High-Resolution Simulations of a Heavy Precipitation Event Over the Australian Snowy Mountains: The Role of Microphysics

Precipitation over the Australian Alps constitutes an important water resource to southeastern Australia, supplying fresh water for agriculture, industry, and domestic use. The current precipitation estimates and operational forecast products available for this region have been shown to have notable biases, part of which is attributed to the poor representation of supercooled liquid water commonly present in this area.

In this talk, a heavy precipitation event associated with the passage of a cold front over the Snowy Mountains on 03 Aug 2018 is investigated using high-resolution Weather Research and Forecasting (WRF) simulations and various ground-based and satellite observational data. The European Centre for Medium-Range Weather Forecasts (ECMWF) operational model forecasts with ~9 km spatial resolution at surface level and 137 model-levels were used to provide the initial and boundary conditions for the 36 h WRF model simulations ensuring 12 h spin-up time. A triple nesting approach was applied to achieve 1 km spatial resolution for the innermost domain covering the Snowy Mountains and its neighbourhood (9 km-3 km -1 km). Observations from an intensive field campaign performed in the Snowy Mountains in austral winter 2018 were employed to evaluate numerical simulation results. The campaign included precipitation observations from 76 pluviometers and 50 weather stations distributed across the Snowy Mountains. Furthermore, observations of vertical profiles of air temperature and humidity, cloud and precipitation vertical structure and microphysical properties were available from radiosondes, three micro rain radars, a cloud radar, two microwave radiometers, and an optical disdrometer distributed among three sites. Sensitivity of the simulations to cloud microphysics parametrization has been investigated using advanced fully double-moment NSSL, Thompson aerosol aware, Morrison, and WDM7 schemes. As expected, the simulated orographic precipitation shows strong sensitivity to cloud microphysics. Most notably, the observed heavy precipitation over windward slopes of the Snowy Mountains is strongly underestimated in the Morrison and WDM7 schemes due to the shift of simulated maximum precipitation toward the mountain peaks. The latter, in turn, leads to an overestimation of observed precipitation amounts above 1600 m above sea-level. In order to better understand the underlying processes driving the simulation biases, a more comprehensive evaluation of the simulations is performed. In particular, the simulated cloud-top temperatures and cloud-top phases are evaluated against Himawari-8 satellite cloud products. High-frequency observed and simulated time-height cross-sections of radar reflectivity and various hydrometeor species (cloud water, rain, snow, ice, graupel) are also examined. Among others, the skill of the simulations in representing the supercooled liquid water clouds and precipitation is assessed. Additional inputs of specific cloud liquid water and ice water content fields from the ECMWF high-resolution forcing data is considered aiming at further improvement of the simulated cloud microphysical properties.