Downward sensible heat flux and decoupling over snow-covered surfaces

The turbulent flux of sensible heat depends on the wind speed, air-surface temperature difference, and surface roughness. If the air is warmer than the surface, increasing temperature difference first results in increasing downward flux, but the thermal stratification increases as well. Finally this starts to dominate and turbulent mixing will be reduced, which generates a positive feedback to the increasing temperature difference. Hence, decoupling may occur with a very large air-surface temperature difference. This is a common problem in numerical models, and it is usually dealt by preventing the surface heat exchange coefficient dropping to zero even in cases of very stable stratification. Analyses are presented on the air-surface temperature difference and the sensible heat flux on the basis of data from the Sodankylä observatory in northern Finland and the SHEBA experiment on the Arctic sea ice. In winter, both represent snow-covered surfaces but with a large difference in the surface roughness. In addition to mutual comparisons between the observation sites, the data are compared against theoretical calculations based on several stability functions utilized in numerical weather prediction and climate models. The relationship between the maximum downward heat flux on the Richardson number and Obukhov length is analyzed, as well as the dependence of this relationship on the wind speed and surface roughness.