Large-eddy simulation (LES) output for a case of thin stratocumulus off the coast of California is examined in a mixed-layer analysis framework to identify the specific mechanisms responsible for governing the evolution of the cloud system. An equation for cloud-base height tendency isolates the individual cloud-modulating mechanisms that control the evolution of boundary-layer liquid-water static energy (S l ) and total water mixing ratio (q T ). With a suitable spin-up procedure, the control simulation performs admirably compared with observed estimates of liquid water content, vertical velocity variance, and radiative fluxes sampled during an aircraft field campaign. Investigation of the cloud response to various environmental forcing scenarios was addressed through a suite of sensitivity simulations, including variations in subsidence velocity, surface fluxes, wind shear near the inversion, and radiative forcing. In the control simulation, rising cloud-base tendencies are associated with entrainment warming/drying and short-wave absorption, whereas lowering cloud-base tendencies are driven by long-wave cooling. Even in the presence of substantial afternoon solar heating, entrainment fluxes remained active. The thin cloud demonstrated unexpected resiliency, with mixed-layer analysis indicating that, as the short-wave flux decreases later in the afternoon, the relative contribution of long-wave cooling often becomes large enough to offset entrainment warming/drying and result in a reversal of cloud-base tendency. The evolution of cloud-base tendency is found to be insensitive to the net radiative flux divergence for most of the simulations (liquid water path ranging from~10-50 g/m 2 ). Error analysis in comparison with LES S l and q T budgets suggests that our method of entrainment flux calculation could be improved by a more complete understanding of entrainment-layer physics. KEYWORDS afternoon, large-eddy simulation, marine, mixed-layer model, stratocumulus 1 Q J R Meteorol Soc. 2019;145:845-866.wileyonlinelibrary.com/journal/qj

The existence of subsiding shells on the periphery of shallow cumulus clouds has major implications concerning the parameterization of shallow convection, with the mass exchange between the shell and cloudy air representing a significant deviation from the commonly used bulk-plume parameterization. We examine the structure and frequency of subsiding shells in shallow cumulus convection using Doppler lidars at the Atmospheric Radiation Measurement Southern Great Plains facility in the central United States and at the Jülich ObservatorY for Cloud Evolution in western Germany. Doppler lidar indicates that the vertical subsiding shell extent is asymmetric, while shell width is typicallỹ 100 m. Large-eddy simulation can reasonably simulate the observed shell structure using a grid spacing of 10 m and suggests that much of the observed asymmetry is not a result of transient cloud evolution. Plain Language Summary Doppler lidars allow for the inference of vertical air motion. On the edges of the shallow "popcorn" cumulus clouds, regions of sinking air (subsiding shells) are observed. If we wish to understand how these clouds interact with their environment, we must understand the structure of the subsiding shells that envelop them. As a cloud passes over the lidar, the front edge of the cloud is sampled first, and the back edge is sampled later. The back-edge subsiding shell descends farther below cloud base than the front-edge shell. High-resolution models can resolve the observed shell structure and suggest that the differences between the front-and back-edge shells do not arise from the evolution of the cloud during the tens of seconds it takes to pass over the lidar.

Vertical wind shear has long been known to tilt convective towers and reduce thermal ascent rates. The purpose of this study is to better understand the physical mechanisms responsible for reduced ascent rates in shallow convection. In particular, the study focuses on cloud-edge mass flux to assess how shear impacts mass-flux profiles of both the ensemble and individual clouds of various depths. A compositing algorithm is used to distill large-eddy simulation (LES) output to focus on up- and down-shear cloud edges that are not influenced by complex cloud geometry or nearby clouds. A direct entrainment algorithm is used to estimate the mass flux through the cloud surface. We find that the dynamics on the up- and down-shear sides are fundamentally different, with the entrainment of environmental momentum and dilution of buoyancy being primarily responsible for the reduced down-shear ascent rates. Direct estimates of fluid flow through the cloud interface indicate a counter-shear organized flow pattern that entrains on the down-shear side and detrains on the up-shear side, resulting from the sub-cloud shear being lifted into the cloud layer by the updraft. In spite of organized regions of entrainment and detrainment, the overall net lateral mass flux remains unchanged with respect to the no shear run, with weak detrainment present throughout cloud depth.