Satellite and airborne optical sensors are increasingly used by scientists, and OPEN ACCESS

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We applied Weather Research and Forecasting (WRF) model's Multilayer Urban Canopy Model (MLUCM) to simulate the urban climate of Singapore during a hot period in April 2016. The high‐resolution local climate zone (LCZ) map was used as urban land use/land cover data in order to study the intraurban variability in different LCZ classes. The LCZ map for Singapore was developed by adopting the World Urban Database and Access Portal Tools (WUDAPT) methodology based on satellite remote sensing imageries and building height data. The coupled WRF/MLUCM model was validated using meteorological data from stations across Singapore. Higher index of agreement compared to observations and lower root‐mean‐squared error of 2‐m temperature, relative humidity, and global horizontal irradiance showed satisfactory model performance. A sensitivity analysis of initial and boundary conditions helped in determining the model configuration with the least error for quantifying the urban heat island (UHI) effect. The diurnal cycle and the spatial pattern of UHI were investigated, and it was found that the mean UHI intensity peaked in the early morning at 2.2 °C, reaching 3.6 °C in the Compact High Rise (LCZ1) areas. The anthropogenic heat due to indoor air conditioning was found to play a major role in all the processes studied, while the effect of the different land use types was most pronounced during nighttime and least visible near noon. The UHI circulation developed near the central catchment is found to prevent sea breezes from further propagating inland.

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Urban geometry and materials combine to create complex spatial, temporal and directional patterns of longwave infrared (LWIR) radiation. Effective anisotropy (or directional variability) of thermal radiance causes remote sensing (RS) derived urban surface temperatures to vary with RS view angles. Here a new and novel method to resolve effective thermal anisotropy processes from LWIR camera observations is demonstrated at the Comprehensive Outdoor Scale MOdel (COSMO) test site. Pixel-level differences of brightness temperatures reach 18.4 K within one hour of a 24-h study period. To understand this variability, the orientation and shadowing of surfaces is explored using the Discrete Anisotropic Radiative Transfer (DART) model and Blender three-dimensional (3D) rendering software. Observed pixels and the entire canopy surface are classified in terms of surface orientation and illumination. To assess the variability of exitant longwave radiation () from the 3D COSMO surface (3), the observations are prescribed based on class. The parameterisation is tested by simulating thermal images using a camera view model to determine camera perspectives of 3 fluxes. The mean brightness temperature differences per image (simulated and observed) are within 0.65 K throughout a 24-h period. Pixel-level comparisons are possible with the high spatial resolution of 3 and DART camera view simulations. At this spatial scale (< 0.10 m), shadow hysteresis, surface sky view factor and building edge effects are not completely resolved by 3. By simulating apparent brightness temperatures from multiple view directions, effective thermal anisotropy of 3 is shown to be up to 6.18 K. The developed methods can be extended to resolve some of the identified sources of sub-facet variability in realistic urban settings. The extension of DART to the interpretation of ground-based RS is shown to be promising. List of symbols and acronyms [units] β, ϕ, ω Euler angles describing a sequence of rotations within the (, ,) coordinate frame BOA Bottom of atmosphere BRF Bidirectional reflectance factor C Non-specific camera COSMO COmprehensive urban Scale MOdel Camera focal plane array size [mm] DART Discrete Anisotropic Radiative Transfer model (Gastellu-Etchegorry et al., 2012) DSM Digital surface model ε Emissivity ↓ Broadband longwave radiation flux (irradiance) downward from sky [W m-2 ] ↓ Broadband shortwave radiation flux (irradiance) downward from sky [W m-2 ] , ↓ 3 Broadband longwave radiation flux (exitance) from discrete points of an urban surface, resolved in 3D [W m-2 ] Camera derived broadband longwave radiation flux (exitance) [W m-2 ] Non-specific broadband longwave radiation flux (exitance) from urban canopy elements [W m-2 ]

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