Estimation Of Aerosol Optical Thickness Research Articles

Estimation of Aerosol Optical Thickness from ADEOS/AVNIR Image Data and Its Atmospheric Correction

Estimation of Aerosol Optical Thickness from ADEOS/AVNIR Image Data and Its Atmospheric Correction

Development, validation, and potential enhancements to the second‐generation operational aerosol product at the National Environmental Satellite, Data, and Information Service of the National Oceanic and Atmospheric Administration

A revised (phase 2) single‐channel algorithm for aerosol optical thickness, τSATA, retrieval over oceans from radiances in channel 1 (0.63 μm) of the advanced very high resolution radiometer (AVHRR) has been implemented at the National Oceanic and Atmospheric Administration's National Environmental Satellite Data and Information Service for the NOAA 14 satellite launched December 30, 1994. It is based on careful validation of its operational predecessor (phase 1 algorithm), implemented for NOAA 11 in 1989. Both algorithms scale the upward satellite radiances in cloud‐free conditions to aerosol optical thickness using an updated radiative transfer model of the ocean and atmosphere. Application of the phase 2 algorithm to three matchup Sun‐photometer and satellite data sets, one with NOAA 9 in 1988 and two with NOAA 11 in 1989 and 1991, respectively, show systematic error is less than 10%, with a random error of στ≈0.04. First results of τSATA retrievals from NOAA 14 using the phase 2 algorithm, and from checking its internal consistency, are presented. The potential two‐channel (phase 3) algorithm for the retrieval of an aerosol size parameter, such as the Junge size distribution exponent, by adding either channel 2 (0.83 μm) from the current AVHRR instrument, or a 1.6‐μm channel to be available on the Tropical Rainfall Measurement Mission and the NOAA‐KLM satellites by 1997 is under investigation. The possibility of using this additional information in the retrieval of a more accurate estimate of aerosol optical thickness is being explored.

Data pre‐processing: Stratospheric aerosol perturbing effect on the remote sensing of vegetation: Correction method for the composite NDVI after the Pinatubo eruption

Stratospheric aerosols produced by the eruption of Mount Pinatubo in the Philippines (6 June, 1991) had a detectable effect on NOAA AVHRR data. Following the eruption, a longitudinally homogeneous dust layer was observed between 20°N and 20°S. The largest optical thickness observed for the dust layer was 0.4–0.6 at 0.5 pm. The amount of aerosols produced by Mount Pinatubo was two to three times greater than that produced by El Chichon and the Stratospheric Aerosol and Gas Experiment (SAGE) on‐board the Earth Radiation Budget Experiment was not able to give quantitative estimate of aerosol optical thickness because of saturation problem. The monthly composite Normalized Difference Vegetation Index (NDVI) (generally bounded between ‐0.1 and 0.6) has systematically decreased by approximately 0.15 two months after the eruption. Such atmospheric effect has never been observed on composite product and is related to the persistence and spatial extent of the aerosol layer causing the composite technique to fail. Therefore, long term monitoring of vegetation using the NDVI necessitates correction of the effect of stratospheric aerosols. In this paper we present an operational stratospheric aerosol correction scheme adopted by the Laboratory for Terrestrial Physics, NASA/GSFC. The stratospheric aerosol distribution is assumed to be only variable with latitude. Each 9 days the latitudinal distribution of the optical thickness is computed by inverting radiances observed in AVHRR channel 1 (0.63 μm) and channel 2 (0.83 μm) over the Pacific Ocean. This radiance data set is used to check the validity of model used for inversion by checking consistency of the optical thickness deduced from each channel as well as optical thickness deduced from different scattering angles. Using the optical thickness profile previously computed and radiative transfer code assuming lambertian boundary condition, each pixel of channel 1 and 2 are corrected prior to computation of NDVI. Comparison between corrected, not corrected, and years prior to Pinatubo eruption (1989, 1990) NDVI composite, shows the necessity and the accuracy of the operational correction scheme.