A quasi-equilibrium tropical circulation model---formulation
J. David Neelin and Ning Zeng
Abstract.
A class of model for simulation and theory of the tropical atmospher
ic
component 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
parameterizations. QE convective closures tend to constrain the vertical
temperature profile in convecting regions. This can be used to generate
analytical solutions for the large scale flow under certain approximations. A
tropical atmospheric model of intermediate complexity is constructed by using
the analytical solutions as the first basis function in a Galerkin
representation of vertical structure. This retains much of the simplicity of
the analytical solutions, while retaining full nonlinearity, vertical momentum
transport, departures from QE and a transition between convective and
nonconvective zones based on convective available potential energy. The
atmospheric model is coupled to a one-layer land-surface model with interactive
soil moisture, and simulates its own tropical climatology. In the
QTCM version presented here, the vertical structure of temperature variati
ons is
truncated to a single profile associated with deep convection. Though designed
to be accurate in and near regions dominated by deep convection, the 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 been carried out
analytically, but the main advantage is relative simplicity of analysis under
certain conditions.
The formulation suggests a slightly different way of looking at the tropical
atmosphere than has been traditional in tropical meteorology. While convective
scales are unstable, the large scale motions evolve with a positive effective
stratification that takes into account the partial cancellation of adiabatic
cooling by diabatic heating. A consistent treatment of the moist static energy
budget aids the analysis of radiative and surface heat flux effects. This is
particularly important over land regions where the zero net surface flux links
land-surface anomalies. The resulting simplification highlights the role of
top-of-the atmosphere fluxes including cloud feedbacks, and illustrates the
usefulness of this approach for analysis of convective regions. Reductions of
the model for theoretical work or diagnostics are outlined.
Acknowledgements. This work was supported in part by National
Science Foundation grant ATM-9521389 and National Oceanographic and Atmospheric
Administration grant NA86-GP0314. Early stages of this work were carried out
while the authors were visiting the Massachusetts Institute of Technology,
Department of Earth, Atmospheric, and Planetary Sciences; JDN acknowledges support from the Houghton Lectureship.