Mapping temperature independent spectral indice of emissivity and directional emissivity in AVHRR channels 4 and 5

This work addresses the retrieval of AVHRR channel 4 and channel 5 emissivities from time series of images over two extended regions, one over Northern Africa and centred on Tunisia, the other centred on the Iberian Peninsula. The retrieval is based on the Temperature Independent Spectral Indice of Emissivity (TISIE) concept, a particular ratio of channel emissivities, which values are computed from a temperature independent combination of infrared channel radiances. This paper is focused on the use of two-channels TISIE indices relating channel 3 emissivity to either channel 4 or channel 5 emissivity. The retrieval of channel 3 directional emissivity from the data sets was achieved and discussed in Part I, whereas this paper emphasizes the determination and characteristics of the TISIE indices. These indices are found to be, within experimental error, (i) temperature independent; (ii) independent of zenithal view angle; (iii) stable over the whole period for a given surface type, allowing for building pixel-wise mean temporal indices. These mean values are then used to determine and model the directional emissivities in both channel 4 and 5. Emissivities exhibit little spectral and angular variations for surfaces covered with vegetation, with values higher than 0.96. Over arid areas (bare soils), emissivities are mostly less than 0.95, slightly higher in channel 5 than in channel 4, and exhibit significant angular variation, up to 3% decrease from nadir to 60° for low emissivity surface. Results suggest that TISIE indices may have a good potential for discriminating between different bare soil types.