Characterizing Subsiding Shells in Shallow Cumulus Using Doppler Lidar and Large‐Eddy Simulation

Abstract

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.

Shallow Cumulus Entrainment Dynamics in a Sheared Environment

“…With the addition of shear, the down-shear subcloud updraft is shifted upward and outward, whereas the up-shear side possesses an enhanced, more vertically oriented subsiding shell (Fig. 4a), consistent with lidar observations (McMichael et al 2020;Griewank et al 2020). There is a pronounced up-to down-shear pressure perturbation gradient force in the subcloud and cloud layer, with a local pressure perturbation maximum near the up-shear cloud base and a local pressure perturbation minimum at 50%-60% cloud depth, just a few grid boxes inside the downshear cloud edge (Fig.…”

“…Numerical and laboratory studies of toroidal circulations injected into sheared, boundary layer-like flows indicate that shear tilts the counterrotating vortices in opposite directions as entrainment of environmental vorticity distorts the initial symmetry, with the down-shear vortex being shifted upward (Cheng et al 2009;Lim et al 2008). Observational studies of moist convection have shown evidence of such tilted toroidal circulations (Damiani and Vali 2007), up-shear displacement of updrafts (Heymsfield et al 1978), enhanced down-shear environmental humidity (Perry and Hobbs 1996;Lu et al 2003;Heus and Jonker 2008), and substantial updraft-and subsiding-shell asymmetry that extends downward well into the subcloud layer (McMichael et al 2020;Griewank et al 2020).…”

Assessing Transboundary‐Local Aerosols Interaction Over Complex Terrain Using a Doppler LiDAR Network

“…In South China, Hou et al (2019) applied a numerical model to assess the contributions of different aerosol species in TAP to Pearl River Delta (PRD). While more physical processes have been recently included in numerical models at much higher spatial resolutions, model accuracy has been substantially improved (Bony et al, 2015;McMichael et al, 2020). However, parameterizations of processes in the ABL in numerical models still cannot fully describe the thermal and dynamical processes related to turbulence at urban scale.…”