A globally applicable model of daily solar irradiance estimated from air temperature and precipitation data

Although not measured at many ground stations, the total daily solar irradi ance (R.) received at the earth's surface is a critical component of ecosys tem carbon, water and energy processes. Methods of estimating R, from other meteorological data, particularly daily temperatures, have not worked as w ell in tropical and maritime areas. At Luquillo, Puerto Rico, the daily atm ospheric transmittance for solar radiation was approximately equal to one m inus the daily-average relative humidity (1 - rh(ave)). From these observat ions, we developed a model (VP-RAD) for estimation of R, with inputs of dai ly maximum and minimum air temperature. daily total precipitation, mean ann ual temperature, mean annual temperature range, site latitude, and site ele vation. VP-RAD performed well over large areas; it showed a good agreement with the site data used for model development and for seven other warm, hum id locations in the southeastern United States. Comparisons with a similar model revealed that predictions using VP-RAD had lower average errors and i mproved day-to-day correlation to measured solar irradiance. In a global co mparison for the year 1987, VP-RAD-estimated and satellite-derived photosyn thetically active radiation converged to within 1.0 MJ m(-2) day(-1) at 72% of the 13072 1 degrees latitude by 1 degrees longitude vegetated grid cell s. Although these comparisons revealed areas where VP-RAD may need improvem ent, VP-RAD should be a useful tool for applications globally. In addition, VP-RAD's similarity in form to the Bristow-Campbell equation provides a co nvenient method to calculate the site-specific coefficients for this model that is widely used when solar irradiance data are not available. (C) 2001 Elsevier Science B.V. All rights reserved.