Evaluation of star in the UV region for the application of GCOM-A1/ODUS

Radiative transfer code plays an important role in climate study and remote sensing by acting as a tool for calculating the radiation received by the land surface and by remote sensors. Though there are many radiative transfe r codes available, little work has been done to compare them. The Ozone Dyn amics Ultraviolet Spectrometer (ODUS) is one of the successors to TOMS and is the core instrument for GCOM-A1 project. ODUS is a spectrometer to measu re the ultraviolet (UV) band from; 306 to 420 nm with a spectral step of 0. 5 nm. The STAR code is one of the candidate codes for the ODUS retrieval al gorithm and was compared with MODTRAN4.0 to test its accuracy. The comparis on revealed major discrepancies between the original STAR and MODTRAN4.0. A fter the solar irradiance data in STAR was modified and the wavenumber was used for solar irradiance calculation, the relative differences that appear like spikes in the spectral domain were greatly reduced. However, relative differences in the UV-B band are still a bit large (up to +/- 6%) and spik es remain. Examining these two codes, we found that the ozone cross section data used in STAR differs from that in MODTRAN4.0 due to a 15 cm(-1) shift toward higher wavenumbers. The relative differences between STAR and MODTR AN4.0 were reduced to 2% for wavelengths exceeding 310 nm. For wavelengths shorter than 310 nm, however, the differences increased as wavelengths decr eased and reached 5.5% at 300 nm. This resulted from dividing by small radi ances because of strong ozone absorption. Increasing the number of atmosphe ric layers from 36 to 50 in MODTRAN4.0 resulted in differences of less than 2% for wavelengths exceeding 306 nm and 4.5% at 300 nm. The STAR code uses a fitting formula for Rayleigh optical depth calculation that may signific antly influence the precision in the UV band. It is modified by using an ac curate calculation of the Rayleigh optical depth instead. The relative diff erences from the fitting formula and the accurate calculation of the Raylei gh optical depth can be as large as 0.5% at 300 nm. (C) 2001 Elsevier Scien ce Ltd. All rights reserved.