Forecasting Northern Australian Summer Rainfall Bursts Using a Seasonal Prediction System

Forecasting Northern Australian Summer Rainfall Bursts Using a Seasonal Prediction System

Rainfall bursts are relatively short-lived events that typically occur over consecutive days, up to a week. Northern Australian industries like sugar farming and beef are highly sensitive to burst activity, yet little is known about the multi-week prediction of bursts. This study evaluates summer (December to March) bursts over northern Australia in observations and multi-week hindcasts from the Bureau of Meteorology’s multi-week to seasonal system, ACCESS-S1 (Australian Community Climate and Earth-System Simulator, Seasonal version 1). The main objective is to test ACCESS-S1’s skill to confidently predict tropical burst activity, defined as rainfall accumulation exceeding a threshold amount over three days, for the purpose of producing a practical, user-friendly burst forecast product. The ensemble hindcasts, made up of 11 members for the period 1990–2012, display good predictive skill out to lead week 2 in the far northern regions, despite overestimating the total number of summer burst days and the proportion of total summer rainfall from bursts. Coinciding with a predicted strong Madden-Julian Oscillation (MJO), the skill in burst event prediction can be extended out to four weeks over the far northern coast in December, however this improvement is not apparent in other months or over the far northeast, which shows generally better forecast skill with a predicted weak MJO. The ability of ACCESS-S1 to skillfully forecast bursts out to 2-3 weeks suggests the Bureau's recent prototype development of a Burst Potential forecast product would be of great interest to northern Australia’s livestock and crop producers, who rely on accurate multi-week rainfall forecasts for managing business decisions.