The effect of solar radiation on occult deposition over hills

A model of the turbulent deposition of cloud droplets to the surface from cap clouds enveloping a ridge has been extended to include the effects of the evaporation of deposited cloud water on the surface of vegetation. It is shown that evaporation driven by solar radiation penetrating the cloud increases the concentration of dissolved species on the leaf surfaces. The concentration effect tends to be a maximum close to cloud base both upwind and downwind of the hill, where deposition rates are low and the optical depth of the cloud is large. In a supercritical flow regime, however, deposition rates are very large on the lee side of the hill where winds are strong. In this region the cloud also has its smallest vertical depth, increasing the radiation density at the ground. It is this region, just to the lee of the summit, where phytotoxic effects are likely to be a maximum. -Authors