Dynamics and Predictability of a Springtime Precipitation Event in the Australian Alps

The prediction of orographic precipitation in regions of complex terrain remains challenging. However, modern weather prediction models are now able to resolve complex terrain features that often play a role in the initiation or enhancement of precipitation, which allows detailed examination of the dynamics of orographic precipitation events and their predictability. Precipitation in southeast Australia is strongly influenced by the Australian Alps, but the dynamics of precipitation events in the region has received relatively little attention. In this study the Weather Research and Forecasting (WRF) model is used to simulate a springtime precipitation event in the Australian Alps using an inner domain with 1.1 km grid spacing to investigate the role of mesoscale phenomena. The event was characterized by the passage of a relatively short-lived pre-frontal event whereby weak convection was triggered upstream and subsequently enhanced over the Alps. This was followed by a more prominent orographically enhanced frontal system.

The dynamics of the pre-frontal event are examined in detail; the timing and location of convection upstream is coincident with the passage of an undular bore propagating along the stable nocturnal layer. The undular bore was generated by a pre-frontal trough and triggers a convective event at the upstream extent of the mountains, and is subsequently advected toward and enhanced by the mountain range. To appreciate the predictability of this pre-frontal event, results from a time-lagged ensemble and a perturbed initial-condition ensemble are also presented. Additionally, these ensemble results provide insight into the sensitivity of alpine precipitation during the passage of a frontal event.