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

Abstract

Abstract. Aircraft measurements are presented from the 27/28 October 2008 case study of the VOCALS Regional Experiment (REx) over the remote subtropical southeast Pacific (18° S, 80° W). 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 a well-mixed MBL containing overcast closed mesoscale cellular 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 with distinctly different structure from the two airmasses 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 d−1, but rates in the closed cell region are not greatly lower than this. This latter finding suggests that precipitation is not a sufficient condition for POC formation from overcast stratocumulus. Despite similar cloud-base precipitation rates in the POC and overcast region, much of the precipitation (>90%) evaporates below cloud in the overcast region, while there is significant surface precipitation inside the POC. In the POC and transition region, although the majority of the condensate is in the form of drizzle, the integrated liquid water path is remarkably close to that expected for a moist adiabatic parcel rising from cloud base to top. The transition zone between the POC and the closed cells often consists of thick "boundary cell" clouds producing mean surface precipitation rates of 10–20 mm d−1, a divergent quasi-permanent cold/moist pool below cloud, a convergent inflow region at mid-levels in the MBL, and a divergent outflow near the top of the MBL. The stratiform clouds in the POC exist within an ultra-clean layer that is some 200–300 m thick. Aerosol concentrations (Na) measured by a PCASP in the diameter range 0.12–3.12 μm in the center of the ultra-clean layer are as low as 0.1–1 cm−3. This suggests that coalescence scavenging and sedimentation is extremely efficient, since Na in the subcloud layer, and droplet concentration Nd in the active cumuli are typically 20–60 cm−3. The droplet concentrations in the quiescent stratiform clouds are extremely low (typically 1–10 cm−3), 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, as do subcloud accumulation mode aerosol concentrations, suggesting that coalescence scavenging is impacting regions in the overcast region as well as inside the POC. Both flights show lower accumulation mode aerosol concentration in the subcloud layer of the POC (Na ∼ 30 cm−3) compared with the overcast region (Na ∼ 100 cm−3), but elevated (and mostly volatile) total aerosol concentrations are observed in the POC at all levels around 20–50 km from the transition zone, perhaps associated with some prior nucleation event. Despite the large differences in cloud and MBL structure across the POC-overcast boundary, the MBL depth is almost the same in the two regions, and increases in concert over the 12 h period between the flights.