Vertical Structure and Ice Production Processes of Shallow Convective Postfrontal Clouds over the Southern Ocean in MARCUS. Part I: Observational Study

A study of the vertical structure of postfrontal shallow clouds in the marine boundary layer over the Southern Ocean is presented. The central question of this two-part study regards cloud phase (liquid/ice) of precipitation, and the asso-ciated growth mechanisms. In this first part, data from the Measurements of Aerosols, Radiation, and Clouds over the South-ern Ocean (MARCUS) field campaign are analyzed, starting with a 75-h case with continuous sea surface-based thermal instability, modest surface heat fluxes, an open-cellular mesoscale organization, and very few ice nucleating particles (INPs). The clouds are mostly precipitating and shallow (tops mostly around 2 km above sea level), with weak up-and downdrafts, and with cloud-top temperatures generally around-18 degrees to-10 degrees C. The case study is extended to three other periods of post -frontal shallow clouds in MARCUS. While abundant supercooled liquid water is commonly present, an experimental cloud -phase algorithm classifies nearly two-thirds of clouds in the 0 degrees to-5 degrees C layer as containing ice (cloud ice, snow, or mixed phase), implying that much of the precipitation grows through cold-cloud processes. The best predictors of ice presence are cloud-top temperature, cloud depth, and INP concentration. Measures of convective activity and turbulence are found to be poor indicators of ice presence in the studied environment. The water-phase distribution in this cloud regime is explored through numerical simulations in Part II.