REDUCTION OF BIDIRECTIONAL EFFECTS IN NOAA-AVHRR DATA ACQUIRED DURINGTHE HAPEX-SAHEL EXPERIMENT

The study presents a model of top of the atmosphere (TOA) and surface reflectances in the visible and near-IR. The reflectance is parameteri zed as the product of an isotropic component (constant) of the reflect ion and normalized temporal and bidirectional functions. The bidirecti onal function uses a simple physical representation of viewing geometr y. The temporal function is represented as a development in modified F ourier series. An iterative scheme is used to adjust the constants of the model. The analysis is applied to three locations consisting of 15 x 15 NOAA-AVHRR pixels acquired in 1992 during the HAPEX-Sahel experi ment (Niger). Clouds were screened using a threshold standard deviatio n of reflectance (visible) and mean brightness temperature (thermal Ch annel 4). The study allowed the highest frequency fluctuations in the dataset to be reduced substantially (about 85% of the variance is expl ained) and allowed the temporal variation of the land surface cover to be detected. Comparisons between results obtained with TOA and atmosp herically corrected surface reflectances show that there is a need to improve the monitoring of aerosols; however, the angular effects were the largest contributors to high-frequency fluctuations in the NOAA-AV HRR data.