Measurement and modelling of cloudwater deposition to a snow-covered forest canopy

Measurements are presented of the flux of cloud droplets, as a function of particle size to a forest canopy (Sitka spruce) before and after a light snowfall. The results are compared to the predivtions of the model of Slinn. Good agreement is found provided that appropriate values for the effective target diameter are chosen, both before and after the snowfall. It is shown that the snowfall results in a considerable reduction in the flux of cloud water to the forest canopy (by a factor of about 2). The snow reduces the surface roughness of the canopy but the most important effect is that it increases the effective target diameter for the droplets impacting on the tree. This effect may considerably reduce the deposition of phytotoxic chemical species to forests at high altitude where snow cover and low cloud are common in winter.