Improved algorithm for global map of water vapor contents

Improved algorithm for global map of water vapor contents S.Mukai, I.Sano, M.Taniguchi(1) and Y.Okada(2) (1)Kinki University,(2)Kobe University An improved retrieval algorithm for water vapor content from satellite data is described. A POLDER sensor, mounted on the Earth observation satellite ADEOS in 1996, is a unique sensor, which can gather multi-directional (up to 14) polarization measurements of one target. The POLDER sensor is mounted on the satellite ADEOS-II launched on 14 December in 2002. Two channels in the near infrared wavelengths are used to estimate the total column water vapor content. The first channel is in the water vapor absorption band of 0.910 mm and the second is in the gas absorption-free band of 0.865 mm. In practice, a ratio of each reflectance for these two channels is used to estimate the total column water vapor content. This procedure has been proposed as a CNES/POLDER standard algorithm [Vesperini et al., 1999], and restricted to the clear-sky pixels over the land and sun-glitter pixels over the ocean. Further assumptions applied to this method are as follows; surface reflectivity is constant over the two channels, scattering by atmospheric aerosols is not significant at these channels, and the total column water vapor content is not significantly affected by the vertical profile of water vapor. This work intends to reduce the restrictions on the method mentioned above. It is shown first that the thermal data is available for retrieval of water vapor contents over the ocean given by ADEOS/OCTS or its successor, ADEOS-II/GLI. Thus the global map of water vapor contents all over the world involving the land and the whole ocean is obtained. Second spectral properties of the land surface are taken into account based on IGBP-DIS/DISCover data. It is found that an assumption of constant surface reflectivity over the two channels is not available, especially for grass-covered land. Furthermore atmospheric effect by aerosols is considered. The obtained satellite derived results are validated with the ground-based data as AERONET, ECMWF etc. Finally the obtained results of water vapor contents are compared with cloud microphysics and aerosol properties to understand the hydrologic cycles in the Earth-atmosphere surface system.