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 inteCorrespondence to: R. Wood (robwood@atmos.washington.edu) grated 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 ultraclean layer that is some 200-300 m thick. Aerosol concentrations (N a ) 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 N a in the subcloud layer, and droplet concentration N d 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 coa...

Modelling the effects of gravity waves on stratocumulus clouds observed during VOCALS-UK

“…Nevertheless, the simulation with four gravity waves displayed an increase in turbulence after the cloud cleared, due to drizzle evaporating and causing cold pools; this led to a cloud layer that was more strongly coupled to the surface moisture fluxes towards the end of the simulation. Cold pools have been shown to be important components of open MCC circulations in an aircraft case study during the same project (Wood et al, 2011a). Our results support the finding of Wood et al (2011a), who suggest that warm rain is not a sufficient condition for the formation of POCs, since at least in our simulations the clearing of the cloud occurs by mixing in of dry air, with cold pools serving to reinvigorate Cu-like convection.…”

“…Cold pools have been shown to be important components of open MCC circulations in an aircraft case study during the same project (Wood et al, 2011a). Our results support the finding of Wood et al (2011a), who suggest that warm rain is not a sufficient condition for the formation of POCs, since at least in our simulations the clearing of the cloud occurs by mixing in of dry air, with cold pools serving to reinvigorate Cu-like convection.…”

“…This is a significant finding and one that is highly relevant to this study. However, it was clear from the work of Wood et al (2011a) that the cells within the POCs were often surrounded by a "boundary cell" with divergent cold pools at lower levels, convergence in the middle of the boundary layer and divergence at the top, thus suggesting that precipitation processes may play a role in maintaining the POCs once they form. This was also confirmed by large eddy simulation LES modelling of the same case (Berner et al, 2011) and is consistent with earlier LES studies of the formation of POCs , which argued that precipitation formation is necessary for the maintenance of long-lived POCs.…”

“…Although the mechanisms responsible for the transition from closed to open cell forms are unclear, it is evident that the two cellular regimes form when the boundary layer is deeper than ∼ 1 km and that closed cells form over cold ocean currents, while open cells form preferentially over warm ocean currents (see Wood and Hartmann, 2006, and references therein). Wood et al (2011a) presented an aircraft study of the transition from closed to open cellular convection observed during the VOCALS campaign. They found that the POCs consisted of intermittent precipitating cumulus (Cu) clouds that detrained into very optically thin stratiform cloud.…”

Real-case simulations of aerosol–cloud interactions in ship tracks over the Bay of Biscay

“…The scarcity of CCN induced the collapse of the boundary layer and a break-up of the stratocumulus cloud deck into an open cell structure with scattered, drizzling shallow cumuli (Ackerman et al, 1993;Wood et al, 2011;Berner et al, 2013).…”