Simple Solar Spectral Model for Direct and Diffuse Irradiance on Horizontal and Tilted Planes at the Earth's Surface for Cloudless Atmospheres

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In previous work (Bird), we presented a simple model to calculate direct normal and diffuse horizontal spectral irradiance at the earth's surface for clear days. In this report, we present a new simple model that incorporates improvements in methodology as well as an algorithm to produce spectra for tilted surfaces. The goal of this work is to give researchers the capability to produce accurate terrestrial spectra using only a microcomputer.

The first model (Bird) was based on models developed by Leckner and Brine and Iqbal. Since that work was completed, Justus and Paris have made improvements to the simple model approach. In the model presented here, we refined the Justus and Paris model and extended it to calculate spectra for tilted surfaces using methods developed by Hay and Davies. Refinements to the Justus and Paris model were based on comparisons with results of rigorous radiative transfer codes and with measured spectra.

The new simple model and results are presented in the sections that follow. Sections 2.0 and 3.0 describe methods for calculating direct normal and diffuse spectral irradiance, respectively. Section 4.0 gives comparisons of the simple model results with rigorous model results and measurements. Examples of the application of the new model are given in Section 5.0.

2.0 Direct Normal Irradiance

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