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J. Otterman, M-D. Chou, and A. Arking

Abstract

The albedo of a forest with snow on the ground is much less than that of snow-covered low vegetation such as tundra. As a result, simulation of the Northern Hemisphere climate, when fully forested south of a suitably chosen taiga/tundra boundary (ecocline), produces a hemispheric surface air temperature 1.9 K higher than that of an earth devoid of trees. Using variations of the solar constant to force climate changes in the GLAS Multi-Layer Energy Balance Model, the role of snow-albedo feedback in increasing the climate sensitivity to external perturbations is reexamined. The effect of snow-albedo feedback is found to be significantly reduced when a low albedo is used for snow over taiga, south of the fixed latitude of the ecocline. If the ecocline shifts to maintain equilibrium with the new climate—which is presumed to occur in a prolonged perturbation when time is sufficient for trees to grow or die and fall—the feedback is stronger than for a fixed ecocline, especially at high latitudes. However, this snow/vegetation-albedo feedback is still essentially weaker than the snow-albedo feedback in the forest-free case.

The loss of forest to agriculture and other land-use would put the present climate further away from that associated with the fully forested earth south of the ecocline and closer to the forest-free case. Thus, the decrease in nontropical forest cover since prehistoric times has probably affected the climate by reducing the temperatures and by increasing the sensitivity to perturbations, with both effects more pronounced at high latitudes.

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W. B. Rossow, F. Mosher, E. Kinsella, A. Arking, M. Desbois, E. Harrison, P. Minnis, E. Ruprecht, G. Seze, C. Simmer, and E. Smith

Abstract

The International Satellite Cloud Climatology Project (ISCCP) will provide a uniform global climatology of satellite-measured radiances and derive an experimental climatology of cloud radiative properties from these radiances. A pilot study to intercompare cloud analysis algorithms was initiated in 1981 to define a state-of-the-art algorithm for ISCCP. This study compared the results of applying six different algorithms to the same satellite radiance data. The results show that the performance of all current algorithms depends on how accurately the clear sky radiances are specified; much improvement in results is possible with better methods for obtaining these clear-sky radiances. A major difference between the algorithms is caused by their sensitivity to changes in the cloud size distribution and optical properties: all methods, which work well for some cloud types or climate regions, do poorly for other situations. Therefore, the ISCCP algorithm is composed of a series of steps, each of which is designed to detect some of the clouds present in the scene. This progressive analysis is used to retrieve an estimate of the clear sky radiances corresponding to each satellite image. Application of a bispectral threshold is then used as the last step to determine the cloud fraction. Cloudy radiances are interpreted in terms of a simplified model of cloud radiative effects to provide some measure of cloud radiative properties. Application of this experimental algorithm to produce a cloud climatology and field observation programs to validate the results will stimulate further research on cloud analysis techniques as part of ISCCP.

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H. L. Kyle, J. R. Hickey, P. E. Ardanuy, H. Jacobwitz, A. Arking, G. G. Campbell, F. B. House, R. Maschhoff, G. L. Smith, L. L. Stowe, and T. Vonder Haar

Three spectrally broadband measurement sets are presently being used for earth radiation budget (ERB) studies. These are the Nimbus-6 ERB (July 1975 to June 1978), the Nimbus-7 ERB (November 1978 to the present), and the Earth Radiation Budget Experiment (ERBE) (November 1984 to present). The measurements yield the incident solar irradiance, absorbed solar energy, outgoing longwave and net radiation. The Nimbus-7 started an accurate record of the solar constant in November 1978, while a nearly continuous record of the earth's radiation budget began in July 1975 with the Nimbus-6. Both the Nimbus-6 and -7 products have, in recent years, been reprocessed with improved processing and calibration algorithms so that the entire dataset can be considered as new. However, because of the use of different calibration and processing procedures, the three datasets for some purposes must be considered as piecewise continuous. Nevertheless, the data have been used in many important climate studies. The Nimbus-7 solar measurements indicate that the sun is a low-level variable star and that the mean annual solar energy just outside the earth's atmosphere was about 0.1% lower in 1984 than in 1979 and 1991. Further, the 9 years of Nimbus-7 ERB measurements show the earth's mean annual energy budget to be stable at the 0.2% level with apparently real changes in the annual emitted longwave at the 0.1% to 0.2% level that are associated with changes in the surface temperature. Other studies deal with the cooling and warming effects of clouds, interregional energy transport, and interannual variations. Our understanding of the sensors and how to derive an accurate mean radiation budget from the measurements has slowly improved over the years. But to date, there has been no consensus on the use of consistent calibration and processing procedures to permit quantitatively consistent analyses across the Nimbus-6, -7, and ERBE products. This report describes some successes and lessons learned during the Nimbus ERB program and the compatibility of the Nimbus and ERBE products.

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