Surface UV Radiation in the South of Portugal: Monitoring and Assessment of Cloud Effects

Abstract

M easurements of both UV B (280 – 315 nm) and UV AB (280 – 400 nm) irradiances taken during about seven years are presented and analyzed here . The UV irradiance data were measured with MACA M radiometers, which are installed in the Atmospheric Physics Observatory of the University of Évora Geophysics Center - CGE (38°34’N, 7°54’W, 300 m above mean sea level) since 200 4 . Special attention is devoted to calibration issues, since the radiometers were calibrated in Spain, at “El Arenosillo” ESAt/INTA laboratory in 2009 and a methodology based on radiative transfer calculations combined with observations from atmospheric quantities, was developed and applied to retro - calib rate the data from 2009 to 2004 . The comparison between the radiative transfer based and the laboratory calibration methodologies yielded a quite promising outcome, with normalized root mean square errors lower than 3 % and mean absolute percentage error s lower than 2 %. Cloud opti cal thickness values derived from ground - based spectral irradiance measurements taken at the CGE observatory, are also used in order to investigate the influence of this cloud quantity o n UV irradiance s .