Parametrization of the effect of drizzle upon the droplet effective radius in stratocumulus clouds



A method is presented to parametrize the effects of drizzle upon the droplet effective radius in stratocumulus clouds. The cloud-droplet size distribution in stratocumulus is represented by the sum of a modified Gamma distribution to represent the small (radius <20 μm) droplets and an exponential Marsh all-Palmer-type distribution to represent the large (drizzle) droplets. Using this approach a relationship is derived to account for the effect of drizzle upon k, the cube of the ratio between the volume and effective radius. Observational evidence from flights in a range of different air-mass types is presented to validate the approach. The results suggest that the value of k pertaining to the small droplets is better parametrized as a function of volume radius rather than of droplet concentration. The results also suggest that, as the ratio of liquid-water content contained in the large droplets to that in the small droplets increases beyond 0.05, the value of it decreases significantly. This results in an underprediction of the effective radius if commonly used parametrizations for k are used.