Systematic long-term comparison of spectral UV measurements and UVSPEC modeling results

For the evaluation of radiative transfer models and for investigations on the influence of parameters like aerosols or clouds on ground level UV irradiance, a combination of spectral measurements and model calculations is required. We show an efficient method for such a combination and present a systematic comparison of the freely available UVSPEC radiative transfer model package with two years of spectrally resolved measurements made at Garmisch-Partenkirchen, Germany (47.48 degrees N, 11.07 degrees E, 730 m above sea level) for cloudless sky and low albedo. More than 1200 spectra have been used for the comparison, covering a wide range of ozone and aerosol conditions. Applying the PSEUDO-SPHERICAL model type, a discrete ordinate model with correction for the sphericity of the Earth, the systematic differences between measurement and model were found to range between -11 and +2% for wavelengths between 295 and 400 nm and solar zenith angles up to 80 degrees. The small observed statistical differences of 2-3% can mostly be explained by the random error of the measurement system. Only two input parameters total ozone column and aerosol optical depth, the latter parameterized by the Angstrom formula, are required to reach this level of agreement. It was further found that knowledge of the aerosol optical depth is essential for obtaining such a good agreement. The evaluated UVSPEC model package, together with the presented interface SDMODEL, provides an efficient tool for the investigation of the processes that control surface UV irradiance.