The relationship between drizzle and cloud morphology, as manifest in transitions to spatially compact regions of open cellular convection, is studied using data collected from recent field studies over the northeast and southeast Pacific.
[1] CGILS-the CFMIP-GASS Intercomparison of Large Eddy Models (LESs) and single column models (SCMs)-investigates the mechanisms of cloud feedback in SCMs and LESs under idealized climate change perturbation. This paper describes the CGILS results from 15 SCMs and 8 LES models. Three cloud regimes over the subtropical oceans are studied: shallow cumulus, cumulus under stratocumulus, and wellmixed coastal stratus/stratocumulus. In the stratocumulus and coastal stratus regimes, SCMs without activated shallow convection generally simulated negative cloud feedbacks, while models with active shallow convection generally simulated positive cloud feedbacks. In the shallow cumulus alone regime, this relationship is less clear, likely due to the changes in cloud depth, lateral mixing, and precipitation or a combination of them. The majority of LES models simulated negative cloud feedback in the wellmixed coastal stratus/stratocumulus regime, and positive feedback in the shallow cumulus and stratocumulus regime. A general framework is provided to interpret SCM results: in a warmer climate, the moistening rate of the cloudy layer associated with the surface-based turbulence parameterization is enhanced; together with weaker VOL. 5, 826-842, doi:10.1002/2013MS000246, 2013 large-scale subsidence, it causes negative cloud feedback. In contrast, in the warmer climate, the drying rate associated with the shallow convection scheme is enhanced. This causes positive cloud feedback. These mechanisms are summarized as the ''NESTS'' negative cloud feedback and the ''SCOPE'' positive cloud feedback (Negative feedback from Surface Turbulence under weaker Subsidence-Shallow Convection PositivE feedback) with the net cloud feedback depending on how the two opposing effects counteract each other. The LES results are consistent with these interpretations.Citation: Zhang, M., et al. (2013), CGILS: Results from the first phase of an international project to understand the physical mechanisms of low cloud feedbacks in single column models, J. Adv. Model. Earth Syst., 5, 826-842,
Radiance measurements made by the Advanced Very High Resolution Radiometer (AVHRR) at 1-km (nadir) spatial resolution were used to retrieve cloud optical depth () and cloud droplet effective radius (r eff ) for 31 marine boundary layer clouds over the eastern Pacific Ocean and the Southern Ocean near Tasmania.In the majority of these scenes (each roughly 256 ϫ 256 km 2 in extent) and r eff are strongly correlated, with linear least squares yielding a regression curve of the form r eff ϰ 1/5 . This relationship is consistent with an idealized model of a nonprecipitating layer cloud in which 1) the average cloud liquid water content increases linearly with height at some fraction of the adiabatic lapse rate in a 1 km 2 vertical column, and 2) the normalized horizontal variability of the cloud liquid water path exceeds the variability of a scaled measure of the cloud droplet number concentration. In contrast, other scenes of similar horizontal extent show little or no correlation between retrieved values of and r eff . These scenes include thicker clouds in which precipitation may be occurring, as well as cloud layers with spatially distinct regions of varying r eff .In situ aircraft measurements were made simultaneously with six AVHRR overpasses as part of the Southern Ocean Cloud Experiment. The clouds sampled by these flights were significantly thicker than the typically 200m-thick eastern Pacific stratocumulus, with large vertical and horizontal variability. On five of the six flights, aircraft measurements of the cloud-top effective radius were well matched by the satellite retrievals, and in two of these layers r eff ϰ 1/5 .
Abstract.A series of laboratory and aircraft measurements have indicated that supercooled liquid water exists to temperatures as low as -70øC. These measurements also show that classical nucleation theory, using standard values for the thermodynamic properties of supercooled water, underestimates the nucleation rate of ice in liquid water at large supercoolings. New theoretical estimates for this •homogeneous nucleation rate are presented, based on a new analytic equation of state for liquid water. The new equation of state, which is accurate over a pressure range of 3000 atmospheres and a temperature range of 1200 K, is used to infer the latent heat of melting, liquid water density, and ice-water surface energy of supercooled water. Predictions of the nucleation rate and the homogeneous freezing temperature made by this equation of state are in agreement with observations at temperatures as cold as -70øC and at pressures as high as 2000 atmospheres. These results indicate that it is not necessary to invoke a phase transition at -45øC to explain aircraft and laboratory observations of homogeneous ice nucleation in supercooled water clouds.
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