An evaluation of the precipitation forecasts of the Poor Man's Ensemble for wintertime rainfall across the southern portion of Australia

Abstract

The Poor Man's Ensemble (PME) forecasts of daily rainfall have been investigated for the eight-year period of 2001-2008 with a focus on wintertime precipitation (May-October) across the southern portion of Australia (south of 30°S). Such rainfall is commonly the product of storm systems that come from the Southern Ocean (SO). The skill of the forecasts is assessed against the surface precipitation observations of the Australian Water Availability Project (AWAP). Moving beyond the basic scores such as the Frequency Bias and the Equitable Threat Score, as well as the Symmetric Extremal Dependency Index, the aim was to explore the sensitivity of the forecasts to geographic factors, specifically orographic enhancement and coastal boundaries. Eight different 1° × 1° grid boxes were chosen across the southern portion of Australia including three in Tasmania. The sensitivity to forecast length was also examined. The PME precipitation product shows considerable skill but is found to fare relatively poorly in the mountain and coastal regions. The PME precipitation for western Tasmania, which can be classified as both orographic and coastal, is roughly half of what is observed. The rainfall was broken down into light (≥1 mm per day) and heavy (≥10 mm per day) days. An analysis of the frequency bias score and equitable threat score finds the PME has particular trouble with the heavy precipitation days.