PNNL

Abstract

A study of the vertical structure of post-frontal 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 associated growth mechanisms. In this first part, data from the Measurements of Aerosols, Radiation, and Clouds over the Southern Ocean (MARCUS) field campaign are analyzed, starting with a 75-hour 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 °C to –10 °C. The case study is extended to three other periods of post-frontal shallow clouds in MARCUS. An experimental cloud phase algorithm classifies xx% of clouds in the 0 to -5?C layer as rain (snow), implying that much of the precipitation grows through warm rain 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.