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John K. Dukowicz, Richard D. Smith, and Robert C. Malone

Abstract

Certain aspects of the Semtner-Chervin version of the Bryan-Cox-Semtner global ocean model are reformulated for improved efficiency on parallel computer architectures and on the Connection Machine CM-2 in particular. These changes involve (a) the switch from a streamfunction to a surface pressure formulation in the barotropic equations, (b) the splitting off of the Coriolis terms from the barotropic equations to produce a symmetric surface pressure equation, which then permits (c) the use of a preconditioned conjugate-gradient method for the solution of this equation. The switch to a surface pressure formulation (a) eliminates global equations associated with island boundary conditions and therefore improves performance as well as allows an unlimited number of islands, (b) reduces sensitivity to rapidly varying bottom topography and therefore obviates the need for smoothing the topography, and (c) makes the surface pressure a prognostic variable, thus potentially making it easier to assimilate surface altimetry data. Care is taken to retain the energetic consistency built into the original model. The nine-point operator for the pressure equation is found necessary to maintain energy consistency, in contrast to the streamfunction formulation where both nine-point and five-point operators are usable. Computational results closely resemble those of the original model, but with significantly improved performance. The barotropic part of the calculations in the surface pressure formulation is newly three times faster than in the streamfunction formulation, and the full calculation involving both barotropic and baroclinic parts is nearly two times faster. These changes are described and details of the performance of the new formulation on the CM-2 are given.

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Robert C. Malone, Eric J. Pitcher, Maurice L. Blackmon, Kamal Puri, and William Bourke

Abstract

We examine the characteristics of stationary and transient eddies in the geopotential-height field as simulated by a spectral general circulation model. The model possesses a realistic distribution of continents and oceans and realistic, but smoothed, topography. Two simulations with perpetual January and July forcing by climatological sea surface temperatures, sea ice, and insulation were extended to 1200 days, of which the final 600 days were used for the results in this study.

We find that the stationary waves are well simulated in both seasons in the Northern Hemisphere, where strong forcing by orography and land-sea thermal contrasts exists. However, in the Southern Hemisphere, where no continents are present in midlatitudes, the stationary waves have smaller amplitude than that observed in both seasons.

In both hemispheres, the transient eddies are well simulated in the winter season but are too weak in the summer season. The model fails to generate a sufficiently intense summertime midlatitude jet in either hemisphere, and this results in a low level of transient activity. The variance in the tropical troposphere is very well simulated. We examine the geographical distribution and vertical structure of the transient eddies. Fourier analysis in zonal wavenumber and temporal filtering am used to display the wavelength and frequency characteristics of the eddies.

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Eric J. Pitcher, Robert C. Malone, V. Ramanathan, Maurice L. Blackmon, Kamal Puri, and William Bourke

Abstract

We describe the results of January and July simulations carded out with a nine-level spectral model, employing a rhomboidal truncation at wavenumber 15. Sea-surface temperature, sea-ice distribution and solar zenith angle are held constant in each simulation. The model includes interactive clouds and radiative processes after Ramanathan et al. (1983). Selected fields are shown which highlight the model's strengths and weaknesses.

The latitude-height distribution of the zonal wind is successfully simulated. The model captures the separation between the wintertime westerly jets in the troposphere and stratosphere and thus simulates the sign reversal in the vertical wind shear across the jet axis in the upper troposphere.

In addition to the zonal wind, we show also the zonally averaged temperature, meridional wind and vertical velocity. Regional distributions of sea-level pressure, surface air temperature, precipitation and a number of other fields defined at various pressure levels are compared in detail with observations. For the most part, the large-scale features of the observed general circulation are successfully simulated, although the sea-level pressure in the subtropics over continental regions in the wintertime is higher than observed, and the model atmosphere tends to be a few degrees colder than observed. We otter a partial explanation for this last deficiency.

There is good agreement between the model stratosphere and the actual stratosphere. Preliminary indications suggest the variability present in the model is comparable to that found in the atmosphere.

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V. Ramanathan, Eric J. Pitcher, Robert C. Malone, and Maurice L. Blackmon

Abstract

We present here results and analyses of a series of numerical experiments performed with a spectral general circulation model (GCM). The purpose of the GCM experiments is to examine the role of radiation/cloud processes in the general circulation of the troposphere and stratosphere. The experiments were primarily motivated by the significant improvements in the GCM zonal mean simulation as refinements were made in the model treatment of clear-sky radiation and cloud-radiative interactions. The GCM with the improved cloud/radiation model is able to reproduce many observed features, such as: a clear separation between the wintertime tropospheric jet and the polar night jet; winter polar stratospheric temperatures of about 200 K; interhemispheric and seasonal asymmetries in the zonal winds.

In a set of sensitivity experiments, we have stripped the cloud/radiation model of its improvements, the result being a significant degradation of the zonal mean simulations by the GCM. Through these experiments we have been able to identify the processes that are responsible for the improved GCM simulations: (i) careful treatment of the upper boundary condition for O3 solar heating; (ii) temperature dependence of longwave cooling by CO2 15 μm bands., (iii) vertical distribution of H2O that minimizes the lower stratospheric H2O longwave cooling; (iv) dependence of cirrus emissivity on cloud liquid water content.

Comparison of the GCM simulations, with and without the cloud/radiation improvements, reveals the nature and magnitude of the following radiative-dynamical interactions: (i) the temperature decrease (due to errors in radiative heating) within the winter polar stratosphere is much larger than can be accounted for by purely radiative adjustment; (ii) the role of dynamics in maintaining the winter polar stratosphere thermal structure is greatly diminished in the GCM with the degraded treatment of radiation; (iii) the radiative and radiative-dynamical response times of the atmosphere vary from periods of less than two weeks in the lower troposphere to roughly three months in the polar lower stratosphere; (iv) within the stratosphere, the radiative response times vary significantly with temperature, with the winter polar values larger than the summer polar values by as much as a factor of 2.5.

Cirrus clouds, if their emissivities are arbitrarily prescribed to be black, unrealistically enhance the radiative cooling of the polar troposphere above ∼8 km. This results in a meridional temperature gradient much stronger than that which is observed. We employ a more realistic parameterization that accounts for the non-blackness of cirrus, and we describe the resulting improvements in the model simulation of zonal winds, temperatures, and radiation budget.

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