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Y. C. Sud, Winston C. Chao, and G. K. Walker

Abstract

A coarse (4° &times 5° × 9-sigma level) version of the Goddard Laboratory for Atmospheres (GLA) General Circulation Model (GCM) was used to investigate the influence of a cumulus convection scheme on the simulated atmospheric circulation and hydrologic cycle. Two sets of integrations, each containing an ensemble of three summer (June, July, and August) simulations, were produced. The first set, containing control cases, included a state-of-the-art cumulus parameterization scheme in the GCM; whereas the second set, containing experiment cases, used the same GCM but without the cumulus parameterization. All simulations started from initial conditions that were taken from analysis of observations for three consecutive initial times that wore only 12 h apart beginning with 0000 UTC 19 May 1988. The climatological boundary conditions—sea surface temperature, snow, ice, and vegetation cover-were kept exactly the same for all the integrations. The ensemble sets of control and experiment simulations are control and differentially analyzed to determine the influence of a cumulus convection scheme on the simulated circulation and hydrologic cycle.

The results show that cumulus parameterization has a very significant influence on the simulated circulation and precipitation. The influence is conspicuous in tropical regions, interior of continents in the Northern Hemisphere, and some oceanic regions. The upper-level condensation heating over the intertropical convergence zone (ITCZ) is much smaller for the experiment simulations as compared to the control simulations; correspondingly, the Hadley and Walker cells for the control simulations are also weaker and are accompanied by a weaker Ferrel cell in the Southern Hemisphere. The rainfall under the rising branch of the southern Ferrel cell (at about 50°S) does not increase very much because boundary-layer convergence poleward reduces the local evaporation. Overall, the difference fields show that experiment simulations (without cumulus convection) produce a cooler and less energetic atmosphere. The vertical profile of the zonally averaged diabatic heating also shows large differences in the tropics that are physically consistent with accompanying differences in circulation. Despite producing a warmer and wetter planetary boundary layer (PBL) in the tropics (20°S–20°N), the control simulations also produce a warmer but drier 400-mb level. The moisture transport convergence fields show that while only the stationary circulation is affected significantly in the PBI, both the stationary and eddy moisture transports are altered significantly in the atmosphere above the PBL. These differences no only reaffirm the important role of cumulus convection in maintaining the global circulation, but also show the way in which the presence or absence of a cumulus parameterization scheme can affect the circulation and rainfall climatology of a GCM.

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Shian-Jiann Lin, Winston C. Chao, Y. C. Sud, and G. K. Walker

Abstract

A generalized form of the second-order van Leer transport scheme is derived. Several constraints to the implied subgrid linear distribution are discussed. A very simple positive-definite scheme can be derived directly from the generalized form. A monotonic version of the scheme is applied to the Goddard Laboratory for Atmospheres (GLA) general circulation model (GCM) for the moisture transport calculations, replacing the original fourth-order center-differencing scheme. Comparisons with the original scheme are made in idealized tests as well as in a summer climate simulation using the full GLA GCM. A distinct advantage of the monotonic transport scheme is its ability to transport sharp gradients without producing spurious oscillations and unphysical negative mixing ratio. Within the context of low-resolution climate simulations, the aforementioned characteristics are demonstrated to be very beneficial in regions where cumulus convection is active. The model-produced precipitation pattern using the new transport scheme is more coherently organized both in time and in space, and correlates better with observations. The side effect of the filling algorithm used in conjunction with the original scheme is also discussed, in the context of idealized tests.

The major weakness of the proposed transport scheme with a local monotonic constraint is its substantial implicit diffusion at low resolution. Alternative constraints are discussed to counter this problem.

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