A quasi-equilibrium tropical circulation model - Formulation

A class of model for simulation and theory of the tropical atmospheric comp onent of climate variations is introduced. These models are referred to as quasi-equilibrium tropical circulation models, or QTCMs, because they make use of approximations associated with quasi-equilibrium (QE) convective par ameterizations. Quasiequilibrium convective closures tend to constrain the vertical temperature profile in convecting regions. This can be used to gen erate analytical solutions for the large-scale flow under certain approxima tions. A tropical atmospheric model of intermediate complexity is construct ed by using the analytical solutions as the first basis function in a Galer kin representation of vertical structure. This retains much of the simplici ty of the analytical solutions, while retaining full nonlinearity, vertical momentum transport, departures from QE, and a transition between convectiv e and nonconvective zones based on convective available potential energy. T he atmospheric model is coupled to a one-layer land surface model with inte ractive soil moisture and simulates its own tropical climatology. In the QT CM version presented here, the vertical structure of temperature variations is truncated to a single profile associated with deep convection. Though d esigned to be accurate in and near regions dominated by deep convection, th e model simulates the tropical and subtropical climatology reasonably well, and even has a qualitative representation of midlatitude storm tracks. The model is computationally economical, since part of the solution has bee n carried out analytically, but the main advantage is relative simplicity o f analysis under certain conditions. The formulation suggests a slightly di fferent way of looking at the tropical atmosphere than has been traditional in tropical meteorology. While convective scales are unstable, the large-s cale motions evolve with a positive effective stratification that takes int o account the partial cancellation of adiabatic cooling by diabatic heating . A consistent treatment of the moist static energy budget aids the analysi s of radiative and surface heat flux effects. This is particularly importan t over land regions where the zero net surface flux links land surface anom alies. The resulting simplification highlights the role of top-of-the-atmos phere fluxes including cloud feedbacks, and it illustrates the usefulness o f this approach for analysis of convective regions. Reductions of the model for theoretical work or diagnostics are outlined.