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Gerald F. Herman

Abstract

The results are reported of pyranometric measurements of solar radiation in Arctic stratus decks made from aircraft flights over the Beaufort Sea during late July 1975. The reflectance of these cloud layers was nearly constant over the range of cloud thicknesses investigated, indicating the importance of the high surface reflectivity. The following ranges of reflectance are obtained: over the total solar spectrum, 60–750%; visible spectrum, 70–85%; near–infrared, 50–65%. Transmittances for the cloud layers are presented as a function of cloud geometrical depth, and the bulk absorptance averaged over all cloud decks was 7% in the total solar spectrum, and 5% and 9% in the visible and near–infrared, respectively.

Additional cloud parameters, namely, single scatter albedo ω˜vand absorption optical depth τv, are derived by fitting the upward and downward flux profiles from each flight to a two–stream approximation to estimate the absorption optical depth. By assuming a linear relation between absorption optical depth and cloud thickness, the scattering parameter βv, which defines the increased path length caused by multiple scattering, is determined from a best fit to the complete set of observed reflectances and transmittances. The following ranges of βv, are estimated: total solar, 8.75–10.7; visible, 9.01–14.3; near–infrared, 6.86–8.12. By assuming an asymmetry factor of 0.85 these values of βv yield estimates of the single scattering albedo (ωv) of 0.994–0.996 over the total solar spectrum, 0.994–0.998 in the visible, and 0.990.993 in the near–infrared. Examples are presented of cloud absorption calculated with these derived values of ω˜v.

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Siegfried D. Schubert and Gerald F. Herman

Abstract

A method is demonstrated for evaluating global and zonally averaged heat balance statistics based on a four-dimensional assimilation with an atmospheric general circulation model (GCM). The procedure, which provides observationally constrained model diagnostics, uses the GCM of NASA's Goddard Laboratory for Atmospheric Sciences to evaluate the atmospheric heat balance for the February 1976 Data Systems Test period. The global distribution of the adiabatic and diabatic components of the heat balance are obtained by sampling the continuous GCM assimilation shortly after the insertion of conventional synoptic observations. Sampling times of 6 and 9 h after data insertion were chosen to provide adequate damping of high-frequency oscillations in the vertical velocity field caused by the data insertion.

Salient features of the February 1976 analysis include the following: Maximum rising motion in the mean vertical velocity field at 500 mb over South America, south-central Africa, Australia and the Indonesian archipelago. These regions also were characterized by large values of diabatic heating due to convective latent heat release. The cyclogenetically active regions over the North Atlantic and North Pacific oceans were characterized by maxima in latent heat release due to supersaturation cloud formation, and also maxima in the upward and northward transient eddy heat fluxes. In contrast, the continental west coasts showed a tendency for large downward and southward transient eddy beat fluxes.

Some differences are obtained between the heating rates calculated with the model parameterizations and through a residual method. Other shortcomings of the procedure include data deficiencies in the Southern Hemisphere, which cause the results to be comparatively more model dependent in the high southern latitudes.

The potential applicability of this method of analysis to the recently acquired FGGE data is noted.

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Gerald F. Herman, Man-Li C. Wu, and Winthrop T. Johnson

Abstract

The effect of global cloudiness on the solar and infrared components of the earth's radiation balance is studied in general circulation model experiments. A wintertime simulation is conducted in which the cloud radiative transfer calculations use realistic cloud optical properties and are fully interactive with model-generated cloudiness. This simulation is compared to others in which the clouds are alternatively non-interactive with respect to the solar or thermal radiation calculations. Other cloud processes (formation, latent heat release, precipitation, vertical mixing) were accurately simulated in these experiments.

We conclude that on a global basis clouds increase the global radiation balance by 40 W m−2 by absorbing longwave radiation, but decrease it by 56 W m−2 by reflecting solar radiation to space. The net cloud effect is therefore a reduction of the radiation balance by 16 W m−2, and is dominated by the cloud albedo effect.

Changes in cloud frequency and distribution and in atmospheric and land temperatures are also reported for the control and for the non-interactive simulations. In general, removal of the clouds’ infrared absorption cools the atmosphere and causes additional cloudiness to occur, while removal of the clouds’ solar radiative properties warms the atmosphere and causes fewer clouds to form. It is suggested that layered clouds and convective clouds over water enter the climate system as positive feedback components, while convective clouds over land enter as negative components.

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