Simulation of Late Summer Arctic Clouds during ASCOS with Polar WRF

Low-level clouds are extensive in the Arctic and contribute to inadequately-understood feedbacks within the changing regional climate. The simulation of low-level clouds, including mixed-phase clouds, over the Arctic Ocean during summer and autumn remains a challenge for both real-time weather forecasts and climate models. Here, improved cloud representations are sought with high-resolution mesoscale simulations of the August-September 2008 Arctic Summer Cloud Ocean Study (ASCOS) with the latest polar-optimized version (3.7.1) of the Weather Research and Forecasting model (Polar WRF) with the advanced two-moment Morrison microphysics scheme. Simulations across several synoptic regimes for 10 August - 3 September 2008 are performed with three domains including an outer domain at 27 km resolution and nested domains at 9 km and 3 km resolution. These are realistic horizontal resolutions for common mesoscale applications. The control simulation produces excessive cloud liquid water in low clouds resulting in a large deficit in modeled incident shortwave radiation at the surface. Incident longwave radiation is less sensitive. A change in the sea ice albedo toward the larger observed values during ASCOS resulted in somewhat more realistic simulations. More importantly, sensitivity tests show that a reduction in specified liquid cloud droplet number to very pristine conditions increases liquid precipitation and greatly reduces the excess in simulated low-level cloud liquid water and improves the simulated incident shortwave and longwave radiation at the surface.