[1] The effect of cloud droplet sedimentation on the entrainment rate and liquid water path of a nocturnal nondrizzling stratocumulus layer is examined using largeeddy simulations (LES) with bulk microphysics. In agreement with a prior study by Ackerman et al. (2004), sedimentation is found to decrease entrainment rate and thereby increase liquid water path. They suggested this is due to reduction of boundary-layer turbulence. Our simulations suggest otherwise. Instead, sedimentation reduces entrainment by removing liquid water from the entrainment zone. This inhibits two mechanisms that promote the sinking of entrained air into the cloud layerentrainment-induced evaporative cooling and longwave radiative cooling. A sensitivity study shows that the radiative effect is less important than the reduced evaporation. A possible parameterization of the effect of sedimentation on entrainment rate in a mixed layer model is proposed and tested. Since the droplet sedimentation rate is inversely related to cloud droplet (and presumably aerosol) concentration and nearly nondrizzling marine stratocumulus are widespread, sedimentation impacts on stratocumulus entrainment efficiency should be considered in climate model simulations of the aerosol indirect effect. Citation: Bretherton, C. S., P. N.Blossey, and J. Uchida (2007), Cloud droplet sedimentation, entrainment efficiency, and subtropical stratocumulus albedo, Geophys. Res. Lett., 34, L03813,

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Southern Ocean (S. Ocean) clouds are important for climate prediction. Yet previous global climate models failed to accurately represent cloud phase distributions in this observation-sparse region. In this study, data from the Southern Ocean Clouds, Radiation, Aerosol, Transport Experimental Study (SOCRATES) experiment is compared to constrained simulations from a global climate model (the Community Atmosphere Model, CAM). Nudged versions of CAM are found to reproduce many of the features of detailed in situ observations, such as cloud location, cloud phase, and boundary layer structure. The simulation in CAM6 has improved its representation of S. Ocean clouds with adjustments to the ice nucleation and cloud microphysics schemes that permit more supercooled liquid. Comparisons between modeled and observed hydrometeor size distributions suggest that the modeled hydrometeor size distributions represent the dual peaked shape and form of observed distributions, which is remarkable given the scale difference between model and observations. Comparison to satellite observations of cloud physics is difficult due to model assumptions that do not match retrieval assumptions. Some biases in the model's representation of S. Ocean clouds and aerosols remain, but the detailed cloud physical parameterization provides a basis for process level improvement and direct comparisons to observations. This is crucial because cloud feedbacks and climate sensitivity are sensitive to the representation of S. Ocean clouds. Plain Language Summary Clouds over the Southern Ocean are important for climate prediction and may influence the evolution of global temperatures. Thus, these clouds are important to represent properly in models; however, recent studies have revealed models inadequately represent Southern Ocean cloud occurrence and phase, which drive large biases in radiation and subsequent climate sensitivity. Observations from research aircraft over the Southern Ocean south of Australia are compared to simulations with a global climate model which is "nudged" to reproduce the day-today cloud systems which are sampled. Despite being a coarse horizontal and vertical resolution, the model is able to reproduce many details of cloud phase and water content during the flights. However, the model has some biases, and these observations have been used to improve the model to better represent cloud phase. These results point to specific observational constraints for improving model simulations.

Abstract.Five pockets of open cells (POCs) are studied using aircraft flights from the VOCALS Regional Experiment (VOCALS-REx), conducted in October and November 2008 over the southeast Pacific Ocean. Satellite imagery from the geostationary satellite GOES-10 is used to distinguish POC areas, and measurements from the aircraft flights are used to compare aerosol, cloud, precipitation, and boundary layer conditions inside and outside of POCs. Conditions observed across individual POC 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 ) and show no remarkable difference from the observed surface and free-tropospheric conditions during the two months of the field campaign. In all cases, compared to the surrounding overcast region the POC boundary layer is more decoupled, supporting both thin stratiform and deeper cumulus clouds. Although cloud-base precipitation rates are higher in the POC than the overcast region in each case, a threshold precipitation rate that differentiates POC precipitation from overcast precipitation does not exist. Mean cloud-base precipitation rates in POCs can range from 1.7 to 5.8 mm d −1 across different POC cases. The occurrence of heavy drizzle (> 0 dBZ) lower in the boundary layer better differentiates POC precipitation from overcast precipitation, likely leading to the more active cold pool formation in POCs. Cloud droplet number concentration is at least a factor of 8 smaller in the POC clouds, and the ratio of drizzle water to cloud water in POC clouds is over an order of magnitude larger than that in overcast clouds, indicating an enhancement of collision-coalescence processes in POC clouds.Despite large variations in the accumulation-mode aerosol concentrations observed in the surrounding overcast region (65 to 324 cm −3 ), the accumulation-mode aerosol concentrations observed in the subcloud layer of all five POCs exhibit a much narrower range (24 to 40 cm −3 ), and cloud droplet concentrations within the cumulus updrafts originating in this layer reflect this limited variability. Above the POC subcloud layer exists an ultraclean layer with accumulationmode aerosol concentrations < 5 cm −3 , demonstrating that in-cloud collision-coalescence processes efficiently remove aerosols. The existence of the ultraclean layer also suggests that the major source of accumulation-mode aerosols, and hence of cloud condensation nuclei in POCs, is the ocean surface, while entrainment of free-tropospheric aerosols is weak. The measurements also suggest that at approximately 30 cm −3 a balance of surface source and coalescence scavenging sinks of accumulation-mode aerosols maintain the narrow range of observed subcloud aerosol concentrations.

Abstract. Convective processes profoundly affect the global water and energy balance of our planet but remain a challenge for global climate modeling. Here we develop and investigate the suitability of a unified convection scheme, capable of handling both shallow and deep convection, to simulate cases of tropical oceanic convection, mid-latitude continental convection, and maritime shallow convection. To that aim, we employ large-eddy simulations (LES) as a benchmark to test and refine a unified convection scheme implemented in the Single-column Community Atmosphere Model (SCAM). Our approach is motivated by previous cloud-resolving modeling studies, which have documented the gradual transition between shallow and deep convection and its possible importance for the simulated precipitation diurnal cycle.Analysis of the LES reveals that differences between shallow and deep convection, regarding cloud-base properties as well as entrainment/detrainment rates, can be related to the evaporation of precipitation. Parameterizing such effects and accordingly modifying the University of Washington shallow convection scheme, it is found that the new unified scheme can represent both shallow and deep convection as well as tropical and mid-latitude continental convection. Compared to the default SCAM version, the new scheme especially improves relative humidity, cloud cover and mass flux profiles. The new unified scheme also removes the well-known too early onset and peak of convective precipitation over midlatitude continental areas.