An aircraft case study of the spatial transition from closed to open mesoscale cellular convection

Aircraft measurements are presented from the 27/28th October 2008 case study of the VOCALS Regional Experiment (REx) over the remote southeast Pacific (18S, 80W). Data from two aircraft that took measurements approximately twelve hours apart but in the same advected airmass are used to document a remarkably sharp spatial transition in marine boundary layer (MBL), cloud, and aerosol structure across the boundary between well-mixed, overcast closed mesoscale cellular marine stratocumulus and a pocket of open cells (POC) with significantly lower cloud cover. Long (~190-250 km) straight and level flight legs at three levels in the marine boundary layer and one level in the lower free troposphere permit sampling of the closed cells, the POC, and a 20-30 km wide transition zone between the two airmasses with distinctly different structure from that on either side. The POC region consists of intermittent active and strongly precipitating cumulus clouds rising and detraining into patches of drizzling but quiescent stratiform cloud which is optically thin especially toward its edges.

Mean cloud-base precipitation rates inside the POC are several mm/day, but rates in the closed cell region are not greatly lower than this, which suggests that precipitation is not a sufficient condition for POC formation from overcast stratocumulus. Despite similar cloud-base precipitation rates in the POC and closed cell region, much of the precipitation (90%) evaporates below cloud in the overcast, while there is significant surface precipitation inside the POC. In the POC and transition region clouds, although the majority of the condensate is in the form of drizzle, the clouds appear to be remarkably adiabatic.

The transition zone between the POC and the closed cells consists of thick "boundary cell" clouds producing mean surface precipitation rates of 10-20 mm/day, a divergent quasi-permanent cold/moist pool below cloud, a convergent inflow region in the middle of the MBL, and divergent outflow near the top of the MBL. There is no evidence to suggest that this boundary moves independently of the mean MBL flow.

The stratiform clouds in the POC exist within an ultra-clean layer that is some 200-300~m thick. Aerosol concentrations N_a in the diameter range 0.1-1 micron in the ultraclean layer are as low as 0.1-1 /cm3 in the center of the ultraclean layer. This suggests that coalescence scavenging by precipitation is extremely efficient, since N_a in the subcloud layer, and droplet concentration N_d in the active cumuli are typically 20-60 /cm3. The droplet concentrations in the quiescent stratiform clouds are extremely low (typically 1-10 /cm3), and most of their liquid water is in the form of drizzle, which mainly evaporates before reaching the surface. The cloud droplet concentration in the overcast region decreases strongly as the transition region is approached, consistent with detrainment of coalescence-scavenged cloud liquid water from the boundary cell. Elevated non-refractory total aerosol concentrations are observed inside the POC at all levels around 20-50 km from the transition zone, perhaps associated with some prior nucleation event.