Shallow Cumulus Entrainment Dynamics in a Sheared Environment

Abstract

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.

The moist halo region around shallow cumulus clouds in large eddy simulations

“…The region with downward motion outside the cloud is usually referred to as a “cloud shell,” but it is not necessarily related to higher water vapour (Savre, 2021). Recent studies (McMichael et al, 2022; Savre, 2021) suggested that, from the composited perspective, the region with downward motion outside the cloud is broader than the halo region with higher water vapour. Thus, the moist halo region seems to be a subset of the cloud shell, and it should be emphasized that the moist halo region investigated in this study is not the same as the downdraft cloud shells studied by Jonker et al (2008); Heus and Jonker (2008); Heus et al (2008), for example.…”

“…More than half of the air parcels within the halo region in the middle of the cloud layer (1000 m, Figure 9b) and near the cloud top (1500 m, Figure 9c) come from higher levels, and they descend slowly to form the halo. However, only 10 min after the reference time, more than $$$ 65\% $$$ of the air parcels have already descended to lower levels, suggesting that the formation of the halo region is accompanied by a downdraft (Heus & Jonker, 2008; McMichael et al, 2022). These results provide evidence to support our hypothesis of length scales associated with the moist halo region in the next section.…”

“…Nair et al (2021) also performed a high‐resolution large eddy simulation with 4.1‐m grid length and found that the size of the “invisible shell” is less than 200 m for a shell defined in terms of enstrophy. Heus et al (2008) performed simulations of shallow cumulus clouds with grid lengths from 12.5–100 m, but they focused mainly on the downdraft shells, which have been found to be wider than the moist halo region (McMichael et al, 2022). The downward mass flux in cloud shells was stronger in finer resolution simulations (Heus et al, 2008) and the integrated mass flux in cloud shells was stronger for larger size clouds (Heus & Jonker, 2008).…”