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Quasi-synchronous multi-angle observations of urban surface temperature (UST) are critical for understanding the urban thermal anisotropy (UTA) regime and validating UTA models. Such observations can be performed by thermal sensors onboard lightweight unmanned aerial vehicles (UAVs), due to their flexibility in sampling from different directions. This study designed a straightforward and efficient protocol to obtain quasi-synchronous multi-angle USTs based on a lightweight UAV for a typical residential area in Nanjing (China). By using this protocol, we retrieved a directional brightness temperature (DBT) dataset with 73 DBTs 12 times throughout the diurnal cycle. Our results exhibited that the diurnal UTA intensity (UTAI) well corresponds to that of air temperature: the maximum UTAI (14.0 °C) occurred around 14:00 to 15:00 local time, and the minimum (3.5 °C) occurred at night. Interestingly, the nighttime DBT variations were found to depend both on viewing zenith and azimuth angles, and a slight hotspot effect was identified. These findings notably differ from previous studies based on modeling, which often assumed the independence of the nighttime DBT of the viewing azimuth angle and the absence of the hotspot effect. The analogous pattern of UTA between the daytime and nighttime (especially before midnight) was seemingly driven by a remarkable temperature contrast among urban surface components that can persist for many hours after sunset. Thus, the designed observation protocol can be applied to retrieve quasi-synchronous multi-angle USTs for other types of urban surfaces, thereby facilitating a complete understanding of diurnal UTA variations and assisting the validations of UTA models. • UAV-based protocol was designed to sample quasi-synchronous DBTs. • Quasi-synchronous multi-angle DBTs were obtained over Nanjing. • DBT dataset was generated for interpreting diurnal UTA variations. • Nighttime DBT varied with both viewing zenith and azimuth angles. • Slight hotspot effect was identified for UTA before/around midnight. [ABSTRACT FROM AUTHOR]

Interpretation of diurnally continuous dynamics of surface urban heat island intensities (SUHIIs) of various local climate zones (LCZs) is essential for deepening the understanding of the urban climate. However, an easily applicable method for quantifying diurnally continuous dynamics of LCZ-based SUHIIs is still lacking, and the understanding of the diurnally continuous patterns of LCZ-based SUHIIs remains largely incomplete. To alleviate these gaps, here we developed a simple yet effective method for producing diurnally continuous land surface temperatures (LSTs) of various LCZ types based on an LCZ map and MODIS LSTs. With the proposed method, we further investigated the diurnally continuous dynamics of LCZ-based SUHIIs in Nanjing, China. We obtained the following findings: First, in terms of the annual mean, the diurnal SUHII patterns among various built LCZ types were similar and had a small valley within 0–2 h after sunrise and a peak within 8–12 h after sunrise. Second, the midrise/high-rise built and heavy industry types (i.e., LCZ 4, 5, and 10) exhibited relatively high SUHIIs during a diurnal cycle and the associated daily duration with a significant SUHII (>0.5 K) was greater than 20 h. Third, the diurnal SUHII patterns in summer were analogous to the annual mean patterns for all built LCZ types, yet the patterns of several built LCZ types in winter contrasted significantly the annual mean ones. Our findings are valuable for deepening the understanding of the diurnally continuous dynamics of LCZ-based SUHIIs. • A method for investigating diurnal dynamics of LCZ-based SUHII was proposed. • Diurnal patterns of annual mean SUHII of various built LCZ types were similar. • Daily duration with a significant SUHII >20 h occurs for LCZ 4, 5, and 10. • Diurnal SUHII patterns of various built LCZ types exhibited large seasonality. [ABSTRACT FROM AUTHOR]