The first quasi-equilibrium tropical circulation model---implementation and simulation
Ning Zeng, J. David Neelin and Chia Chou
Abstract.
The first quasi-equilibrium tropical circulation model QTCM1 is implemented and tested. The
formulation, described by Neelin and Zeng, uses a Galerkin framework in the vertical, but with basis
functions tailored to quasi-equilibrium deep convective physics via analytical solutions. QTCM1
retains a single vertical structure of temperature and humidity. For a balanced treatment of dynamics
and sub-grid scale physics, a physics parameterization package of intermediate complexity is
developed. These include a linearized longwave radiation scheme, a simple cloud prediction
method, simple shortwave radiation schemes, and an intermediate land-surface model.
The simulated climatology has a reasonable spatial pattern and seasonal evolution of the tropical
convergence zones, including over land regions. Outgoing longwave radiation and net surface heat
flux both appear satisfactory. The Asian monsoon is slightly weak but depicts the northward
progression of the monsoon onset, and a monsoon wind shear index exhibits interannual variability
associated with observed SST that is similar to general circulation model (GCM) results. The extent
and position of the main El Ni\~no/Southern Oscillation rainfall anomalies are simulated, as well as
a number of the observed tropical and subtropical teleconnections. The seasonal cycle and
interannual variability of the Amazon water budget, including evapotranspiration, interception loss,
surface and subsurface runoff, illustrate reasonable simulation of the hydrologic cycle. Sensitivity
studies on effects of topography, evaporation formulation, and land-surface processes are also
conducted. While the results are imperfect with respect to observations, many aspects are
comparable to or better than GCMs of the previous generation. Considering the complexity of these
simulated phenomena, the model is computationally light and easy to diagnose. It thus provides a
new tool filling the niche between GCMs and simpler models.
Acknowledgements. This work was supported in part by National
Science Foundation grant ATM-9521389 and National Oceanographic and Atmospheric
Administration grant NA46GP-0244. 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.