Relationship between UVB and broadband solar radiation in Spain.

ABSTRACT The daily values of UVB irradiation (290-315 nm), I_UVB, and the broadband total irradiation (300-2800 nm), I_T, measured on a horizontal plane have been correlated for the period 2000-2008 at 16 measurement sites in Spain. The results have been compared with the daily experimental values registered at the same sites during the period 2009-2011. The coefficients of determination R² obtained by applying a linear regression are higher than 0.88 for all sites and increase to 0.94 when using a quadratic regression. When all data are considered together, the values of R² are 0.91 and 0.97 for the linear and quadratic regressions, respectively. Three different clearness indices, which are dimensionless parameters, have been also used in order to reduce the uncertainties of the estimations and to reduce the local nature of the coefficients: (1) the UVB clearness index, k_TUVB = I_UVB/ I_0UVB; (2) the modified UVB clearness index, k′_TUVB and (3) the estimated UVB clearness index, k*_TUVB. The results show a strong dependence on the solar zenith angle. The bi-dimensional regression of k_TUVB as a function of the solar zenith angle and the clearness index, k_T, substantially improves the correlation, with R² values higher than 0.81 for all sites. Finally, a formula relating k_TUVB and k_T, valid for latitudes between 36°N and 44°N, has been obtained: k_TUVB = 0.064 k_T - 0.0011 θ_z - 1.24 E-05 ϕ + 7.6 E-05 λ + 0.063, where θ_z represents the solar zenith angle (in degrees), ϕ is the latitude (in degrees) of the site and λ is the distance to the sea (in km). We have compared the experimental values for the period 2009-2011 with those estimated using this expression. For this, the coefficient of determination (R²) is 0.87, with a MBD of -3.4%, a RMSE of 21% and a NRMSE of 8.7%. [ABSTRACT FROM AUTHOR]