The Arctic Snow and Air Temperature Budget Over Sea Ice During Winter

Abstract

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° to −35°C, a broad temperature maximum layer of −20° to −25°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°N show polar air mass characteristics by early November, but numerous storms interrupted this air mass during December. Snow temperature changes of 15°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.