Water vapor retrievals using Moderate Resolution Imaging Spectroradiometer (MODIS) near‐infrared channels

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Abstract
At present, two Moderate Resolution Imaging Spectroradiometer (MODIS) instruments on board the NASA Terra and Aqua Spacecraft platforms are operational for global remote sensing of the land, ocean, and atmosphere. In this paper, we describe an algorithm for water vapor derivations using several MODIS near‐IR channels. The derivations are made over areas that have reflective surfaces in the near‐IR, such as clear land areas, clouds, and oceanic areas with Sun glint. The algorithm relies on observations of water vapor attenuation of near‐IR solar radiation reflected by surfaces and clouds. Techniques employing ratios of water vapor absorbing channels centered near 0.905, 0.936, and 0.940 μm with atmospheric window channels at 0.865 and 1.24 μm are used. The ratios partially remove the effects of variation of surface reflectance with wavelengths and result in the atmospheric water vapor transmittances. The column water vapor amounts are derived from the transmittances based on theoretical calculations and using lookup table procedures. Typical errors in the derived water vapor values are in the range between 5% and 10%. The daily “pixel‐based” near‐IR water vapor product, which is a standard MODIS level 2 data product, at the 1‐km spatial resolution of the MODIS instrument, and the daily, 8‐day, and monthly near‐IR water vapor products, which are standard MODIS level 3 products, at a 1° by 1° latitude‐longitude grid globally are now routinely produced at a NASA computing facility. We present samples of water vapor images and comparisons to ground‐based measurements by microwave radiometers.

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