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Abstract:
The role of wind speed on shallow marine cumulus convection is explored using large-eddy simulations and
concepts from bulk theory. Focusing on cases characteristic of the trades, the equilibrium trade wind layer is
found to be deeper at stronger winds, with larger surface moisture fluxes and smaller surface heat fluxes.
The opposing behavior of the surface fluxes is caused by more warm and dry air being mixed to the surface as
the cloud layer deepens. This leads to little difference in equilibrium surface buoyancy fluxes and cloud-base
mass fluxes. Shallow cumuli are deeper, but not more numerous or more energetic. The deepening response is
necessary to resolve an inconsistency in the subcloud layer. This argument follows from bulk concepts and
assumes that the lapse rate and flux divergence of moist-conserved variables do not change, based on simulation
results. With that assumption, stronger winds and a fixed inversion height imply larger surface
moisture and buoyancy fluxes (heat fluxes are small initially). The consequent moistening tends to decrease
cloud-base height, which is inconsistent with a larger surface buoyancy flux that tends to increase cloud-base
height, in order to maintain the buoyancy flux at cloud base at a fixed fraction of its surface value (entrainment
closure). Deepening the cloud layer by increasing the inversion height resolves this inconsistency by allowing
the surface buoyancy flux to remain constant without further moistening the subcloud layer. Because this
explanation follows from simple bulk concepts, it is suggested that the internal dynamics (mixing) of clouds is
only secondary to the deepening response.
