The UV-A and visible solar irradiance spectrum: inter-comparison of absolutely calibrated, spectrally medium resolution solar irradiance spectra from balloon- and satellite-borne measurements

摘要

Within the framework of the ENVISAT/-SCIAMACHY satellitevalidation, solar irradiance spectra are absolutely measured atmoderate resolution in the UV/visible spectral range (in the UVfrom 316.7-418 nm and the visible from 400-652 nm at a fullwidth half maximum resolution of 0.55 nm and 1.48 nm,respectively) from aboard the azimuth-controlled LPMA/DOAS balloongondola at around 32 km balloon float altitude. After accountingfor the atmospheric extinction due to Rayleigh scattering andgaseous absorption (O₃ and NO₂), the measured solar spectraare compared with previous observations. Our solar irradiancespectrum perfectly agrees within +0.03% with the re-calibratedKurucz et al. (1984) solar spectrum (Fontenla et al., 1999, calledMODTRAN 3.7) in the visible spectral range (415-650 nm), but itis +2.1% larger in the (370-415 nm) wavelength interval, and-4% smaller in the UV-A spectral range (316.7-370 nm), whenthe Kurucz spectrum is convolved to the spectral resolution of ourinstrument. Similar comparisons of the SOLSPEC (Thuillier et al.,1997, 1998a, b) and SORCE/SIM (Harder et al., 2000) solar spectrawith MODTRAN 3.7 confirms our findings with the values being-0.5%, +2%, and -1.4% for SOLSPEC -0.33%, -0.47%, and -6.2%for SORCE/SIM, respectively. Comparison of the SCIAMACHY solarspectrum from channels 1 to 4 (- re-calibrated by the Universityof Bremen -) with MODTRAN 3.7 indicates an agreement within-0.4% in the visible spectral range (415-585 nm), -1.6% withinthe 370-415 nm, and -5.7% within 325-370 nm wavelength interval, in agreementwith the results of the other sensors. In agreement withfindings of Skupin et al. (2002) our study emphasizes that thepresent ESA SCIAMACHY level 1 calibration is systematically +15%larger in the considered wavelength intervals when compared to allavailable other solar irradiance measurements.