Aircraft observations of aerosol, cloud, precipitation, and boundary layer properties in pockets of open cells over the southeast Pacific

摘要

Five pockets of open cells (POCs) are studied using aircraft flights from theVOCALS Regional Experiment (VOCALS-REx), conducted in October and November 2008 over thesoutheast Pacific Ocean. Satellite imagery from the geostationary satelliteGOES-10 is used to distinguish POC areas, and measurements from the aircraftflights are used to compare aerosol, cloud, precipitation, and boundary layerconditions inside and outside of POCs. Conditions observed across individualPOC cases are also compared.POCs are observed in boundary layers with a wide range of inversion heights(1250 to 1600 m) and surface wind speeds (5 to 11 m s−1) andshow no remarkable difference from the observed surface and free-troposphericconditions during the two months of the field campaign. In all cases,compared to the surrounding overcast region the POC boundary layer is moredecoupled, supporting both thin stratiform and deeper cumulus clouds.Although cloud-base precipitation rates are higher in the POC than theovercast region in each case, a threshold precipitation rate thatdifferentiates POC precipitation from overcast precipitation does not exist.Mean cloud-base precipitation rates in POCs can range from 1.7 to5.8 mm d−1 across different POC cases. The occurrence of heavydrizzle (> 0 dBZ) lower in the boundary layer better differentiates POCprecipitation from overcast precipitation, likely leading to the more activecold pool formation in POCs. Cloud droplet number concentration is at least afactor of 8 smaller in the POC clouds, and the ratio of drizzle water tocloud water in POC clouds is over an order of magnitude larger than that inovercast clouds, indicating an enhancement of collision–coalescenceprocesses in POC clouds.Despite large variations in the accumulation-mode aerosol concentrationsobserved in the surrounding overcast region (65 to 324 cm−3), theaccumulation-mode aerosol concentrations observed in the subcloud layer ofall five POCs exhibit a much narrower range (24 to 40 cm−3), andcloud droplet concentrations within the cumulus updrafts originating in thislayer reflect this limited variability. Above the POC subcloud layer existsan ultraclean layer with accumulation-mode aerosol concentrations< 5 cm−3, demonstrating that in-cloud collision–coalescenceprocesses efficiently remove aerosols. The existence of the ultraclean layeralso suggests that the major source of accumulation-mode aerosols, and henceof cloud condensation nuclei in POCs, is the ocean surface, while entrainmentof free-tropospheric aerosols is weak. The measurements also suggest that atapproximately 30 cm−3 a balance of surface source and coalescencescavenging sinks of accumulation-mode aerosols maintain the narrow range ofobserved subcloud aerosol concentrations.