The Sydney Hailstorm of April 14, 1999: Synoptic description and numericalsimulation

During the evening of April 14, 1999 an intense hailstorm struck the most d ensely populated region of Australia, the eastern suburbs of Sydney. This t hunderstorm, which transformed into a high precipitation supercell when it moved into a region of enhanced surface moisture convergence and increased helicity on the coast, maintained its identity for 5.5 hours. It produced t he largest verified hail in Australia's history with the biggest stones bei ng 11 cm in diameter. A microburst was recorded at Sydney Airport. The dama ge inflicted by this hailstorm was immense with three deaths, numerous inju ries and insured losses exceeding $1.7 billion Australian dollars. This sto rm is the most expensive Australian natural disaster since severe weather r ecords commenced in 1975. The thunderstorm initially formed from surface heating of relatively dry ai r in a low shear environment but was advected by middle level west to south westerly winds into a region where the surface to 500 hPa wind shear had i ncreased to 17 ms(-1) west south westerly. The storm relative helicity in t his region was -180 m(2)s(-2), in the layer between the surface and 700 hPa . Diagnostics from the 1500 Australian Eastern Standard Time (AEST) radioso nde released from Sydney Airport. 150 km north of where the thunderstorm in itially formed, are thought to be representative of the pre-storm environme nt. The Convective Available Potential Energy (CAPE) was moderately high at 1713 J/kg with a relatively low Convective Inhibition (CIN) value of 50 J/ kg. The Total Totals Index (TT) was 55 and the Surface Lifted Index (SLI) w as -5.51 capable of supporting severe convection. The freezing level was at 2900 m. near average for the time of the year. Convective cloud tops would be expected to reach the tropopause at 250 hPa. The coastal environment wa s assessed as being, able to support a supercell thunderstorm. A preliminary high resolution numerical simulation of the severe thundersto rm has been conducted. The model was triply nested, with its highest resolu tion grid spacing being 1 km. It incorporates a multi (six) water - ice pha se microphysics, enabling it to simulate hail growth associated with superc ell developmet. The initial generation and subsequent northward propagation of a hail-producing thunderstorm are captured in this simulation.