Using dynamical downscaling to simulate rainfall for East Coast Low events

Abstract

East Coast Lows (ECLs) are intense low-pressure systems which occur on average several times each year off the eastern coast of Australia, in particular southern Queensland, New South Wales (NSW) and eastern Victoria. East Coast Lows will often intensify rapidly overnight making them one of the more dangerous weather systems to affect and damage the eastern coast of Australia each year. They are also a major source of water for the reservoirs serving coastal communities and ECL events were identified as being responsible for most of the high inflow days in the NSW coastal catchments. In this study, four East Coast Low events, covering the various ECL categories, are simulated using the Weather Research and Forecasting (WRF) model in order to assess the model's ability to capture the ECL events. By choosing various combinations of two Planetary Boundary Layer (PBL) schemes, two Cumulus (Cu) schemes, three microphysics (mp) schemes and three radiation (ra) schemes, a 36 member multi-physics ensemble is created and evaluated against observations. The uncertainties among the 36 ensemble members are analysed, rainfall probability assessment undertaken and the best estimation of the simulated rainfall are investigated. The results suggest that the Betts-Miller-Janjic cumulus scheme and the Yonsei University planetary boundary layer scheme can be chosen with some robustness to simulate rainfall pattern and the intensity and position of rainfall centres. Results further indicate that the selection of the cumulus scheme options has the largest impact on model performance during the more intensive rainfall events. The conclusions from the study can be applied to better estimate rainfall amount and its distribution from dynamical downscaling of climate change projections.