Regional sensible and radiative heat flux estimates for the winter Arctic during the Surface Heat Budget of the Arctic Ocean (SHEBA) experiment

We estimate the influence of the mosaic distribution of surface temperatures of sea ice on regional surface sensible and radiative heat fluxes on the basis of advanced very high resolution radiometer (AVHRR) temperatures. The AVHRR data were used to derive similar to 1 km(2) surface temperature values for 100 x 100 km(2) regions. Regional flux estimates are compared to direct flux measurements taken during the Surface Heat Budget of the Arctic Ocean (SHEBA) experiment in the Beaufort Sea. We describe 48 cases of clear-sky conditions between December 1997 and February 1998. The distribution of surface temperatures within each region is skewed: most temperatures fall near the value observed at the ice camp, but the distribution has a warm tail corresponding to subregions with thinner ice. Sensible heat fluxes at the ice camp were downward, from the air to the ice; Although the camp was representative of the majority of the region, the upward flux from thin ice generally balanced the downward flux to thicker ice, suggesting a regional equilibrium of near-zero sensible heat flux. This was true whether the net downward flux at the camp was large or small or whether the ice was less compact, as in January, or more compact, as in December and February. The only exception to this generalization occurred when the ice was under strong compression. The net regional radiative loss from the ice surface was similar to 22% greater than that measured on the flee at the SHEBA camp. This increase is due to the fourth-power temperature dependence of the radiative flux acting on the skewed distribution of surface temperatures. The implications for modeling are that sea ice tends toward small net surface sensible heat flux on an aggregate scale, an emergent property of the system, and that parameterizing the regional mosaic of surface temperatures to resolve correctly the radiative balance in winter is important.