On the simulation of regional scale sublimation over boreal and agricultural landscapes in a climate model

Abstract

In this study, a detailed evaluation of regional scale sublimation over two contrasting surface types is performed, based on observations and simulations from a regional climate model. New observed estimates of regional scale evaporative fractions (sublimation/precipitation) based on remotely sensed and gridded surface climate data suggest values of 0.16 and 0.09 respectively for boreal forest and agricultural regions during midwinter. While not dominating the water balance, this does indicate that sublimation is a significant component at the regional scale. On the other hand, simulated results were substantially less: 0.02 and ‐0.02 (i.e., net vapour deposition) respectively. Further analysis revealed that in both cases a net vapour deposition onto the ground snowpack was found, though in the forested case this was offset by a positive vapour flux off the canopy yielding a small net positive flux overall. Even over the agricultural region, sublimation was simulated during daytime in January, but this was overwhelmed by night‐time frost, resulting in a net daily vapour deposition. The source of the surface vapour flux bias was further investigated. An attempt was made to enhance sublimation by increasing the sensible heat flux based on the “windless exchange” concept, which has been found helpful in studies of stand‐alone snowpack models. However, in the coupled atmosphere‐land surface model used here, increases in sensible heat flux into the snow were largely offset by increases in upward surface longwave flux: the net impact on sublimation and surface temperature bias was small. A proposed cold snow‐vapour deposition feedback mechanism, under clear sky conditions, was determined to be unlikely for the simulation under consideration. The impact of cloud cover appears to be much more relevant to our simulation of the surface energy balance over snow. Satellite and limited surface radiation observations suggest that insufficient cloud radiative forcing simulated in the model was responsible for the excessive night‐time frost.