A synoptic climatology of heavy rainfall in Hobart

While a generally dry site, Hobart in Tasmania is subject to occasional heavy rainfall events that can cause significant damage and disruption. In this paper, we investigate the synoptic characteristics of such events at a number of temporal scales using composites derived from large-scale reanalysis data.

At a daily timescale, heavy rainfall events – defined for the purposes of this paper as the 50% Annual Exceedance Probability (AEP) – are dominated by the passage of deep low pressure systems directing moist southeasterly air onto the foothills of the Wellington range. At the three-hourly timescale of synoptic observations, such lows are still the dominant mechanism leading to heavy rainfall with a minor component contributed by convective events. By the scale of hourly rainfall (at which the Hobart 50% AEP is 12 mm), however, convective rainfall events, characterised by open troughs, have begun to dominate the climatology, although there is still a strong influence of the broader synoptic events identified earlier.

In this paper, we examine secular, seasonal and diurnal features of these events, highlighting:a strong decline in the number of heavy rain events at Hobart at the three-hourly time scale in the last several decades; the occurrence throughout the year of such events, but with a summer/autumn peak; and an early morning peak in synoptic time-scale events.

We show that even at the relatively broad scale of the reanalyses available, it is clear that many heavy rainfall events occur with pulses of tropical moisture into the Tasmanian region. More detailed individual moisture analyses (available for recent events) suggest that often these tropical incursions can be quite narrow bands of moisture associated with the synoptic troughs or lows moving through the Tasmanian region.