Estimating solar irradiance and components

Abstract

The performance of models to estimate solar irradiance and its components is assessed using data for six Canadian stations for nine years (1968–1976). Greatest emphasis is placed on a model, the MAC model, which uses cloud information from different layers. Here, effects on model estimates of using observed cloud layer opacities instead of amounts is examined because after 1976 the latter quantity is no longer recorded. Effects of aerosol are also examined. Two other models are considered here; a version of the MAC model which uses total cloud information rather than layer information, and Rietveld's[1] sunshine-based model. In general, the layer model yielded the best results. This is in agreement with a previous Canadian study[2]. RMSE values are between 11 and 15 percent for global irradiance on a daily basis and decrease below 10 percent for averaging periods greater than 2–4 days. Daily RMSE values for the direct beam and diffuse components are 25 per cent. These decrease below 10 per cent for averaging periods larger than 10–15 days. Errors increase when layer opacities are used but results are still acceptable. Aerosol effects can be neglected for much of the country except in Montreal and Toronto. Rietveld's model gives results of lower accuracy than the MAC model but may be useful for quick, easy estimates. The MAC model using total cloud amount performed poorly.