Date of Award:

5-1979

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Plants, Soils, and Climate

Department name when degree awarded

Soil Science and Biometeorology

Committee Chair(s)

Inge Dirmhirn

Committee

Inge Dirmhirn

Committee

Jack Chatelain

Committee

Gene Wooldridge

Abstract

A study of the parameters effecting diffuse irradiance in mountainous terrain was made. Ground/location parameters of site elevation, ground albedo, elevated horizon and surface tilt were examined under the pertinent atmospheric conditions of clear, polluted, overcast, and cloudy. Measured diffuse irradiance data used for comparisons were taken from fall 1977-spring 1979 at four sites located in northern Utah.

Diffuse irradiance varied with elevation according to the existing atmospheric conditions. For clear sky, diffuse irradiance decreased with elevation due to the shorter optical pathlength, thus less scatter of the direct beam. However, with completely overcast sky, diffuse irradiance increased with elevation. The thinner cloud cover associated with higher elevations causes less adsorption of the diffuse irradiance. For partially cloudy conditions a correlation of diffuse irradiance with duration of sunshine showed curved relationships with maximum diffuse irradiance at 70 percent cloud cover. At high elevation, the curves were much steeper than at low elevation, becoming almost linear.

Climatic changes in ground albedo were found to modify diffuse irradiance considerably. This occurred mainly through multiple reflection between ground and atmosphere with maximum enhancement for snow covered ground and overcast sky. Even for clear days snow cover shifted the maximum in the annual cycle of diffuse irradiance day totals towards the spring months. A comparison of March (snow cover) and September (bare ground) diffuse irradiance values for various amounts of cloud cover showed that diffuse enhancement increased steadily with cloud amount to a maximum at overcast conditions.

The effects of elevated horizon such as might be found in a mountain valley were examined through validation of a physical model describing a V-shaped valley. Comparisons between calculated and measured diffuse irradiance showed excellent results for clear days during winter, spring, and summer.

The ratio of measured diffuse irradiance on a south facing 50° tilt to that on the horizontal was plotted against duration of sunshine for different seasons. For clear days the 50° tilt enhanced diffuse irradiance values as much as two times the horizontal values during winter while during summer 50° values dropped to .8 of the horizontal values. For overcast days the ratio varied from 1.0-0.8 for winter and summer, respectively.

A model predicting diffuse irradiance on a tilted surface for clear day was developed and validated using measured data. A good comparison is shown for spring and fall days using high and low elevation data.

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