Fundamental Ingredients of Australian Rainfall Extremes

Satellite observations of Australian rainfall are used with reanalysis fields to identify the fundamental ingredients that distinguish extreme rain days from non-extreme rain days. We analyze regions from the tropics to the mid-latitudes, seeking to address whether regions with very different large-scale circulations have common factors required for extreme rainfall, and if so, what synoptic conditions lead to the development of the extremes. In all regions, extreme rain days occur in locally strong ascent, high moisture, and an atmosphere closer to saturation throughout the depth of the free troposphere than on weaker intensity rain days, although the synoptic processes leading to the development of these local conditions vary regionally. The concentration of moisture over several days preceding an extreme is important to the development of the extreme. We show that the local values of precipitable water and mid-level ascent can indicate whether a rain day will be extreme. The analysis uses 20 years of satellite-derived rain estimates covering the period 1998–2017, averaged daily and over 100 × 100 km² areas centered on the Australian cities of Darwin (tropical), Brisbane (subtropical), and Melbourne (mid-latitude). The daily mean rainfalls are ranked by intensity, and the top 100 days in each region are selected as extremes. The large-scale conditions of these extreme rain days are taken from reanalysis and compared against those of the median and low intensity rain days in each region.