Actualization of Semi-Empirical Equation for UV Index

Abstract UV radiation at ground level undergoes many variations in the present, due to the ozone column, aerosols, clouds, surface reflectivity, both positive and negative variations, depending on latitude. This work reports on the updating of the semi-empirical equation proposed by Madronich in 2007 for estimating the UV index, by updating its coefficients, as they encapsulate the state of the atmosphere. Data for Bucharest in 2019-2021 was used for fitting the equation. It was found that the new equation improves the accuracy of the estimation of the UVI in Bucharest five times for 2021 (from nRMSE = 17.3% to nRMSE=3.3%). The validity of the obtained equation was assessed in various locations, showing a great improvement of the estimation accuracy in most of them (in 12 of the 14 chosen locations), regardless of latitude. The equation produces a negative bias for locations having Koppen climate types A and B, a positive bias for the investigated locations situated in climate type D and no definite bias for temperate climates (type C). More studies are needed for drawing a definite conclusion and further improving the applicability of the equation.