The Urban Thermal Anisotropy and Its Impact on Urban Heat Storage Estimation

The urban surface temperature is an essential variable for urban energy and the urban thermal environment. However, in general in-situ or satellite surface observations measure the two dimensional (2-D) land surface temperature (LST) which does not capture the whole surface thermal energy of the urban system. This study observed a whole surface temperature of buildings in Beijing, China using iButton loggers in June 2018. The diurnal cycle of the complete surface temperature (T_c), which represents the integration of surface temperatures across the volume produced an equivalent 3-D LST of a building. Additionally, the building heat storage (Q_G) was estimated using the T_c. Results showed that: (1) The roof temperature (T_roof) is much higher than any other building surfaces with maximum differences of 18K between T_roof and north wall temperature at 12:00 local standard time; T_c is much lower than T_roof but higher than north wall temperature, and is nearly same with south and west walls values and the diurnal maximum difference between T_c and T_roof is up to 13 K. (2) The diurnal maximum roof Q_G is 325 W/m2 at 12:00 local standard time while the building Q_G using T_c is only 137 W/m2, evidence of the miss-estimation when using the 2-D LST only as proxy of the building heat storage energy. T_c may be a better choice for urban energy and urban thermal environmental studies.