Recovery of atmospheric water vapor total column abundance from imaging spectrometer data around 940 nm — sensitivity analysis and application to Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) data

Abstract

Two simple techniques to retrieve path precipitable water from the Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) (Vane et al., 1993) high spectral resolution radiance data (Continuum Interpolated Band Ratio, CIBR, and Narrow/Wide ratio, N/W), using the 940 nm water absorption band, are compared. Since the shape and depth of the atmospheric water bands are influenced not only by the water present but also by surface (background) reflectance, atmospheric scattering, and instrument radiance by calibration, a sensitivity analysis was performed using the radiative transfer code LOWTRAN 7 (Kneizys et al., 1988) to determine which one of these two approaches will provide a better estimate over land and water areas. The CIBR proved to be the technique less sensitive to perturbing effects, except for errors in visibility estimate. Both techniques were applied to AVIRIS radiance data acquired over Salton Sea, California. Resulting images confirmed that the use of a constant gray reflectance in the model led to a higher overestimation of the amount of water retrieved for N/W over vegetated areas. Validation was performed through comparison between an independent estimate of water vapor from concurrent Reagan sunphotometer measurements and AVIRIS estimates. Amounts retrieved using the N/W approach match more closely in situ measurements, even after adjusting model parameters for background reflectance, viewing geometry and type of aerosol at the site. The 13% underestimation observed for the CIBR was explained by small differences ΔL(λ i) between AVIRIS and LOWTRAN 7 modeled radiances. Results from this study emphasizes the importance of accurate instrument calibration in flight and correct physical modeling of atmospheric absorptions.