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Modeling directional anisotropy of thermal infrared measurements over a pine forest canopy

Kurz B., Lagouarde J.P., Moreau P., Guyon D., Champion I., Ogée J., Boudon F., Caraglio Y., Godin C., Pradal C.. 2006. In : Sobrino José A. (ed.). Second recent advances in quantitative remote sensing (RAQRS'II), Auditori de Torrent, Spain, 25-29 September 2006. Valence : Universitat de Valencia, p. 320-325. International Symposium on Recent Advances in Quantitative Remote Sensing. 2, 2006-09-25/2006-09-29, Valence (Espagne).

Experimental airborne thermal infrared (TIR) measurements performed over a maritime pine stand in the Landes forest in the southwest of France revealed important hot spot effects and directional anisotropy reaching up to ±2 K in summer conditions. The generalization of such results requires modelling efforts with the scope of different practical applications such as eventual angular correction of large swath satellite data, or assimilation of TIR data in forest growth models. The directional TIR anisotropy model is based on a 3D structural modelling of pine canopies combined with a ray tracing program (POV-ray). The images generated in a given viewing configuration are used to derive the percentages of sunlit and shadowed elements of the canopy; the resulting directional temperature is then computed by weighing their elementary temperatures. In a first step the 3D canopy model itself is validated against gap frequency measurements. The TIR anisotropy modelling approach is validated in a second step prescribing elementary temperatures from ground based measurements and comparing derived directional temperature against airborne data. Finally the coupling with a forest soil-vegetation model MuSICA developed at the laboratory providing the elementary temperatures at different levels inside the canopy and the possibilities opened for assimilating actual TIR satellite data are discussed.

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