Resumen de Characterization of the surface emissivity and temperature at heterogeneous sites for different zenith viewing angles using remote sensing
For an accurate retrieval of the land surface temperature (LST) it is required an accurate characterization of the surface emissivity. The complexity on the estimation of the canopy emissivity is higher due to the multiple reflections of the radiance (emitted and reflected) between the canopy components (soil and plant elements). In this work, several radiative transfer models (RTMs) to obtain the directional canopy emissivity were analyzed and evaluated. These RTMs were: FR97 (François et al., 1997), Mod3 (François, 2002), Rmod3 (Shi, 2011), REN15 (Ren et al., 2015), CE-P (Cao et al., 2018) and 4SAIL (Verhoef et al., 2007).
The RTMs were evaluated against in-situ emissivity measurements over a set of rose plants and two soils with different features: an organic soil with high emissivity and an inorganic soil with low emissivity. Two CIMEL Electronique CE312 radiometers were used to take in-situ radiance measurements for five narrow bands in the 8 – 14 µm spectral range. The Temperature and Emissivity Separation (TES) method was applied to obtain the in-situ directional emissivity at 7 different observation zenith angles and six different leaf area index (LAI) values. Over the sample with organic soil, no significant variation with LAI was found from the TES emissivities. Nevertheless, over the sample with inorganic soil, it was observed an increase of the emissivity with LAI. The Mod3 RTM showed the best agreement with the TES emisivities. Little variation with the viewing angle was observed for the emissivity derived from the RTMs but also for the measured emissivity.
The RTMs were used to generate directional emissivity maps over the Iberian Peninsula for a spatial resolution of 500 m. The canopy emissivity derived from the RTMs with satellite data was compared with the emissivity obtained by the MYD11A1, MYD11B1 and MYD21A1 MODIS products over the different surface types found at the Iberian Peninsula. In global terms. The Mod3 obtained the best agreement with the MYD21A1 emissivity at channels centered on 11 and 12 µm. The LST retrieved with the Mod3 RTM obtained a better result than the LST retrieved with the other RTMs or the MODIS products at the field of olives trees site. However, similar results were obtained for all the RTMs and MODIS products over the Cortes de Pallás validation site and the vineyard site.
The RTMs showed a good performance on the canopy emissivity estimation, providing good results in their application for the LST retrieval with MODIS data, which were similar or better than those obtained with the MODIS operational products data. Specifically, the MTR Mod3 showed the best performance considering the results for both in terms of emissivity and TST. These results make interesting the extension of the RMTs, especially the Mod3 RTM, to other satellite thermal sensors, since they allow estimating the canopy emissivity considering changes over time in the surface vegetation.
