The effects of vertical wind shear on radiative-convective equilibrium states

Abstract

Using a three-dimensional cloud ensemble model, a systematic exploration is undertaken of radiative-convective equilibrium states as a function of the structure and magnitude of an imposed background flow with vertical shear. In such simulations, mesoscale organization appears naturally, independent of the particulars of the initial condition. As the magnitude of an imposed low-level shear increases, the convection becomes increasingly organized in lines or arcs, propagating broadly downshear, as predicted by earlier work. When the shear is very strong, the convection tends to organize into lines at an angle to the shear, such that the line-normal component is not far from its theoretical optimal value. Midlevel shear favors shear-parallel lines, but if it occurs in conjunction with sufficiently strong low-level shear, the convection can become very strongly organized into lines or arcs generally orthogonal to the low-level shear. Optimal organization occurs when the depth of the shear layer is comparable to that of the cold pools associated with the convective downdrafts. As the vertical shear is increased, the domain-averaged convective available potential energy (CAPE) at first increases but then decreases at stronger shear values. Associated with these changes, the lower to middle troposphere becomes drier at low shear values and more humid when the shear is strong. This relationship between humidity and CAPE is broadly consistent with recently developed CAPE theories. The authors also confirm previous work that shows that the transport of momentum by the simulated convection, though usually down the gradient of the background flow, is nonlocal in character. Finally, some simulations are performed with an imposed hodograph taken from a tropical cyclone. Convective arcs form with an orientation similar to observed outer spiral bands, but the simulated bands propagate more rapidly than observed, perhaps because of a dry middle troposphere in the simulations.