Remote Sensing of Atmospheric Water Vapor

Abstract

Remote sensing of the total column water vapor (in cloud free conditions) from the proposed MODIS instrument on the NASA’s Earth Observing System requires special “water vapor” channels in the near infrared (IR). By using 2 near-IR “water vapor” channels (0.905 pm and 0.940 Irm) in addition to existing “window” channels (0.865 pm and 1.24 pm), it will be possible to derive the total column water vapor amount from MODIS data. A ratio of the measured radiance in an absorbing channel at 0.940k0.025 pm or 0.905+0.015 pm to that in a nonabsorbing channel at 0.865+0.020 pm is proposed to retrieve the column water vapor over land. LOWTRAN-7 code was used to simulate remote sensing of water vapor over 20 different surface covers. The simulation was used to optimize the water vapor channel selection and to test the accuracy of the remote sensing method. The channel selection minimizes the uncertainty in the derived water vapor due to variations in the spectral dependence of the surface reflectance. The selection also minimizes the sensitivity of the selected channels to possible drift in the channel position. In a sensitivity analysis it is shown that the error in the directly derived water vapor amount is A3%. The use of additional MODIS channels reduces the errors due to the effect of haze, subpixel clouds and uncertainties in the temperature profile to *7%. Remote sensing of the variation of water vapor from day to day will be more accurate, because the surface reflectances vary slowly with time. The method was applied to Airborne Visible Infrared Imaging Spectrometer (AVIRIS) data acquired from the NASA-ER2 aircraft and compared with other measurements.