Geophysical parameters derived from thermal infrared AVHRR data

The definition and the measurement from space of the surface emissivity and temperature over heterogeneous surfaces raise several difficulties addressed in the first part of this paper. It is shown, in the second part of the paper, that AVHRR is suitable to solve these difficulties using the mid-infrared channel (channel 3 at 3.7 μm): Thanks to a particular intercomparison between day and night images based on the concept of TISI, the bi-directional reflectivity in channel 3 and its angular variations are determined by extracting the contribution of solar reflection occurring in this channel are modeled. Atmospheric corrections are performed using MODTRAN code and atmospheric data extracted from outputs of ARPEGE, a GCM developed by Meteo-France. The bi-directional reflectivity in channel 3 is calculated with errors of the order of ±0.01. The emissivity in channel 3 defined as (1-reflectivity) is then calculated with accuracy better than 3% thanks to an angular integration of the bidirectional reflectivity based on a realistic model of angular variations. Using the spectral properties of TISI, emissivities in channels 4 and 5 are extracted. The surface temperature could be retrieved with these emissivities by a “local split window”. This methodology is applied to images covering a Northern Africa region and the Iberian Peninsula. Emissivities in channels 3, 4 and 5 are shown for different view angles and their angular characteristics are studied