Sea ice treatment in the Arctic System Reanalysis

Polar WRF, which is employed for the Arctic System Reanalysis (ASR), uses specified sea ice concentration. Beginning with version 3.1.1, it can ingest specified variable sea ice thickness. Version 3.2 can ingest sea ice albedo and snow cover over sea ice. Unfortunately, gridded sea ice thickness histories have generally not been available until very recently. Through the use of satellite data and drifting buoys, however, the formation, movement, and disappearance of sea ice can be observed, enabling the estimation of sea ice age and thickness. Platforms employed include daily satellite images acquired by the Scanning Multichannel Microwave Radiometer (SMMR), the Special Sensor Microwave/Imager (SSM/I), and the series of Advanced Very High Resolution Radiometer (AVHRR) sensors. For more recent years, high resolution Advanced Microwave Scanning Radiometer for EOS (AMSR-E) retrevals are employed. For the ASR time period (2000-2010), values of sea ice thickness at 25-km resolution are obtained for January 2000 to June 2002. Sea ice thickness at 6.25-km resolution is obtained beginning in June 2002. These values are used for the sea ice representation in the ASR. Additional values are also obtained for snow cover on sea ice and sea ice albedo in the Arctic. Sea ice albedo is based upon a seasonal Arctic trend. Prior to the onset of snow melt over sea ice, the sea ice albedo is set at 0.82. Onset of snow melt over sea ice is determined for 16 sectors of longitude each year from passive microwave through a dataset supplied by Mark Anderson of the University of Nebraska-Lincoln. During a 30-day snow melt period, sea ice albedo decreases linearly to 0.5. During July, melt ponds are presumed to grow deeper and darker until they are represented as "open water". The albedo of the remaining bare ice is set at 0.65 for late July and early August. Between August 15th and September 4th, freeze-up occurs and the sea ice albedo increases to 0.82. For snow depth on sea ice, monthly values are based upon 1954-1991 observations at Russian sea ice stations over the Arctic Ocean, along with additional constraints.