Measurements of the size dependence of cloud droplet deposition at a hill site.

Abstract

Measurements have been made of the deposition of cloud droplets and dissolved chemical species to moorland vegetation. Measurements of liquid water fluxes have been made by the gradient technique using Knollenberg FSS probes and by a weighing lysimeter. Deposition velocities for cloud liquid water in the range 0.1 to 0.24 g m -3 at a height of 1.0m were close to those for momentum. The FSS probes enabled the determination of deposition velocities as a function of droplet size. It was found that the deposition velocity increased with droplet radius from 2.5 m to 6.5 m radius. Between 6.5 and 12.5 m radius, deposition velocities exceeded those for momentum, but decreased sharply between 12.5 and 15.5 m. In periods of thin cloud (0.1 g m -3), evaporation from the surface occurred despite continued depositio