Size-resolved measurements of cloud droplet deposition velocity to a forest canopy using an eddy correlation technique

Abstract

Two case studies are presented of the direct measurement of cloud droplet fluxes to a forest canopy by the eddy correlation technique. Two instruments have been used for the liquid water measurements, and close agreement for values of liquid water content and liquid water flux have been obtained. The case studies were conducted in different weather conditions: case 1 with high values of liquid water content (typically 0.3 g m−3) and low wind speeds (2–3 m s−1), and case 2 with low liquid water content (0.1 g m−3) and higher wind speeds (typically 6 m s−1). In both cases the deposition velocity for liquid water was markedly less than for momentum. For case study 2 the cause of the small deposition velocity for liquid water was the small droplet size; the typical modal radius was 4.4 μm. The deposition velocity increased as a function of droplet size, exceeding that for momentum for droplets larger than about 6 μm. When normalized, the results agreed reasonably well with those obtained over grassland using the gradient technique. The results are compared with the predictions of the models of both Lovett and Slinn.