Microphysical mechanisms of wintertime postfrontal precipitation enhancement over the Australian Snowy Mountains

Abstract

A heavy orographic precipitation event associated with the postfrontal sector of a mid-latitude cyclone over the Australian Snowy Mountains (ASM) was analysed using field observations and numerical simulations. This event, observed during a 2018 intensive field campaign, was of particular interest as three distinct precipitation episodes were identified within a prolonged postfrontal period. Deep mixed-phase clouds (MPCs) characterised by cold cloud-top temperatures (colder than -30°C) and the presence of updrafts extending 3.5–4.5 km above the boundary-layer height produced the three enhanced precipitation events over the windward slopes of the ASM. The presence of conditional instabilities and deep updrafts were also found in the sounding and Doppler velocity observations, respectively, while the cloud radar observations show the deep MPCs with cloud-tops reaching to 6–7 km above sea-level. Orographic convection invigoration (OCI) was found to be the main mechanism producing the precipitation enhancement over the windward slopes and higher terrain.