is shifted by one month (September to January). The majority of hailstorms occur during the late afternoon between 3pm and 7pm with hailstorms in the metropolitan area starting to occur about one hour earlier. The most active hailfall regions are located in northern NSW - northern Tablelands, Northern Rivers as well as parts of the Northwest Slopes. In metropolitan Sydney the most hail-prone suburbs are located at the edge of the Sydney basin and along the coast.

Case study

As property values increase, so does exposure to storm and hail losses. The main determinants of hail damage to property are: frequency of hailstorms; intensity of hailstorms, dependant on number, size and kinetic energy of hailstones; wind speed and construction attributes such as roof type and windows, that determine a building’s vulnerability. A methodology using weather radar to estimate hail damage to buildings is introduced. Results are presented for one recent severe hailstorm: a damaging supercell that passed over the western suburbs of Sydney in the late afternoon of 28th October 1995. Hailstone sizes of between 4.5cm to 8cm were reported.

The Bureau of Meteorology’s Weather Watch Radar “Letterbox”, located 60km southwest of Sydney, provided the radar data. This S-Band non-Doppler radar operates with a 1.9° bandwidth, 10min time steps and a 1km range resolution. The radar reflectivity was projected onto a low storm level close to the surface where hailstones occur. Radar reflectivities greater than 55 dBZ are assumed to contain a significant hail fall. An empirical relationship was used to relate the radar reflectivity to the flux of the kinetic energy of hailstones. The resulting hail intensity per radar pixel [J/m²] represents the entire volume of hail received per surface unit over time and it is this quantity that is correlated with hail damage. Results of the comparison of the radar derived hail intensity to the resulting hail damage in the form of insurance claims are presented.

Since ground measurements of hail are rather scarce, incomplete and often prone to exaggeration, the advantages of an objective remote sensing tool like radar are apparent. As well as having applications in the insurance industry, near real-time estimates of hail damage could also assist State Emergency Services in locating the worst affected areas and assessing manpower and material requirements to protect damaged roofs and house contents.