Full text: Mesures physiques et signatures en télédétection

Greenbelt, MD, 20771 (U.S.A.) 
Stratospheric aerosols produced by the eruption of Mount Pinatubo in the Philippines (6 June, 1991) have 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 0.5pm. 
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 
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|im) and channel 2 (0.83pm) 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, non corrected, and years prior to Pinatubo eruption (1989,1990) NDVI composite, shows the 
necessity and the accuracy of the operational correction scheme. 1 
The importance of remote sensing to provide a quantitative estimate of Earth resources and to monitor global 
change has been well demonstrated (Becker et al, 1988; Gatlin et al, 1983; Justice et al,1985). The global 
vegetation index derived from the NOAA Advanced Very High Resolution Radiometer (AVHRR) gave the first 
means for scientists to study large scale natural cycles of vegetation and carbon (Tucker et al, 1985,1985). 
Research to clarify the limitations of the AVHRR are in progress (Townshend and Justice, 1988, Kaufman et al, 
1992 ;Holben et al, 1992). For example, the importance of perturbations induced by tropospheric aerosol 
particles has been clearly demonstrated and that subsequent reduction through compositing is being understood 
(Kaufman et al,1992; Tanrd et al,1992). However, the effect of the presence of a large amount of aerosols in the 
stratosphere on the NDVI has never been assessed. Here, we present an operational method of correction using 
AVHRR data itself for derivation of aerosol optical depth. 
On June 6 , 1991 Mt. Pinatubo, in the Philippines, erupted. It injected approximately two to three 
times more SO 2 into the stratosphere than the El Chichon eruption as estimated by SAGE (Me Cormick and 
Veiga,1992). Within a short time, the application of NDVI data for operational drought monitoring as part of

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