【摘要】 城市热辐射方向性限制了遥感地表温度反演精度,阻碍了对全球城市气候(如城市地表能量通量估算和城市热环境评估)的准确理解.然而,全球不同气候背景城市热辐射方向性时空特征及其造成的城市地表能量通量和热岛强度遥感估算偏差仍不清楚,在全球尺度是否存在与城市热辐射方向性密切相关且易获取的地表和气候参量尚不明晰.本研究结合多角度卫星观测地表温度和遥感云平台,分析了全球5500多个城市的热辐射方向性特征,量化了其造成的全球城市地表能量通量和热岛强度遥感估算偏差,厘清了全球尺度城市热辐射方向性与关键地表和气候参量的关系.研究发现全球城市平均热辐射方向性强度在夏季白天下午高达5.4 K.在极端情况下,当地表温度观测天顶角位于±60°时,热辐射方向性使得全球城市地表显热通量和热岛强度分别低估45.4%和43.0%.研究进一步发现全球城市热辐射方向性与地表不透水面百分比和空气温度具有良好的统计关系,这为卫星遥感城市地表温度角度纠正提供了新思路.更多还原

【Abstract】 Urban thermal anisotropy(UTA) drastically impacts satellite-derived urban surface temperatures and fluxes, and consequently makes it difficult to gain a more comprehensive understanding of global urban climates. However, UTA patterns and associated biases in observed urban climate variables have not been investigated across an adequate number of global cities with diverse contexts; nor is it known whether there are globally measurable factors that are closely related to the UTA intensity(UTAI, quantified as the maximum difference between the nadir and off-nadir urban thermal radiation). Here we investigate the UTAI over more than 5500 cities worldwide using multi-angle land surface temperature(LST) observations from 2003 to 2021 provided by Moderate Resolution Imaging Spectroradiometer(MODIS). The results show that the global mean UTAI can reach 5.1, 2.7, 2.4, and 1.7 K during summer daytime, winter daytime, summer nighttime, and winter nighttime, respectively. Using nadir LST observations as a reference, our analysis reveals that UTA can lead to an underestimation of satellite-based urban surface sensible heat fluxes(H) by 45.4% and surface urban heat island intensity(Is) by 43.0% when using LST observations obtained from sensor viewing zenith angles(VZAs) of ±60°. Practitioners can limit the biases of H and Iswithin ±10% by using LSTs from sensor VZAs within ±30°. We also find that UTAI is closely related to urban impervious surface percentage and surface air temperature across global cities. These findings have implications for angular normalization of satellite-retrieved instantaneous LST observations across cities worldwide.更多还原