Student Seminar Series
Department of Atmospheric & Oceanic Sciences
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Sensitivity of Shallow-Cumulus Dilution to Vertical Wind Shear
Shallow cumulus clouds are prevalent over land and ocean and play an essential part in weather and climate by regulating the thermodynamic structure of the atmosphere. The vertical growth of shallow cumuli is inhibited by entrainment, the process by which environmental air is ingested into the clouds. It causes cloud dilution and the evaporation of cloud droplets, which reduces the liquid water content (LWC), buoyancy, and vigor of convection. Cumulus dilution is potentially sensitive to numerous environmental conditions, including humidity, static stability, aerosol loading, surface conditions, and vertical wind shear. Of particular interest to this study is the role of vertical shear, which has been assumed to enhance cumulus entrainment for decades. However, the large-eddy simulations (LES) of Brown (1999) have shown that this is not always the case.
As part of an ongoing study on the environmental sensitivities of dilution, we have conducted LES of observationally constrained shallow-cumulus cloud fields over the ocean and over land, based on two LES intercomparison projects (BOMEX over the ocean and ARM-SGP over land). By modifying the wind profiles in these test cases, we systematically investigate the sensitivity of cloud dilution to vertical wind shear. The maritime BOMEX case is found to exhibit a strong sensitivity to cloud-layer shear with up to 60% larger dilution rates in the strongly sheared experiments. In these flows, a pressure dipole develops across the cloud with high pressure on the upshear side and low pressure on the downshear flank. Upshear, it tends to divert the flow around the cloud, whereas it enhances the inflow into the downshear cloud flank. As a result of the dynamic entrainment downshear, the cumuli are more strongly diluted on the downshear flank. While a stronger pressure dipole as a consequence of the higher vertical velocities in the cloud core is observed in the continental case, the dilution rate exhibits a weaker sensitivity. This weaker sensitivity is explained by the larger surface heat fluxes and higher levels of turbulence in the continental case.