THE ARCTIC SNOW AND AIR-TEMPERATURE BUDGET OVER SEA ICE DURING WINTER

Arctic cooling through the fall-winter transition is calculated from a coupled atmosphere-sea ice thermal model and compared to temperature soundings and surface measurements made north of Svalbard during the Coordinated Eastern Arctic Experiment (CEAREX). A typical winter, clear-sky vertical temperature structure of the polar air mass is composed of a surface-based temperature inversion or an inversion above a very shallow (30-180 m) mechanically mixed boundary layer with temperatures -30-degrees to -35-degrees-C, a broad temperature maximum layer of -20-degrees to -25-degrees-C between 0.5 and 2 km, and a negative lapse rate aloft. Because the emissivity of the temperature maximum layer is less than that of the snow surface, radiative equilibrium maintains this low level temperature inversion structure. A 90-day simulation shows that heat flux through the ice is insufficient to maintain a local thermal equilibrium. Northward temperature advection by transient storms is required to balance outward longwave radiation to space. Leads and thin ice (< 0.8 m) contribute 12% to the winter tropospheric heat balance in the central Arctic. CEAREX temperature soundings and longwave radiation data taken near 81-degrees-N show polar air mass characteristics by early November, but numerous storms interrupted this air mass during December. Snow temperature changes of 15-degrees-C occurred in response to changes in downward atmospheric longwave radiation of 90 W m-2 between cloud and clear sky. We propose that the strength of boundary layer stability, and thus the degree of air-ice momentum coupling, is driven by the magnitude of the radiation deficit (downward-outward longwave) at the surface and the potential temperature of the temperature maximum layer. This concept is of potential benefit in prescribing atmospheric forcing for sea ice models because a surface air temperature-snow temperature difference field is difficult to obtain and it may be possible to obtain a radiation deficit field via satellite sensors.