According to the authors,
Even though the monitoring of solar radiation experienced a vast progress in the recent years both in terms of expanding the measurement networks and increasing the data quality, the number of stations is still too small to achieve accurate global coverage. Alternatively, various models for estimating solar radiation are exploited in many applications. Choosing a model is often limited by the availability of the meteorological parameters required for its running. In many cases the current values of the parameters are replaced with daily, monthly or even yearly average values. This paper deals with the evaluation of the error made in estimating global solar irradiance by using an average value of the Angstrom turbidity coefficient instead of its current value.
If you believe Trenberth's global energy budget, annualized global solar irradiance at Earth's surface is 161 W/m2. A 20% error could account for up to 32.2 W/m2 in annualized global solar irradiance at Earth's surface, which is 18 times greater than the alleged 1.88 W/m2 increase in CO2 forcing since 1750.
It is shown that the relative errors in estimating global solar irradiance (GHI) due to inadequate consideration of Angstrom turbidity coefficient may be very high, even exceeding 20%. However, when an hourly or a daily average value is used instead of the current value of the Angstrom turbidity coefficient, the relative errors are acceptably small, in general less than 5%. All results prove that in order to correctly reproduce GHI [global solar irradiance] for various particular aerosol loadings of the atmosphere, the parametric models should rely on hourly or daily Angstrom turbidity coefficient values rather than on the more usual monthly or yearly average data, if currently measured data is not available.
|Mean solar irradiance at the same latitudes can vary greatly as a result of aerosol concentrations and altitude, and thus is extremely difficult to model