Drizzle rates and large sea-salt nuclei in small cumulus

The role of large sea-salt condensation nuclei generated by wind blowing over the ocean surface is evaluated by applying a Lagrangian parcel model to a range of conditions based on observations made during NCAR research flight RF12 of the Rain in Cumulus over the Ocean (RICO) trade wind cumulus (Cu) study in the Caribbean near Antigua. The model utilizes droplet condensation growth, a simplified droplet sedimentation scheme, and quasi-stochastic coalescence to calculate drizzle rates 1100 m above Cu base. The calculations are repeated without the sea-salt solution droplets to permit calculation of a drizzle rate enhancement factor (Df) owing to the large nuclei. The model predicts a small effect of the large nuclei on the RF12 drizzle rate, as well as suggesting the same for other RICO flights in agreement with radar studies of the same Cu that also show at most a small effect on precipitation due to the large nuclei. These findings are contrary to those some other studies of the Cu. The present study agrees with several previous studies that large nuclei affect the drizzle rate for wind speeds greater than about 10 m s(-1), that the rate increases as wind speed increases, and that the rate increases as droplet concentration becomes larger at constant wind speed. Df values are fit with an analytical expression relating drizzle rate with wind speed and in-cloud droplet concentration.