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J. Otterman

Abstract

A model is developed that treats, in a simplified way, the reflection of the direct solar radiation by a surface consisting of a soil-plane and protruding vertical plant elements, such as needles of pine trees or stalks of a wheat field. Such a surface is treated as a Lambertian reflectivity soil-plane (reflectivity ri) and thin, vertical cylinders are also characterized as Lambertian reflectors (reflectivity rc). Each cylindrical section is randomly located with respect to the other sections. The first hemispheric reflection of the direct beam from this surface is computed exactly, expressed as diri + dcrc, where di and dc are the geometrical factors governing the reflection from the soil-plane and from the protrusions respectively. These geometrical factors are a function of the solar zenith angle θ0, and the area s of the projections on a vertical plane of the cylindrical plant elements per surface unit area. The light trapping in such a complex surface is assessed by analyzing di and dc. For the case rc = ri, we analyze the combined geometrical factor de = di + dc. In the range 20° < θ0 < 80°, the factor de decreases with increasing zenith angle for low values of s, but for a large s it increases, sharply so for θ0 > 60°. When s tanθ0 ≫ 1, the albedo accrues from the protrusions only and can be stated as a product of rc and an explicit function of θ0, which increases monotonically with θ0.

The above analytical results apply to the direct solar beam. For the scattered component of the irradiance or for the global irradiance under cloudy conditions, a constant albedo, independent of the solar zenith angle, seems applicable. A comparison with the albedo measurements reported over a pine forest indicates that our simple analytic model reproduces quite well the dependence of the albedo on the solar zenith angle and the overall light trapping characteristics of such a complex surface. Determination of the parameter s for any vegetated area over which albedo measurements are taken will enhance the meaning of the measurements for meteorological-climatological studies.

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J. Otterman and C. J. Tucker

Abstract

Measurements of surface parameters in an arid steppe (the semi-desert of the northern Sinai) were made from the NOAA-6 satellite to assess the effects of the vegetation recovery in a fenced-off area. The radiances measured in the solar wavelengths over the vegetated area were about 25% lower than those measured over the surrounding bare sandy soil (where the surface albedo measured from Landsat is about 0.42). This implies a reduction in the albedo by the vegetation also by about 25% if both surfaces are regarded as Lambertian, but by as much as 42% if the vegetated area is modeled as a plane of soil with vertically protruding plants. The radiation temperatures in the 11 μm channel at ∼0730 LST measured over the vegetated area were by as much as 2.5 K higher than over the surrounding sands.

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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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