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Mixing between shallow cumulus clouds and their environment is studied using large-eddy simulations. The origin of in-cloud air is studied by two distinct methods: 1) by analyzing conserved variable mixing diagrams (Paluch diagrams) and 2) by tracing back cloud-air parcels represented by massless Lagrangian particles that follow the flow. The obtained Paluch diagrams are found to be similar to many results in the literature, but the source of entrained air found by particle tracking deviates from the source inferred from the Paluch analysis. Whereas the classical Paluch analysis seems to provide some evidence for cloud-top mixing, particle tracking shows that virtually all mixing occurs laterally. Particle trajectories averaged over the entire cloud ensemble also clearly indicate the absence of significant cloud-top mixing in shallow cumulus clouds.

In this study large-eddy simulations (LES) are used to gain more knowledge on the shell of subsiding air that is frequently observed around cumulus clouds. First, a detailed comparison between observational and numerical results is presented to better validate LIES as a tool for studies of microscale phenomena. It is found that horizontal cloud profiles of vertical velocity, humidity, and temperature are in good agreement with observations. They show features similar to the observations, including the presence of the shell of descending air around the cloud. Second, the availability of the complete 3D dataset in LES has been exploited to examine the role of lateral mixing in the exchange of cloud and environmental air. The origin of the subsiding shell is examined by analyzing the individual terms of the vertical momentum equation. Buoyancy is found to be the driving force for this shell, and it is counteracted by the pressure-gradient force. This shows that evaporative cooling at the cloud edge, induced by lateral mixing of cloudy and environmental air, is the responsible mechanism behind the descending shell. For all clouds, and especially the smaller ones, the negative mass flux generated by the subsiding shell is significant. This suggests an important role for lateral mixing throughout the entire cloud layer. The role of the shell in these processes is further explored and described in a conceptual three-layer model of the cloud.

Phase 1 of the CGILS large-eddy simulation (LES) intercomparison is extended to understand if subtropical marine boundary-layer clouds respond to idealized climate perturbations consistently in six LES models. Here the responses to quadrupled carbon dioxide ( 'fast adjustment ') and to a composite climate perturbation representative of CMIP3 multimodel mean 2×CO2 near-equilibrium conditions are analyzed. As in Phase 1, the LES is run to equilibrium using specified steady summertime forcings representative of three locations in the Northeast Pacific Ocean in shallow well-mixed stratocumulus, decoupled stratocumulus, and shallow cumulus cloud regimes. The results are generally consistent with a single-LES study of Bretherton et al. () on which this intercomparison was based. Both quadrupled CO2 and the composite climate perturbation result in less cloud and a shallower boundary layer for all models in well-mixed stratocumulus and for all but a single LES in decoupled stratocumulus and shallow cumulus, corroborating similar findings from global climate models (GCMs). For both perturbations, the amount of cloud reduction varies across the models, but there is less intermodel scatter than in GCMs. The cloud radiative effect changes are much larger in the stratocumulus-capped regimes than in the shallow cumulus regime, for which precipitation buffering may damp the cloud response. In the decoupled stratocumulus and cumulus regimes, both the CO2 increase and CMIP3 perturbations reduce boundary-layer decoupling, due to the shallowing of inversion height. [ABSTRACT FROM AUTHOR]

The article discusses the development of an autonomous observation system for operational use onboard cargo and research vessels to provide a solid basis for monitoring energy and material exchanges between ocean and atmosphere. Based on a network of expertise from OCEANET the fluxes of heat, momentum, humidity and carbon dioxide are measured to close the energy and mass budget at the sea surface. The OCEANET system was also used to quantify the role of clouds and sea surface wave on temporal and spatial variability of solar radiation below sea surface.

The dynamics of a subsiding shell at the edges of actively growing shallow cumulus clouds with updrafts is analyzed using direct numerical simulation with grid sizes of up to 3072 x 1536 x 1536. The actively growing clouds have a fixed in-cloud buoyancy and velocity. Turbulent mixing and evaporative cooling at the cloud edges generate a subsiding shell which grows with time [1]. A self-similar regime is observed for first and second order moments when normalized with their respective maximum values. Internal scales derived from integral properties of the flow problem are identified [2]. Self-similarity analysis conducted by normalizing using these scales reveal that contrary to classical self similar flows, the Turbulent Kinetic Energy (TKE) budget terms and the velocity moments scale according to the buoyancy and not with the mean velocity. The shell thickness is observed to increase linearly with time. The shell buoyancy scale remains constant as it thickens and is set by the initial thermodynamics of the cloud and environment. The shell accelerates ballistically with a magnitude defined by the saturation value of the buoyancy of the cloud-environment mixture. In this regime, the shell is buoyancy driven and independent of the in-cloud velocity. The shell thickness and the velocity continue to grow indefinitely and could possibly be limited only by the lifetime of the cloud or thermal.Relations are obtained for predicting the shell thickness and minimum velocities by linking the internal scales with external flow parameters. The values of shell thickness and velocities calculated using these derived relations are consistent with the values of a typical shallow cumulus cloud [3]. The entrainment coefficient, which is predicted by studying the rate of growth of the shell thickness, is observed to be a function only of the initial state of the cloud and the environment. This coefficient is linked to the fractional entrainment rate used in cumulus parameterization schemes for large scale models and is shown to be of the same order of magnitude [4].References:[1] Heus, T., and H. Jonker, 2008: Subsiding shells around shallow cumulus clouds. Journal of the Atmospheric Sciences, 65, 1003–1018, doi:10.1175/2007JAS2322.1.[2] Craske, J., and M. van Reeuwijk, 2015: Energy dispersion in turbulent jets. part 1. Direct simulation of steady and unsteady jets. Journal of Fluid Mechanics, 763, 500537, doi:10.1017/jfm.2014.640.[3] Rodts, S., P. Duynkerke, and H. Jonker, 2003: Size distributions and dynamical properties of shallow cumulus clouds from aircraft observations and satellite data. Journal of the Atmospheric Sciences, 60,1895–1912.[4] Romps, D., 2010: A direct measure of entrainment. Journal of the Atmospheric Sciences, 67, 1908–1927, doi:10.1175/2010JAS3371.1. [ABSTRACT FROM AUTHOR]