Drizzle and Cloudiness Transitions in Southeast Pacific Marine Stratocumulus

The representation of boundary layer clouds in global climate models (GCMs) has been identified as a leading cause driving uncertainties in future climate change scenarios. Low cloud properties are highly variable in space and time, with the climatically important shortwave cloud forcing differing widely between cloudy and clear regions. Mostly visibly notable in marine boundary layer systems are pockets of open cells (POCs), regions of low cloud fraction, high precipitation rate, and low aerosol concentration, most commonly embedded in extensive regions of solid stratocumulus. The casual mechanisms explaining the evolution from solid cloud to POC, however, are not well understood. Specifically of interest are the transition regions between solid and broken cloud, since these are thought to be the focal point for driving mesoscale variability.

The Variability of the American Monsoon Systems [VAMOS] Ocean-Cloud-Atmosphere-Land Study — Regional Experiment (VOCALS-Rex) field campaign took place over the southeast Pacific (SEP) region during Oct-Nov 2008. The proposed research takes advantage of the fact that the C-band radar on board the NOAA R/V Ronald H. Brown was the sole observational platform in VOCALS able to sample the near-instantaneous three-dimensional mesoscale structure of the precipitation field, while also capturing evolution of individual cells over their lifecycles. The VOCALS radar observations include an unexpected discovery: the frequent occurrence of regions of very high reflectivity (>40 dBZ), untruecedented for boundary layer clouds. The VOCALS data, along with preliminary simulations, suggests the following overarching question: Fundamentally, what drives cloud system variability in marine boundary layer clouds?

The proposed research seeks to evaluate, in a numerical modeling framework, various hypotheses related to boundary layer drizzle processes and cloud variability over the SEP. These hypotheses are formulated to address the following questions directly related to SEP cloud systems: i. What are the leading factors in establishing drizzle and mesoscale cloud variability?; ii. What are the predominant mechanisms active at the transition between solid stratocumulus and POCs?; iii. What are the dynamic and microphysical processes associated with cloud field evolution from unbroken stratocumulus to POC and back to solid cloud? The study proposes a “near-LES” simulation framework, combined with observational constraint on the model.

This proposal direction addresses the CPPA goal to “improve understanding and process modeling of cloud, planetary boundary layer, and microphysics…,” with the VOCALS field campaign being specifically mentioned in the original call. Furthermore, this proposal would leverage undergoing CPPA-funded work, particularly the ship-based observational efforts.