Observation and Modelling of the Thermal Boundary Layer over Snow and soil Patches

The melting of the snow cover, in most cases, is characterized by a transition from complete snow cover, to the appearance of bare soil patches, to the increase in size of the bare patches, and the decrease in size and subsequent disappearance of the snow patches. Thus, for a significant portion of the snowmelt period, the local advection of energy plays an important role, and the correct determination of the melting of a patchy snow cover requires an understanding of this advection process. The energy advected from the exposed bare soil to the snow surface depends on the fetch and size of snow and bare ground patches. Lightweight, portable instrument masts were designed and constructed for the purpose of measuring the growth of the thermal boundary layer over snow patches and bare soil patches within a snow field. The results of a field study are presented. The thermal boundary layer height is shown to be a power function of the fetch distance; the coefficient and exponent are related to the upwind surface roughness. A simple boundary-layer model is developed for calculating the advection of energy from bare ground to melting snow patches. Results from the model are compared with observations of temperature profiles and boundary-layer heights. The model is then used to investigate advection for a wider range of atmospheric stability and surface roughness.