Experimental characterization and modelling of the nighttime directional anisotropy of thermal infrared measurements over an urban area: Case study of Toulouse (France)

Abstract

The measurements of surface temperature are prone to important directional anisotropy in relation to the structure of the urban canopy and the radiative and energy exchanges inside it. Following the work of Lagouarde et al. 2010 that describes the daytime conditions, this paper focuses on the experimental analysis and modelling of the nighttime directional anisotropy. An extensive data set of airborne thermal infrared (TIR) measurements was collected over the city of Toulouse, France in the framework of the CAPITOUL project (Masson et al., 2008). The TIR measurements use a pair of thermal cameras equipped with wide angle lenses installed aboard a small aircraft. Three flights were made between 21:45 and 23:15 UTC, one in autumn and 2 in winter during 2004 and 2005 intensive operation periods (IOPs). The experimental results show that (i) the nighttime TIR directional anisotropy remains lower than 1°C for zenithal view angles up to 50°, and (ii) the nighttime anisotropy is insensitive to the azimuthal viewing direction. A modelling approach that combines a simplified 3D representation of the urban canopy with 2 energy transfer models, TEB and SOLENE, is then proposed. It confirms the experimental results quite well. Possible uncertainties on the geometry of the canopy and on most important thermo-radiative surface parameters are shown to have only little impact on the modelled TIR directional anisotropy. The time evolution of TIR anisotropy throughout night is simulated: for the fall and winter conditions studied the effects of thermal inertia completely vanish about 3 to 4 h after sunset, and the anisotropy never exceeds 2°C up to 60° zenithal view angles.