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R. N. Byrne
,
R. C. J. Somerville
, and
B. Subaşilar

Abstract

Observations cited by Ramanathan et al. and Cess et al. indicate systematic errors in the solar radiation parameterizations of the current atmospheric general circulation models. Cloudy scenes have an observational excess (or calculational deficit) of atmospheric absorption. Pilewskie and Valero have also reported anomalously large absorption.

A simple model is presented here to show how fields of broken clouds cause average photon pathlengths to be greater than those predicted by homogeneous radiative transfer calculations of cloud-atmosphere ensemble with similar albedos, especially under and within the cloud layer. This one-sided bias is a contribution to the anomalous absorption. The model is illustrated quantitatively with a numerical stochastic radiative transfer calculation. More than one-half the anomaly is explained for the parameters used in the numerical example.

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P. H. Stone
,
W. J. Quirk
, and
R. C. J. Somerville

Abstract

Several experiments are described in which the sub-grid-scale vertical eddy viscosity in the GISS global general circulation model was varied. The results show that large viscosities suppress large-scale eddies in middle and high latitudes, but enhance the circulation in the tropical Hadley cell and increase the extent of the tropical easterlies. Comparison with observations shows that the GISS model requires eddy viscosities ∼1 m2/s or less to give realistic results for middle and high latitudes, and eddy viscosities ∼100 m2/s to give realistic results for low latitudes. A plausible mechanism for the implied increase in small-scale mixing in low latitudes is cumulus convection.

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F. Malvagi
,
R. N. Byrne
,
G. C. Pomraning
, and
R. C. J. Somerville

Abstract

A radiation treatment of the broken-cloud problem is presented, based upon various stochastic models of the equation of radiative transfer that consider the clouds and clear sky as a two-component mixture. These models, recently introduced in the kinetic theory literature, allow for non-Markovian statistics as well as both vertical and lateral variations in the cloudiness. Numerical results are given that compare different models of stochastic radiative transport and that point out the importance of treating the broken-cloud problem as a stochastic process. It is also shown that an integral Markovian model proposed within the atmospheric radiation community by Titov is entirely equivalent to a special case of a simple low-order different model. The differential form of Titov's result should be easier than the integral form to implement in any general circulation model.

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R.C.J. Somerville
,
P.H. Stone
,
M. Halem
,
J.E. Hansen
,
J.S. Hogan
,
L.M. Druyan
,
G. Russell
,
A.A. Lacis
,
W.J. Quirk
, and
J. Tenenbaum

Abstract

A model description and numerical results are presented for a global atmospheric circulation model developed at the Goddard Institute for Space Studies (GISS). The model version described is a 9-level primitive-equation model in sigma coordinates. It includes a realistic distribution of continents, oceans and topography. Detailed calculations of energy transfer by solar and terrestrial radiation make use of cloud and water vapor fields calculated by the model. The model hydrologic cycle includes two precipitation mechanisms: large-scale supersaturation and a parameterization of subgrid-scale cumulus convection.

Results are presented both from a comparison of the 13th to the 43rd days (January) of one integration with climatological statistics, and from five short-range forecasting experiments. In the extended integration, the near-equilibrium January-mean model atmosphere exhibits an energy cycle in good agreement with observational estimates, together with generally realistic zonal mean fields of winds, temperature, humidity, transports, diabatic heating, evaporation, precipitation, and cloud cover. In the five forecasting experiments, after 48 hr, the average rms error in temperature is 3.9K, and the average rms error in 500-mb height is 62 m. The model is successful in simulating the 2-day evolution of the major features of the observed sea level pressure and 500-mb height fields in a region surrounding North America.

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