Low-Cloud, Boundary Layer, and Sea Ice Interactions over the Southern Ocean during Winter

Abstract

During austral winter, a sharp contrast in low-cloud fraction and boundary layer structure across the Antarctic sea ice edge is seen in ship-based measurements and in active satellite retrievals from Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations ( CALIPSO), which provide an unprecedented view of polar clouds during winter. Sea ice inhibits heat and moisture transport from the ocean to the atmosphere, and, as a result, the boundary layer is cold, stable, and clear over sea ice and warm, moist, well mixed, and cloudy over open water. The mean low-cloud fraction observed by CALIPSO is roughly 0.7 over open water and 0.4–0.5 over sea ice, and the low-cloud layer is deeper over open water. Low-level winds in excess of 10 m s−1 are common over sea ice. Cold advection off of the sea ice pack causes enhanced low-cloud fraction over open water, and thus an enhanced longwave cloud radiative effect at the surface. Quantitative estimates of the surface longwave cloud radiative effect contributed by low clouds are presented. Finally, 10 state-of-the-art global climate models with satellite simulators are compared to observations. Near the sea ice edge, 7 out of 10 models simulate cloudier conditions over open water than over sea ice. Most models also underestimate low-cloud fraction both over sea ice and over open water.