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 leve l UV irradiance, a combination of spectral measurements and model calc ulations is required. We show an efficient method for such a combinati on 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 al bedo. More than 1200 spectra have been used for the comparison, coveri ng a wide range of ozone and aerosol conditions. Applying the PSEUDO-S PHERICAL model type, a discrete ordinate model with correction for the sphericity of the Earth, the systematic differences between measureme nt and model were found to range between -11 and +2% for wavelengths b etween 295 and 400 nm and solar zenith angles up to 80 degrees. The sm all observed statistical differences of 2-3% can mostly be explained b y the random error of the measurement system. Only two input parameter s total ozone column and aerosol optical depth, the latter parameteriz ed by the Angstrom formula, are required to reach this level of agreem ent. 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, provide s an efficient tool for the investigation of the processes that contro l surface UV irradiance.