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Robert D. Cess

Abstract

It is suggested that vegetational modification of the earth’s surface albedo, a process which occurs during a change in global climate, could produce a significant albedo-climate coupling or feedback mechanism. Employing the ice age of 18 000 years ago as a comparative climate, it is estimated that such a long-term biosphere-albedo feedback might roughly double the sensitivity of the global climate to factors which produce climatic change.

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Robert D. Cess

Abstract

The sensitivity of the earth's surface temperature to factors which can induce long-term climate change, such as a variation in solar constant, is estimated by employing two readily observable climate changes. One is the latitudinal change in annual mean climate, for which an interpretation of climatological data suggests that cloud amount is not a significant climate feedback mechanism, irrespective of how cloud amount might depend upon surface temperature, since there are compensating changes in both the solar and infrared optical properties of the atmosphere. It is further indicated that all other atmospheric feedback mechanisms, resulting, for example, from temperature-induced changes in water vapor amount, cloud altitude and lapse rate, collectively double the sensitivity of global surface temperature to a change in solar constant. The same conclusion is reached by considering a second type of climate change, that associated with seasonal variations for a given latitude zone. The seasonal interpretation further suggests that cloud amount feedback is unimportant zonally as well as globally. Application of the seasonal data required a correction for what appears to be an important seasonal feedback mechanism. This is attributed to a variability in cloud albedo due to seasonal changes in solar zenith angle. No attempt was made to individually interpret the collective feedback mechanisms which contribute to the doubling in surface temperature sensitivity. It is suggested, however, that the conventional assumption of fixed relative humidity for describing feedback due to water vapor amount might not be as applicable as is generally believed. Climate models which additionally include ice-albedo feedback are discussed within the framework of the present results.

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Robert D. Cess

Abstract

No abstract available.

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James A. Coakley Jr. and Robert D. Cess

Abstract

We insert the effect of naturally occurring tropospheric aerosols on solar radiation into the NCAR Community Climate Model (CCM). The effect of the aerosol depends on concentration and type (continental, maritime), surface albedo, solar zenith angle and cloud cover. The experiments were performed for perpetual July boundary conditions. Globally averaged, the aerosol reduces the solar radiative flux absorbed at the top of the model atmosphere by 3.0 W m−2, at the surface by 4.4 W m−2 and in the lower troposphere it increases the flux absorbed by 1.4 W m−2. Owing to fixed sea surface temperatures the climate simulated by the CCM is hardly affected by these perturbations. The global surface temperature change is −0.08 K (−0.27 K for continents), zonal surface temperature changes are limited to a few tenths of a degree and regional surface temperature changes rarely surpass −1 K. Between 30°S and 60°N the aerosol suppresses convective activity by reducing solar beating for land surfaces. As a result the upper troposphere, which for these regions and time of year is heated largely through moist convective processes, cools more so than the surface and lower troposphere which are directly affected by the interaction of the aerosol with solar radiation. Small but significant changes in climate are obtained for isolated portions of the globe. The most notable changes occurred for a region of Africa just north of the equator where the aerosol pushed the model towards “desertification.” That is, for this region the radiative forcing due to the aerosol gave rise to changes in convection and wind fields which in turn led to a significant reduction in precipitation. The processes involved in the changes wore like thaw discussed by Charney et al. for the role of surface albedo in desertification.

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Inna L. Vulis and Robert D. Cess

Abstract

An atmospheric solar radiation model has been coupled with surface reflectance measurements for two vegetation types, pasture land and savannah, in order to address several issues associated with understanding the directional planetary albedo; i.e., the dependence of planetary albedo upon solar zenith angle. These include an elucidation of processes that influence the variation of planetary albodo with solar zenith angle, as well as emphasizing potential problems associated with converting narrowband planetary albodo measurements to broadband quantities. It is suggested that, for vegetated surfaces, this latter task could be somewhat formidable, since the model simulations indicate that narrowband to broadband conversions strongly depend upon vegetation type. A further aspect of this paper is to illustrate a procedure by which reciprocity inconsistencies within a bidirectional reflectance dataset, if they are not too severe, can be circumvented.

