Numerical model simulations of boundary-layer dynamics during winter conditions

A mesoscale numerical model, incorporating a land-surface scheme based on D eardorffs' approach, is used to study the diurnal variation of the boundary layer structure and surface fluxes during four consecutive days with air t emperatures well below zero, snow covered ground and changing synoptic forc ing. Model results are evaluated against in-situ measurements performed dur ing the WINTEX field campaign held in Sodankyla, Northern Finland in March 1997. The results show that the land-surface parameterization employed in t he mesoscale model is not able to reproduce the magnitude of the daytime se nsible heat fluxes and especially the pronounced maximum observed in the af ternoon. Additional model simulations indicate that this drawback is to a l arge extent removed by the implementation of a shading factor in the origin al Deardorff scheme. The shading factor, as discussed in Gryning et al. (20 01), accounts for the fact that in areas with sparse vegetation and low sol ar angles, both typical for the nor-them boreal forests in wintertime, abso rption of direct solar radiation is due to an apparent vegetation cover whi ch is much greater than the actual one (defined as the portion of the groun d covered by vegetation projected vertically). Moreover, the observed asymm etry in the diurnal variation of the sensible heat flux indicates that ther e might be a significant heat storage in the vegetation. The implementation of an objective heat storage scheme in the mesoscale model explains part o f the observed diurnal variation of the sensible heat flux.