2 NASA / GODDARD SPACE FLIGHT CENTER / Code 913
Greenbelt, MD, 20771 (U.S.A.)
3 NOAA CLIMATE MONITORING AND DIAGNOSTICS LABORATORY
BOULDER, COLORADO, 80303 (U.S.A.)
ABSTRACT:
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
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|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
1. INTRODUCTION
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
