User:Daniel/Characteristics of the 14 April 1999 Sydney hailstorm based on ground observations, weather radar, insurance data and emergency calls

Natural Hazards and Earth System Sciences, 5, 613–620, 2005 SRef-ID: 1684-9981/nhess/2005-5-613 European Geosciences Union

S. S. Schuster, R. J. Blong, R. J. Leigh, and K. J. McAneney Risk Frontiers – Natural Hazards Research Centre, Macquarie University, Sydney, NSW 2109, Australia Received: 26 May 2005 – Revised: 2 August 2005 – Accepted: 2 August 2005 – Published: 11 August 2005

Abstract
Hailstorms occur frequently in metropolitan Sydney, in the eastern Australian State of New South Wales, which is especially vulnerable due to its building exposure and geographical location. Hailstorms challenge disaster response agencies and pose a great risk for insurance companies. This study focuses on the Sydney hailstorm of 14 April 1999 – Australia’s most expensive insured natural disaster, with supporting information from two other storms. Comparisons are drawn between observed hailstone sizes, radarderived reflectivity and damage data in the form of insurance claims and emergency calls.

The “emergency response intensity” (defined by the number of emergency calls as a proportion of the total number of dwellings in a Census Collection District) is a useful new measure of the storm intensity or severity experienced. The area defined by a radar reflectivity [greater than or equal to] 55 dBZ appears to be a good approximation of the damage swath on ground. A preferred area for hail damage is located to the left side of storm paths and corresponds well with larger hailstone sizes. Merging hail cells appear to cause a substantially higher emergency response intensity, which also corresponds well to maximum hailstone sizes. A damage threshold could be identified for hailstone sizes around 2.5 cm (1 cm), based on the emergency response intensity (insurance claims). Emergency response intensity and claims costs both correlate positively with hailstone sizes. Higher claim costs also occurred in areas that experienced higher emergency response intensities.

1. Introduction
Climatologically, the Sydney metropolitan area experiences an average of 10 hailstorms per year with the hail season lasting from August to February. Hailstorms generally occur during the late afternoon between 02:00 p.m. and 06:00 p.m. In Sydney the most hail-prone suburbs are concentrated in the most densely populated areas and corridors (Schuster et al., 2005).

Over the last 38 years, hailstorms have contributed over one third of the total insured costs caused by all natural hazards in Australia, with 75% of these losses occurring in the south eastern State of New South Wales (NSW) (Insert, Fig. 1) (Insurance Disaster Response Organisation, 2005). With an insured loss of approximately AUD 1700 million (EUR 1000 million; USD 1100 million) and total estimated economic costs of approximately AUD 2300 million, the Sydney hailstorm of 14 April 1999 ranks as the costliest natural disaster in Australian insurance history. Blong et al. (2001) considered this event to have an estimated return period of only 25 to 30 years based on the hail footprint and maximum reliable hailstone size (9 cm). Loss return periods for houses (vehicles) can be expected on average more (less) frequent than once in one hundred years (Blong et al., 2001). The Sydney hailstorms of 18 March 1990 and 3 October 1986 and the Brisbane hailstorm of 6 January 1985 also rank amongst the top ten insured loses in Australia in the period of record, 1967 to 2005 (Insurance Disaster Response Organisation, 2005).

The April 1999 hailstorm was outstanding from a climatological point of view because of large hailstone sizes, its evening occurrence, outside the hail season and during a time period of (statistically significant) lower hailstorm frequency. Moreover, the damage cost of this particular supercell storm is comparable to the costliest hailstorms worldwide, e.g. the April 2001 tri-state hailstorm that affected Kansas, Missouri and Illinois, USA and resulted in the most costly losses on record with USD 1900 million (Changnon and Burroughs, 2003); severe storms in 1998 in the Minneapolis – St. Paul area, Minnesota, USA that caused insured losses of USD 1350 million; the May 1995 Dallas – Ft. Worth hailstorm in Texas, USA with insured costs of USD 1135 million and the July 1984 Munich hailstorm in Germany that produced insured losses of USD 480 million (Munich Re, 1984; Heimann and Kunz, 1985). All monetary values refer to the year of storm occurrence.

Weather radar allows consistent measurements of potential hail intensity across large areas with high spatial and temporal resolution. Hailstone size observations for the April 1999 hailstorm are available (see next section). Combining these data on the hazard with damage data held by emergency services and the insurance industry results in the most detailed and comprehensive description of the impacts of this storm to date. This study draws comparisons and links between observed hailstone sizes, radar-derived reflectivity and damage data in the form of insurance claims and emergency calls. Supporting evidence from two other storms is also described.