Observations and kinematic modeling of drizzling marine stratocumulus

This paper presents observations and simple numerical modeling of drizzle processes in subtropical marine stratocumulus. Flight-long statistics from DYCOMS-II flights are derived and examples of small-scale cloud and drizzle observations are presented. The small-scale observations highlight the complexity of the natural system and the deficiency of an approach where single values of relevant parameters are used to describe the entire cloud system. Together with previously published analysis of ACE-2 flights, DYCOMS-II statistics are compared to the output of a kinematic (prescribed flow) model of drizzle processes in stratocumulus. Such a model has previously been used to validate simplified approaches to cloud microphysics, and it can be used in the future as a "superparameterization" of microphysical processes within stratocumulus-topped marine boundary layer. The focus is on separating the impact of the cloud depth from the impact of the cloud droplet concentration, with deeper and lower-droplet-concentration clouds both leading to more drizzle. The kinematic model reduces the stratocumulus-topped marine boundary layer dynamics to a single circulation cell that spans the entire boundary-layer depth. The model produces significantly less drizzle than estimated from the observations, and possible reasons for such a discrepancy are discussed. However, the model does reproduce observationally-based relationships between drizzle and such cloud parameters as cloud depth, droplet concentration, or maximum droplet size near the cloud top. To quantify the observed reductions of the cloud water and droplet concentration, a twin experiment approach is used, with and without collision/coalescence. The results show that observed reduction of cloud water content and droplet concentration may result from scavenging of cloud droplets by drizzle drops, in addition to processes by design excluded from the model, such as entrainment. (C) 2011 Elsevier B.V. All rights reserved.