Thermodynamic and aerosol controls on eastern Pacific stratocumulus precipitation processes in VOCALS

The representation of low marine clouds in global climate models is recognized as a leading uncertainty in simulations of future climate scenarios. The VOCALS Regional Experiment (VOCALS–Rex), which took place from 6 October to 2 December 2008, was driven by the need to document the complicated land–atmosphere–ocean interactions over the southeast Pacific (SEP) region off the coast of Peru and Chile. The NOAA ship Ronald H. Brown (RHB) obtained a variety of remote sensing and in situ data sets of aerosol, cloud, and precipitation properties during 35 days in the SEP. Included in the suite of instruments was a scanning C-band radar that observed the mesoscale structure of precipitation within 60 km of the ship. The C-band radar observed over 400 hours of drizzle > 0 dBZ, including 7 strong events containing two or more cells with reflectivity > 20 dBZ. Analysis of soundings launched from the RHB reveals that these cases of strong precipitation were associated with boundary layers that were both moist and deep (1.4–2 km in depth). In contrast, there was no clear association between the occurrence of high reflectivities and aerosol concentration.

In order to explore the relative importance of boundary layer depth and aerosol concentration on the precipitation process, we conducted a series of simulations in a “near–LES” framework that is able to capture mesoscale variability while also resolving the salient aspects of boundary layer dynamics. A case of unusually strong convection observed in VOCALS serves as the basis for the control simulation. This case is contrasted with sensitivity simulations where we reduce boundary layer depth and increase CCN concentration. For this specific case of strong convection, results suggest that precipitation is to a first order thermodynamically controlled via processes that determine boundary layer depth, moisture, and stratification. The decrease in drizzle associated with a modest reduction in boundary layer depth (200 m reduction from 1650 m to 1450 m) is on par with a doubling of cloud condensation nuclei concentration (from 135 cm^-3 to 270 cm^-3). Quantifying the first-order controls on precipitation is important for ultimately understanding aerosol–cloud–precipitation interactions over the SEP.