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

Two simple techniques to retrieve path precipitable water from the Air borne Visible/Infrared Imaging Spectrometer (AVIRIS) (Vane et al., 199 3) high spectral resolution radiance data (Continuum interpolated Band Ratio, CIBR, and Narrow/Wide ratio, NIW), using the 940 nm water abso rption band, are compared. Since the shape and depth of the atmospheri c water bands are influenced not only by the water present but also by surface (background) reflectance, atmospheric scattering, and instrum ent radiance by calibration, a sensitivity analysis was performed usin g the radiative transfer code LOWTRAN 7 (Kneizys et al., 1988) to dete rmine which one of these two approaches will provide a better estimate over land and water areas. The CIBR proved to be the technique less s ensitive to perturbing effects, except for errors in visibility estima te. 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. Validat ion was performed through comparison between an independent estimate O f water vaporfrom concurrent Reagan sunphotometer measurements and AVI RIS estimates. Amounts retrieved using the N/W approach match more clo sely in situ measurements, even after adjusting model parameters for b ackground reflectance, viewing geometry and type Of aerosol at the sit e. The 13% underestimation observed for the CIBR was explained by smal l differences AL(lambda(i)) between AVIRIS and LOWTRAN 7 modeled radia nces. Resultsfrom this study emphasizes the importance Of accurate ins trument calibration in flight and correct physical modeling of atmosph eric absorptions.