J. Atmos. Sci. , 71, 2976-2993.
Usama Anber
Department of Earth and Environmental Sciences,
Columbia University, New York, NY.
Shuguang Wang
Department of Applied Physics and Applied Mathematics,
Columbia University, New York, NY.
Adam H. Sobel
Department of Applied Physics and Applied Mathematics, Department of Earth and Environmental Sciences, and Lamont-Doherty Earth Observatory,
Columbia University, New York, NY.
Abstract
It is well known that vertical wind shear can organize deep convective systems
and greatly extend their lifetimes. We know much less about the influence of shear on the bulk properties of tropical convection in statistical equilibrium. To address the latter question, the authors present a series of cloud-resolving simulations on a doubly periodic domain with parameterized large scale dynamics. The horizontal mean horizontal wind is relaxed strongly in these simulations towards a simple unidirectional linear vertical shear profile in the troposphere. The strength and depth of the shear layer are varied as control parameters. Surface enthalpy fluxes are prescribed.
The results fall in two distinct regimes. For weak wind shear, time-averaged rainfall decreases with shear and convection remains disorganized. For larger wind shear, rainfall increases with shear, as convection becomes organized into linear mesoscale systems. This non-monotonic dependence of rainfall on shear is observed when the imposed surface fluxes are moderate. For larger surface fluxes, convection in the unsheared basic state is already strongly organized, but increasing wind shear still leads to increasing rainfall. In addition to surface rainfall, the impacts of shear on the parameterized large-scale vertical velocity, convective mass fluxes, cloud fraction, and momentum transport are also discussed.