Drizzle Rates and Giant Sea-Salt Nuclei in Small Cumulus

A modeling study is performed to estimate the effect of giant sea-salt condensation nuclei on the drizzle precipitation rate in small cumulus clouds. Quasi-stochastic droplet coalescence calculations are applied to a parcel model to calculate the precipitation rate 1000-m above cumulus base. Initial conditions for the model use a large number of droplet and sea-salt nuclei size spectra and concentrations varied about values constrained by measurements made in RICO (Rain in Cumulus Over the Ocean) trade-wind cumuli The effect of the sea-salt nuclei is found by ratioing the precipitation rates with and without the sea salt nuclei, resulting in a precipitation enhancement factor.

We find, in agreement with some earlier work, that wind-generated giant sea-salt nuclei have a negligible effect on producing precipitation in most RICO trade-wind cumuli. However, the modeling shows that the enhancement factor is a continuous function of droplet concentration and near sea-surface wind speed. Several RICO flights with the highest wind speed show a modest increase in the enhancement factor. The factor increases for large values of droplet concentration and large values of wind speed. This suggests that sea-salt nuclei, produced by a given wind speed, have their largest affect on precipitation in cloudy air with higher concentrations of other condensation nuclei, such as, for example, in continental air blowing off or maritime air blowing onto land areas. These modeling results permit parameterization of the precipitation enhancement factor in the form of an analytical expression that depends only on cloud droplet concentration and Beaufort wind scale over the ocean. (GSI effort supported by NSF Grant ATM-0630671)