Characteristics and Variability of Precipitation Across Different Sectors of an Extra-Tropical Cyclone: A Case Study Over the High-Latitudes of the Southern Ocean

Shipborne observations from the CAPRICORN-2018 field campaign were used to investigate the characteristics and variability of precipitation across different sectors of an extra-tropical cyclone on 16 February 2018, over the Southern Ocean (SO). Three distinct time periods—frontal, post-frontal, and cyclone—were identified during the day. The frontal passage recorded a total accumulation of 1.9 mm, where the precipitation phases were primarily composed of rain (96%), while the cyclone period recorded the largest precipitation (4.0 mm), where the precipitation phases varied with snow (10%), mixed-phase (40%), and rain (50%). The BASTA radar suggests the freezing level was shallow (∼500 m) with snow present above. The cloud top heights, observed by a C-band radar, were shallower in the cyclone period, although deeper cloud depths of ∼6 km were sporadically recorded. Increased surface fluxes and a southerly wind direction indicate that cold air advection, was likely the main cause of high precipitation during the cyclone period. A non-precipitating multi-layer cloud structure with a geometrically thin (200 m) homogeneous layer of supercooled liquid water (SLW) overlaying shallow boundary layer convection was seen during the post-frontal period. The ship-borne observations were used to evaluate Weather Research & Forecasting (WRF) simulations with different microphysics settings. We found the frontal precipitation intensity is well reproduced, but it is underestimated during the cyclone period. This study represents a unique set of observations and highlights the need for understanding how ice processes and potentially horizontal advection contribute to the development of precipitation and convection over the SO.