A Study on the Low-elevation Clouds over the Southern Ocean with A-Train Observation and WRF Simulations

The thermodynamic phase of the prevailing low-altitude clouds (tops below 3km) has a strong effect upon the radiative budget over the Southern Ocean (Mace et al. 2010), which has been found to be poorly represented in both state-of-the-art reanalysis and coupled global climate models” (Trenberth and Fasullo, 2010).

A 4-yr climatology of the thermodynamic phase of the clouds over the Southern Ocean (40-65°S,100-160°E) has been constructed with the A-Train merged product DARDAR-MASK for winter and summer. Low-elevation clouds (tops below 1km) with a weak seasonal cycle dominate this climatology. Such clouds predominantly reside in the temperature range from freezing to -20°C and are difficult to observe as a result of the physical limitation of the Cloud Profiling Radar (CPR) at this altitude and the lidar signal of CALIPSO commonly being attenuated. A cloud-top phase climatology comparison between CALIPSO, DARDAR-MASK and MODIS highlights the extensive presence of supercooled liquid water over the SO, particularly during summer. Glaciated and mixed phase cloud tops are appreciably recorded by DARDAR-MASK especially during winter.

A-Train observations have also been used to evaluate the Weather Research and Forecasting (WRFV3.3.1) simulations over Southern Ocean, which is a pristine environment essentially free of anthropogenic aerosol emissions. These simulations show skill in capturing the overall structure and phase composition of the convective clouds associated with frontal passages, although they are sensitive to the microphysics scheme and boundary layer scheme employed. The simulations, however, have greater difficulty capturing the wide spread low-elevation clouds that are not immediately associated with the frontal passage, even at high resolution.