Importance of vertical velocity variations in the cloud droplet nucleation process of marine stratus clouds

Eleven cloud cases through marine stratus, obtained during two field experiments in the North Atlantic Ocean, are used to study the sensitivity of cloud droplet nucleation to the vertical gust velocity. Selected cloud microphysical data, size-distributed aerosol properties and particle chemistry are applied in an adiabatic parcel model. The nucleated cloud droplet number concentrations (N) predicted using the probability density function (PDF) of the measured in-cloud vertical velocities are compared to predictions using a characteristic velocity value. In this study, the model-predicted N from the PDF of the measured in-cloud vertical velocities agrees with the observed maximum N (N-max) to within 8.6%. The average N (N-avg) can be related to N-max using a power law (Leaitch et al., 1996). If a relationship between N-max and N-avg based on the measurements is applied to obtain the average N from the model-predicted N, then the model-predicted average N agrees with the observed average N to within 13.3%. When a characteristic vertical velocity (0.8 times the standard deviation of the vertical velocity distribution in this study) is used in the parcel model to simulate N, the model-predicted N agrees with the observed maximum N to within 5.7% and the model-predicted average N agrees with the observed average N within 8.8%. This indicates that using a characteristic value of the vertical velocity distribution instead of its PDF is a good approximation for simulating the nucleated cloud droplet number of marine stratus on a cloud scale.