Effect of surface heterogeneity on energy fluxes during the SHEBA winter

Overland et al. (2000) showed histograms of AVHRR-derived surface temperatures (Ts) over the Arctic pack ice during clear-sky wintertime conditions. These histograms were remarkable in that they showed a range of Ts of up to 19°C (-42° to -23°C) within a 100 X 100 km area centered on the Surface Heat Budget of the Arctic Ocean (SHEBA) site. This study also showed that surface sensible heat fluxes (Hs) aggregated over this area are 5-12 W m greater than those measured at the SHEBA site during these clear-sky conditions because of the areas of higher Ts. Questions motivated by the Overland et al. results include: 1) is this large Ts range over such a small area physically realistic? 2) if so, what is the spatial distribution of the warm and cold areas and what are the implications of this distribution? , 3) are the clear-sky Ts and Hs variations representative of the variations during cloudy conditions? , and 4) is there another method that can show the spatial distribution of Ts and Hs during cloudy conditions so long-term mean aggregate Ts and Hs values can be computed? We have developed a method to estimate the wintertime aggregate Ts and Hs under all sky conditions. The method combines synthetic aperture radar (SAR) data and a simple one-dimensional snow and ice model. The SAR data show the time of the formation of firstyear ice (FYI), the size of the FYI areas, and their subsequent movements. The 1-D model provides the evolution of the ice thickness, snow depth, Ts, and Hs for freezing leads. In addition to providing estimates of aggregate Ts and Hs, the technique can provide the spatial distribution of ice thickness and snow depth necessary for 3-D mesoscale simulations to explore other aggregation techniques and the flux contribution of mesoscale circulations resulting from the larger scales of surface heterogeneity. Ice that forms in leads during the 9-month freezing season is known as FYI, whereas ice that survives the summer melt season is defined as multi-year ice (MYI). The purpose of this paper is to illustrate the heterogeneity of the surface characteristics near the ____________________________________________ *Corresponding author address: P. Ola G. Persson, CIRES/NOAA/ESRL/PSD3, 325 Broadway, Boulder, CO 80305; e-mail: opersson@cires.colorado. edu; tel: 303-4975078; fax: 303-497-6181 SHEBA site, and to determine the impact of this heterogeneity on the local and aggregate surface sensible heat fluxes for all conditions. Sections 2 and 3 describe the 1-D model and the combination of it with the SAR data to form the SAR/1D method of determining the spatial distribution of surface characteristics. Hence, these sections provide the necessary background information. Sections 4 and 5 describe the aggregation methods, and quantify the effect of this heterogeneity on the aggregate surface sensible heat flux. Time series of aggregate values of Ts and Hs are computed using the SAR/1D technique and different assumptions regarding the Hs transfer coefficient. These aggregate values are compared with each other, the AVHRR clear-sky measurements, and with those from the long-term SHEBA site on multi-year ice (MYI) to assess which assumptions are best. These sections, along with the discussion and conclusions in section 6, present the key concepts.