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 irradiance (Rs) received at the earth’s surface is a critical component of ecosystem carbon, water and energy processes. Methods of estimating Rs from other meteorological data, particularly daily temperatures, have not worked as well in tropical and maritime areas. At Luquillo, Puerto Rico, the daily atmospheric transmittance for solar radiation was approximately equal to one minus the daily-average relative humidity (1−rhave). From these observations, we developed a model (VP-RAD) for estimation of Rs with inputs of daily maximum and minimum air temperature, daily total precipitation, mean annual temperature, mean annual temperature range, site latitude, and site elevation. 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, humid locations in the southeastern United States. Comparisons with a similar model revealed that predictions using VP-RAD had lower average errors and improved day-to-day correlation to measured solar irradiance. In a global comparison for the year 1987, VP-RAD-estimated and satellite-derived photosynthetically active radiation converged to within 1.0 MJ m−2 day−1 at 72% of the 13072 1° latitude by 1° longitude vegetated grid cells. Although these comparisons revealed areas where VP-RAD may need improvement, 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 convenient method to calculate the site-specific coefficients for this model that is widely used when solar irradiance data are not available.