Remote estimation of complete urban surface temperature using only directional radiometric temperatures

Complete urban surface temperature (Tc) is a key variable for quantifying the urban surface - atmosphere interactions. The estimation of Tc usually requires detailed knowledge of surface geometry and facet temperatures, which are difficult to obtain by remote sensing. Therefore, simple but efficient strategies for the remote estimation of Tc are currently lacking. Facing this challenge, we proposed a comprehensive solution to estimate Tc remotely using only the directional radiometric temperatures (DRTs) without the assistance of ground-based information. By comparing Tc with the nadir temperature (T0), DRTs within the entire hemisphere, and solid-angle integral temperature (SIT), we obtain the following results: (1) The optimal viewing azimuth and zenith angles at which the DRTs are the closest to Tc are φs±90° (φs is the solar azimuth angle) and 45-60°, respectively. (2) The estimation of Tc could be further improved once the DRTs from all-direction are available, and the MAE for the SIT that most closely approximated Tc is only 0.87 K. (3) The MAE of the SIT that best approximates Tc by combining only four DRTs and a kernel-driven model is only slightly higher than that of the specific SIT based on the original DRTs, indicating that the remote estimation of Tc using limited DRTs is plausible. These proposed strategies are generally simple and effective, and can help facilitate the estimation of Tc and therefore help advance the thermal remote sensing of urban climate and environment.