Large-scale dynamics, dry katabatic winds and snowfall microphysics: two contrasted precipitation events over Davis, East Antarctica

Abstract:

Precipitation in Antarctica is a crucial component of the ice sheet mass balance, which directly impacts sea-level rise. On the Antarctic coast, extratropical cyclones contribute to most of the precipitation accumulation. However, snowfall associated with these systems often sublimates in the dry katabatic layer present over the East Antarctic coast, leading to a decrease of up to 35% of the total snowfall accumulation. Davis station is located in one of the driest regions of Antarctica and experiences katabatics despite being sheltered by low-level hills from the plateau. Nevertheless, sublimation of snowfall has not yet been properly documented there. In this study, we analyse two contrasted snowfall events that took place in January 2019 in Davis station Antarctica, using data from a scanning X-band polarimetric radar, a W-band Doppler cloud profiler, a multi-angle snowflake camera (MASC) and a Raman lidar collected during the PLATO campaign. The first event took place on 06 January 2019. A local-scale circulation developed offshore Davis bringing a shallow precipitating system with cloud top at about 3800 m a.s.l. The katabatic layer was weak with wind speeds below 10 m/s. Partial sublimation can be observed as a slight decrease in reflectivity and increase in mean Doppler vertical velocity values, but most of the precipitation reached the ground. A hydrometeor classification based on the polarimetric variables shows the dominance of crystals, as well as aggregates below 2000 m a.s.l. (T > -16°C). After the passage of this shallow cloud, isolated precipitating cells containing rimed particles were observed, consistent with supercooled liquid water layers identified in the lidar data. The hydrometeor classification from the MASC shows a majority of aggregates followed by small particles and graupels, consistent with the radar-based classification. The second event took place between 08-09 January 2019. An extratropical cyclone advected moist unstable air from the Southern Ocean. The warm front reached Davis with cloud top at 8000 m a.s.l and reflectivity values up to 30 dBZ at X-band. Despite the intense reflectivity, most of the snowfall sublimated completely, due to a strong katabatic layer with wind speed up to 39 m/s and relative humidity as low as 60%. These two case studies suggest that the strength of the katabatic layer is more decisive than the amount of advected moisture: while shallow precipitating clouds can lead to significant precipitation amount, strong frontal snowfall can almost totally sublimate in the katabatic layer.