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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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Claire L. Parkinson and Gerald F. Herman

Abstract

A four-month simulation of the thermodynamic portion of the Parkinson-Washington sea ice model was conducted using atmospheric boundary conditions that were obtained from a pre-computed seasonal simulation of the Goddard Laboratory for Atmospheric Sciences' Gencral Circulation Model (GLAS GCM). The sea ice thickness and distribution were predicted for the 1 January–30 April period based on the GCM-generated fields of solar and infrared radiation, specific humidity and air temperature at the surface, and snow accumulation. The sensible heat and evaporative fluxes at the surface are mutually consistent with the ground temperatures generated by the ice model and the air temperatures generated by the atmospheric model.

In general, in the Northern Hemisphere the predicted ice distributions and the wintertime accretion and southward advance of the pack ice are well simulated. The computed ice thickness in the Southern Hemisphere appears reasonable, but the Antarctic melt season is extended, causing ice coverage to be less than observed in late March and April. During the Northern Hemisphere winter, the simulated ice accretion is the result of the net deficit of longwave radiation, heat gained from the ocean, am sensible heat lost to the atmosphere. In the early part of the Southern Hemisphere summer, the melting essentially balances the excess of solar over longwave radiation at the surface, while later in the simulation accretion balances the longwave and convective heal losses.

The results show that the Parkinson–Washington sea ice model produces acceptable ice concentrations and thicknesses when used in conjunction with the GLAS GCM for the January to April transition period. These results suggest the feasibility of fully coupled ice-atmosphere simulations with these two models.

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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 and Winthrop T. Johnson

Abstract

A series of experiments were conducted with the Goddard general circulation model to determine the effect of variation in the location of Arctic sea ice boundaries on the model's mean monthly climatology. A control was defined as the mean of six January-February simulations with ice boundaries corresponding to climatologically minimum ice cover occurring simultaneously in the Davis Strait, East Greenland Sea, Barents Sea, Sea of Okhotsk and Bering Sea. An anomaly was similarly defined as the mean of two simulations with maximum ice conditions occurring simultaneously in the same regions. The extremes were estimated from 17 years of observed conditions in the Atlantic sector, and from five years of data in the Pacific sector.

When sea ice boundaries were at their maximum extent the following differences resulted in the January-February climatology as compared with minimum boundaries: Sea level pressure was higher by as much as 8 mb over the Barents Sea, by more than 4 mb in Davis Strait, and by slightly less than 4 mb in the Sea of Okhotsk. Pressure was lower by as much as 8 mb in the north Atlantic between Iceland and the British Isles, and in the Gulf of Alaska. Pressure rises of as much as 4 mb in the eastern subtropical regions of the North Atlantic and North Pacific accompanied pressure falls in the Gulf of Alaska and Icelandic region. Pressure also increased over the Mediterranean region. Geopotential heights at 700 mb were more than 80 gpm lower in the Gulf of Alaska, and more than 100 gpm lower in the Icelandic region. Changes of opposite sign occurred over the subtropics. Zonally averaged temperatures were cooler by 2°C below 800 mb between 50 and 70°N with little change elsewhere. The poleward flux of total energy was a maximum of 13% greater between latitudes 40 and 53°N.

The computed 700 mb geopotential differences were more than twice the inherent variability or "noise" of the model over a broad region of the North Pacific, and between Iceland and the British Isles. Pressure differences were more than twice the inherent variability in the Davis Strait, Gulf of Alaska and Barents Sea, and in the eastern subtropical Atlantic and Pacific. Statistical significance of zonally averaged differences was largest between 50 and 70°N where the confidence levels were as follows: for geopotential height differences, 99% (850 mb), 99% (700 mb) and 97% (500 mb); for temperature, 97% (850 mb) an 92% (700 mb); and sea level pressure, 94%. Confidence levels were high for changes in the Azores region.

On the basis of model results we conclude that ice margin anomalies are capable of altering local climates in certain regions of the high and mid-latitudes. Possible interactions between high latitudes and subtropical regions also are suggested.

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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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