Overview

The Neelin-Zeng Quasi-Equilibrium Tropical Circulation Model (QTCM1), developed at the UCLA Department of Atmospheric Sciences by J. David Neelin and Ning Zeng [1, 2], is an intermediate-level atmospheric model that assumes the atmosphere is in a state of ``quasi-equilibrium.'' By quasi-equilibrium, we mean that the changes in energy at the large-scales are roughly balanced by changes in energy associated with small-scale convective activity. Thus, the vertical profiles of temperature are assumed to be ``defined'' by a ``reference'' profile; the model will restore departures from that profile back to the reference profile.

The assumption of quasi-equilibrium is valid in the tropical deep convective regions, where the atmosphere is unstable and highly convective throughout the tropospheric column. Away from the deep tropics, dry dynamics predominate, and the quasi-equilibrium assumption is generally no longer valid. However, because the QTCM1 provides a robust treatment of the moist dynamics that dominate in the tropics, the QTCM1 should also provide a reasonable description of atmospheric dynamics in areas where quasi-equilibrium does not locally apply, up to approximately one Rossby radius away from the tropics.

The QTCM1 does not divide the atmosphere into multiple vertical layers as most general circulation models do, but rather uses a Galerkin-like representation of vertical structure, with the assumption of quasi-equilibrium. This reduces the dynamical system to as few a number of degrees of freedom as possible, consistent with a realistic simulation of climate. This gains simplicity and dramatically cuts computational time. The current version takes approximately 5 minutes of CPU time to run one year of model simulation on a SUN Ultra2 workstation.

The current version of the model includes the following major features:

• Two vertical components (one barotropic and one baroclinic) of the general circulation are represented. These components are calculated from 1000 mb to 200 mb. The ``1'' in ``QTCM1'' refers to there being only one baroclinic mode simulated by the model.
• Convective heating is parameterized by a version of the Betts-Miller [15, 16, 17] parameterization scheme.
• Land surface processes are represented by the simple scheme ``SLand1'' of Zeng and Neelin [13] and Zeng et al. [2].
• Two different radiation cloud packages are available in the model: a simpler parameterization (Zeng et al. [2]) and a more complicated parameterization developed by Chia Chou [3, 4, 5] at UCLA. They differ mostly in the shortwave radiation scheme and the choice of predicted vs. prescribed cloud types.
• All surface fluxes are calculated. NB: a sensitivity of model climatology and anomalies on the equatorial Pacific and Indian Ocean to the evaporation formulation has been observed.
• The effects of topography are represented but turned off in the standard version.