Trees are the most important natural factor to alleviate the urban heat island effect and the latent heat flux (LE) they release contributes significantly to urban cooling. In this study, the model ENVI-met was used to study the influence of building geometry on the LE exchanged by trees at the block scale in compact urban areas. The building density (BD), building height (BH) and sky view factor (SVF) were used to characterize building geometry. The sensitivity of LE to building geometry was estimated by multi-linear regression analysis. The following conclusions were drawn:(1) the LE of trees is sensitive to building geometry, higher during daytime than night-time in winter and the higher during night-time than daytime in summer; (2) LE changes as expected across the seasons, with LE larger in summer; (3) The shade affects the LE of a tree by influencing the solar irradiance; (4) In winter the average LE of a single tree is larger with 0.06 than 0.12 fractional vegetation cover (FVC). This study can provide useful leads towards further research to explore latent heat exchanges by trees at the street scale.

Thermal remote sensor data have been widely used in urban climate and environmental research. The urban geometry, however, hinders a nadir-looking radiometer to observe all urban facets, which makes the observed urban radiometric surface temperature (Tr) different from the urban complete surface temperature (Tc). The T c of all urban facets conveys complete information on the response of the urban surface to radiometric and convective forcing. In this chapter, we firstly examined the limitations of three current most popular surface temperature retrieval methods for radiometric surface temperature retrieval. Then, we discussed a methodology using precisely designed numerical experiments through an urban micro-climate model to help understand thermal radiative transfer within the built-up space and the relationship between observed T r and T c. Specifically, T r is firstly retrieved from radiance observed at the top of atmosphere over urban areas by applying corrections for atmospheric and emissivity effects. Numerical experiments are designed to evaluate different combinations of urban geometry with the configuration of radiometric observations to construct the relationships between T r and T c. Finally, T c is estimated by using such relationships. T c can then be used for urban climate and environment research. We further present a case study to demonstrate the methodology discussed above, which was based on the estimation of T c from satellite TIR data under the condition that the urban areas have no vegetation or negligible vegetation. Lastly, we discussed some challenges that need to be addressed in the future for improving the applicability of thermal infrared remote sensing in urban areas.

Sensible heat exchange has important consequences for urban meteorology and related applications. Directional radiometric surface temperatures of urban canopies observed by remote sensing platforms have the potential to inform estimations of urban sensible heat flux. An imaging radiometer viewing the surface from nadir cannot capture the complete urban surface temperature, which is defined as the mean surface temperature over all urban facets in three dimensions, which includes building wall surface temperatures and requires an estimation of urban sensible heat flux. In this study, a numerical microclimate model, Temperatures of Urban Facets in 3-D (TUF-3D), was used to model sensible heat flux as well as radiometric and complete surface temperatures. Model data were applied to parameterize an effective resistance for the calculation of urban sensible heat flux from the radiometric (nadir view) surface temperature. The results showed that sensible heat flux was overestimated during daytime when the radiometric surface temperature was used without the effective resistance that accounts for the impact of wall surface temperature on heat flux. Parameterization of this additional resistance enabled reasonably accurate estimates of urban sensible heat flux from the radiometric surface temperature.