A Description and Alternative Hypothesis for the Sydney Hailstorm - 14th April 1999

Clyve Herbert (ASWA - Victoria)

As the weak cold front pushed through the Shoalhaven River valley and started to collide with the higher ground to the northwest, deep cumulus developed, with the first major echoes becoming apparent  by approximately 1630hrs just to the north of Nowra. From this time, the storm complex started to show steady state features and reached severe levels at about 1715hrs AEST. Leaving the land and moving seaward near Kiama at approximately 1725hrs, the storm pushed NNE off shore from Wollongong by 1800hrs, then recrossed the coast near Bundeena at about 1905hrs, then continued northward to dissipate offshore of Gosford at approximately 2210hrs.

It has been established that the storm reached supercell status for most of its existence producing phenomenal hail and what has now become Australia's most costly natural disaster. To describe some of the interesting features of this storm probably takes us into a hypothetical discussion on why the storm behaved the way it did.

As the weak surface cold front moved landward near Nowra, the first deep convective development was detected possibly resulting from orographic and diurnal maximum interplay. It is apparent that the storm system intensified rapidly from this location.

Wind Pattern over southeast Australia at the 500hPa level (approx 5.5km) for 4pm EST 14 April 1999. 
This image shows the strengthening upper westerly flow south of Sydney.

Upper air dynamics may then have interacted with the surface feature. At 500hpa there appears to have been a strengthening in the west-east flow just ahead of the surface front. This upper flow feature may have been effected by the Great Dividing Range to produce a standing lee wave or dynamic lee trough just off the coast of NSW. As the storm intensified just south of Wollongong, the system then continued to backbuild strongly towards the north northeast, anchoring the main updraft focus near the surface cold front and growing explosively within the dynamic lee wave mechanism that may have been present. (see previous diagrams) This dynamic lee wave possibly extended northwards for some distance north of Sydney just near the coastal regions of central NSW at between 600 & 300 hpa.

Sea surface temperatures were probably in the order of 20+C and coupled with an upper cold pool over the NSW south coast (-47C at 300hpa), this may have provided strong lapse rate potential through a deep layer of the atmosphere, with considerable latent heat becoming available through the condensation process.

To summarise this hypothesis: it appears that the weak surface cold front pushing northward along the south coast may have interacted with a standing lee wave east of the Great Dividing Range with maximum updraft occurring within this lee wave location just off the south coast of NSW. The focus point of the main updraft continued to move northward near to the surface cold front more or less under the upper lee wave feature present at between 600 and 300hpa. This developmental process allowed the storm to move at a relatively sharp angle to the left of the middle & upper geostrophic flow. Other contributing factors may have been a cold pool at between 400 & 250 hpa which may have moved northeastward from southern NSW the previous day.

Updraft strength & hail size relationship

The actual concept of hail generation surprisingly is one of the least understood aspects of the storm environment.  The mind boggles at the thought of how strong an updraft is required to suspend a single hailstone as large as 9cm; but not just 1 hailstone, but millions of these iceballs are suspended and moved around the updraft core in a severe supercell hail-generating storm.  The updraft core or updraft jet in a thunderstorm must at least match or exceed the terminal velocity of each hailstone.  Naturally, the larger the ice stone the stronger the updraft required.  Other aspects to consider are that hailstones are not always spherical, but can take on a multitude of shapes thus altering their drag and air resistance status.  Maximum updrafts required for lifting very large hail through the main updraft core probably would exceed 50 metres per second (mps) and possibly reach 60mps or more.  Such an updraft intensity is generated by the actual storm environmental potential, but an aspect not discussed is the mass vapour conversion (MVC) - that is the conversion of water vapour to solid ice which may assist in the provision of heat exchange and the drying of the middle and upper parts of the updraft core allowing further enhancement of the updraft jet in certain situations. 

The Sydney hailstorm supercell is known to have generated 6-9cm stones and probably larger examples greater than 9cm may have developed, thus we may deduce updraft core speed of greater than 50mps and possibly greater than 60mps occurred. 

Another interesting aspect of hail, is when hailstones fall through the FZL (freezing level) towards the surface layers, the outer parts of the stone start melting.  This induces evaporation which results in a short period of additional cooling and allowing the stone to refreeze which may lower the local FZL to below the environmental FZL and additionally raising the storm environmental lapse rate.

Clyve Herbert - ASWA - Victoria June 1999