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Robert D. Cess and Inna L. Vulis

Abstract

Desert regions are employed in somewhat of a tutorial mode for the purpose of addressing several issues associated with understanding the dependence of planetary (top-of-the-atmosphere) albedo upon solar zenith angle, i.e., the directional planetary albedo. It is emphasized that in evaluating this quantity from satellite data, and with reference to land surfaces, spurious results may be obtained if geographical variations of the planetary, albedo are not isolated from the albedo's solar zenith angle dependence. An atmospheric solar radiation model is then coupled with desert surface bidirectional reflectance measurements to test for consistency with satellite-derived directional planetary albedos. The model is further used to address issues such as the use of narrowband versus broadband instruments, the impact of desert aerosols upon the directional planetary albedo, and to interpret potential differences in the directional planetary albodo associated with different types of deserts. The model results show consistency with satellite measurements, while further suggesting that over desert regions, narrowband instruments should replicate broadband measurement of the directional planetary albedo, as is also consistent with observations. The model shows that the directional planetary albedo is dominated by the directional surface albedo, although surface brightness is a second factor since it influences atmospheric limb brightening and limb darkening processes.

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Robert D. Cess and Gerald L. Potter

Abstract

The Oregon State University/Lawrence Livermore National Laboratory general circulation model has been employed as a vehicle for suggesting and exploring various means of converting narrow-band measurements of reflected solar radiation from the earth-atmosphere system to broad-band quantities. For purely illustrative purposes we have adapted, within the model's solar radiation routine, a narrow-band filter function consisting of a square-wave window extending from 0.5 to 0.9 μm. A limitation of the model, for this sort of endeavor, is that it does not include the wavelength dependence of surface albedos. Nevertheless the model simulations tend to mimic the calibration of a narrow-band instrument, utilizing reflected solar radiation from the earth-atmosphere system as simultaneously measured by a collocated broad-band instrument; for the model, however, this is done in terms of fluxes, in contrast to instrument-measured radiances. The model results suggest that it might be preferable to perform narrow- to broad-band conversions in terms of planetary albedo (or an equivalent quantity), rather than in terms of reflected fluxes or radiances. Further improvement is achieved if, for instruments that can differentiate between clear and overcast conditions, separate clear and overcast calibrations are performed.

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Robert D. Cess and Inna L. Vulis

Abstract

An atmospheric solar radiation model, in conjunction with a variety of surface albedo models, has been employed to address several issues related to inferring the surface solar radiation budget from satellite measurements. With reference to albedo determinations using narrowband instruments such as GOES and METEOSAT, it is suggested that converting these to broadband quantities, either at the surface or at the top of the atmosphere, could be a formidable task for vegetated surfaces. To do this operationally it would seem necessary to subdivide vegetated surface into a large number of categories, incorporate some appropriate means of scene identification, account for probable seasonal variations in surface albedo and surface anisotropy, and then devise a quantitative method for actually performing the conversions.

It would be preferable to evaluate broadband quantities from broadband measurements. A further point of this paper, however, is that a commonly used linear conversion between broadband planetary and surface albedos is also strongly dependent upon vegetation type. It is then alternatively proposed that a linear slope-offset relationship exists between surface and surface-atmosphere solar absorption. For clear skies this relationship is only modestly dependent upon scene type, requires only a rudimentary correction for variations in atmospheric water vapor, and, with the exception of deserts, necessitates only a modest correction for tropospheric aerosols over land areas. No correction for maritime aerosols over ocean area is required. This study also elucidates problems and possible approaches for dealing with overcast areas and regions containing broken clouds.

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Robert G. Ellingson, Robert D. Cess, and Gerald L. Potter
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James A. Coakley Jr., Robert D. Cess, and Franz B. Yurevich

Abstract

Guided by the results of doubling-adding solutions to the equation of radiative transfer, we develop a simple technique for incorporating in climate models the effect of the background tropospheric aerosol on solar radiation. Because the atmosphere is practically nonabsorbing for much of the solar spectrum the effects of the tropospheric aerosol on the reflectivity, transmissivity and absorptivity of the atmosphere are adequately accounted for by the properties of a two-layered system with the atmosphere placed above the aerosol layer. The two-stream and delta-Eddington approximations to the radiative transfer equation then provide reasonably accurate estimates of the changes brought about by the aerosol. Furthermore, results of the doubling-adding calculations lead to a simple parameterization for the distribution of absorption by the aerosol within the atmosphere. Using these simple techniques, we calculate the changes caused by models for the naturally occurring tropospheric aerosol in a zonal mean energy balance climate model. The 2–30°C surface cooling caused by the background aerosol is comparable in magnitude but opposite in sign to the temperature changes brought about by the current atmospheric concentrations of N20 and CH4 and by a doubling of CO2. The model results also indicate that even though the background aerosol may decrease the planetary albedo at high latitudes, it causes cooling at all latitudes. We also use the simple techniques to calculate the influence of dust on the planetary albedo of a desert. Here we demonstrate that the interaction of the aerosol scattering with the angular dependence of the surface reflectivity strongly influences the planetary albedo.

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