Analysis of intraday urban thermal anisotropy and downward shortwave radiation through UAV experiment and coupled model

The pronounced thermal radiation anisotropy observed in rapidly urbanizing areas arises from the complex interplay of solar downward shortwave radiation (DSWR), building geometry, and local topography. To address this, we propose a novel framework, termed GRay-AU3SM-IRT, for the analysis of urban thermal anisotropy (UTA). The study was conducted in a representative mountainous city in Chongqing, China. A specific 0.4 × 0.4 km 2 area, characterized by buildings and terrain on 10° slopes, was selected for analysis at hourly resolution. The simulations showed strong agreement with measured data, achieving a root mean square error below 0.46 K and a coefficient of determination exceeding 0.79. The analysis reveals that UTA shows pronounced diurnal variations: it intensifies gradually in the morning, peaks at solar noon with clear hotspot formation, and generates secondary hotspots in the afternoon due to changing solar azimuth and slope effects. The morning striped distribution pattern caused by building geometry diminishes with declining solar irradiance and disappears by afternoon. Diurnal trends in the UTA Effect (UTAE), directional brightness temperature (DBT), DBT Effect (DBTE), and DSWR are generally aligned. Notably, UTAE and DBTE peak before DSWR, while both maximum and minimum DBT peak later. The proposed framework offers key advantages: precise multi-lap UAV navigation, minimized motion artifacts, consistent temporal sampling via adaptive velocity control, gap-filling through simulation, and spatio-temporal outlier correction. It provides critical data for angle normalization and correction across diverse urban environments.