Case Studies of Shallow Convection and Precipitation over the Southern Ocean during the CAPRICORN field campaign

Persistent biases in the energy budget over the Southern Ocean (SO) within climate simulations and reanalysis products have been linked to the poor representation of the unique clouds over the region, particularly in regions of shallow, post-frontal convection. Satellite observations suggest that this region is fundamentally different than over comparable oceans in the Northern Hemisphere, with supercooled liquid water being observed more frequently and at colder temperatures.

In response to these challenges, a primary aim of the 2016 CAPRICORN (Clouds, Aerosols, Precipitation, Radiation, and atmospherIc Composition Over the southeRn oceaN) field campaign has been to characterize the cloud, aerosol, and precipitation properties and boundary layer structure over the remote SO. To meet this primary aim, the Australian R/V Investigator undertook a 35-day cruise (March-April) in 2016 to make observations across the lower latitudes of the Southern Ocean from Hobart (~43°S) to near the Southern Ocean polar front (~53°S) during which time nine frontal passages were encountered, as well as a warm and a cold ocean eddy. The ship was instrumented with a vertically pointing cloud radar, a lidar and microwave rain radar, a 2-channel microwave radiometer and a disdrometer. In addition to these surface observations, regular radiosondes were launched throughout the campaign.

Two cases are examined in this study with a focus on shallow convective clouds that were commonly observed during the cruise. Shipborne measurements, Himawari-8 satellite observations, and high-resolution numerical simulations using the Weather Research and Forecasting (WRF) model are integrated to investigate the dynamical and microphysical characteristics of the targeted cloud fields. In the first case (21 – 23 March), a rapid succession of two fronts were encountered, separating fields of shallow convective warm clouds. Light precipitation (~2 mm) originating from the prefrontal shallow convection (cloud-top height below 1 km) was recorded by the ship. This precipitation is underrepresented in the simulations, which is linked to a deficit of the low-cloud cover. The second case (26 – 28 March) focusses on a sustained period of open mesoscale cellular convection in a post-frontal environment. Different from the first case, the observed cloud field resided primarily below 2.5 km and in the sub-freezing temperature range (0 to -8°C), where mixed phase cloud tops was suggested by both the shipborne radar-lidar and the Himawari-8 retrieval products. Relatively heavy precipitation (18 mm over 24 hours) was observed to be generated from these clouds. Despite the relatively good representation of some surface meteorological variables (e.g. surface temperature and humidity), WRF simulations have difficulties in producing both the low-level cloud field, mixed-phase cloud tops, and surface precipitation. An evaluation of the thermodynamic profiles using radiosonde soundings suggests that the model does not simulate the growth of the boundary layer depth prior to the onset of precipitation.

Sensitivity experiments with different physical parameterization schemes are performed to investigate the impact of boundary layer and microphysical processes on the simulations of the shallow convective clouds. Possible causes of the model deficiencies and possible pathways for model improvements will be discussed.