In situ surface temperature retrieval in a boreal forest under variable cloudiness conditions

Abstract

Canopy temperature retrieval was one of the purposes during the Solar Induced FLuorescence EXperiment (SIFLEX‐2002) of the European Space Agency, carried out in a Finnish boreal forest. In this work, we describe the strategy used to determine this temperature from ground thermal infrared (TIR) data under skies with variable cloud cover. TIR radiance was measured by a CIMEL Electronique CE 312 radiometer. An analysis of the radiative transfer equation showed which terms were necessary to obtain accurate surface temperatures during the campaign. Atmospheric correction was considered negligible due to the small atmospheric path, but hemispheric downwelling sky radiance determination was needed for the emissivity correction. Since most days during the campaign the sky showed partial cloudiness, a methodology to estimate this last term was proposed, using continuous information of cloudiness amount and cloud height given by a weather station. These thermal data were used to analyse some correlations between canopy and air temperatures and plant‐activity‐related variables in the context of the SIFLEX‐2002 campaign.

Acknowledgement

This work was supported by the European Space Agency through the SIFLEX‐2002 project (ESA/ESTEC Contract No. 16026/02/NL/SF), the Spanish Ministerio de Ciencia y Tecnología by means of Contracts REN2001‐3116/CLI and ‘Ramon y Cajal’ of Dr. E. Valor, the European Union (FEDER funds) and the Generalitat Valenciana (project GV2004‐B‐084). The Spanish Ministerio de Educación, Cultura y Deportes is acknowledged for the research grant received by Dr. R. Niclòs, and also the University of Valencia for the grant ‘V Segles’ to J. M. Sánchez and the current contract of Dr R. Niclòs (Ministerio de Educación y Ciencia, CGL2004‐06099‐C03‐01/CLI). The authors also wish to thank the Finnish Meteorological Institute (FMI, Helsinki, Finland) and the Laboratoire pour l'Utilisation du Rayonnement Electromagnétique (LURE, Orsay, France) for their essential contributions during campaign development, and for the weather, PRI and PAR data used in the study.