J. Atmos. Sci. , 72, 3378-3388.

Effect of surface fluxes versus radiative cooling on tropical deep convection

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

The effects of turbulent surface fluxes and radiative cooling on tropical deep convection are compared in a series of idealized cloud-system resolving simulations with parameterized large scale dynamics. Two methods of parameterizing the large scale dynamics are used; the Weak Temperature Gradient (WTG) approximation and the Damped Gravity Wave (DGW) method. Both surface fluxes and radiative cooling are specified, with radiative cooling taken constant in the vertical in the troposphere. All simulations are run to statistical equilibrium.

In the precipitating equilibria, which result from sufficiently moist initial conditions, an increment in surface fluxes produces more precipitation than equal increment of column-integrated radiative cooling. This is straightforwardly understood in terms of the column-integrated moist static energy budget with constant normalized gross moist stability. Under both large-scale parameterizations, the gross moist stability does in fact remain close to constant over a wide range of forcings, and the small variations which occur are similar for equal increments of surface flux and radiative heating.

With completely dry initial conditions, the WTG simulations exhibit hysteresis, maintaining a dry state with no precipitation for a wide range of net energy inputs to the atmospheric column. The same boundary conditions and forcings admit a rainy state also (for moist initial conditions), and thus multiple equilibria exist under WTG. When the net forcing (surface fluxes minus radiative cooling) is increased enough that simulations which begin dry eventually develop precipitation, the dry state persists longer after initialization when the surface fluxes are increased than when radiative cooling is decreased. The DGW method, however, shows no multiple equilibria in any of the simulations.