Case studies of orographic precipitation in the Brindabella Ranges: model evaluation and prospects for cloud seeding

Abstract

Recent confirmatory results in cloud seeding trials in the Australian Snowy Mountains have generated interest in performing similar experiments elsewhere. The Brindabella Ranges, which form the western border and catchment watershed of the Australian Capital Territory, share similarities in both climate and topography to the Snowy Mountains, so there is some prospect of conducting cloud seeding operations there. This paper presents an analysis of observations and high-resolution weather research and forecasting (WRF) model simulations of two wintertime storms from 2008 with the purpose of (1) evaluating the performance of the WRF model in simulating wintertime storms in the Australian alpine environment, and (2) investigating the nature of these storms from the perspective of cloud seeding research. The WRF model results compare favourably with much of the meteorological data used. There was a tendency to simulate too much moisture in the lower levels, and as a result the simulated low-level cloud coverage was somewhat more extensive than observed. Precipitation amounts were generally well represented, but the paucity of the observational network in the Brindabella Ranges made a comprehensive evaluation impossible. Cloud liquid water path (LWP), observed with a mountain top dual-channel microwave radiometer, was surprisingly well represented by the WRF model, especially in the post-frontal conditions. Radar reflectivity in the analysis region showed significant differences upwind and downwind of the Brindabella Ranges, suggesting that the mountains played an important role in the modulation of precipitation structures. Both of the case study storms were characterised by extended periods with appreciable quantities of supercooled liquid water, which is central to the glaciogenic cloud seeding hypothesis. However, further research would need to be conducted to determine whether such conditions occur frequently enough to permit cloud seeding operations, and whether it would be feasible to target the catchment regions with seeding material.