Intercomparison and combination of low-cost urban air temperature measurement approaches [Dataset]

Measurements of urban air temperatures (Tair) are vital to successful adaptation and mitigation policies to increasing urban heat stress. However, in-situ measurements in cities are often scarce and costly, and therefore low-cost approaches are increasingly used to study urban Tair. This allows for inexpensive, yet still highly spatially and temporally resolved observations of urban Tair. Despite their merits, a common issue of such low-cost approaches is lacking data quality and potential measurement errors. Here, we compare three low-cost measurement approaches regarding their ability to capture intra-urban variability of Tair over a period of 24 hours in Bern, Switzerland: a) A network of 79 low-cost measurement devices (LCD), b) bicycle mounted mobile measurement (BCY) transects, and c) 581 Netatmo citizen weather stations (CWS). As the BCY sensor is actively ventilated, it is used as the reference for intercomparisons with LCD and CWS. Compared to the BCY, the median difference of Tair for LCD is found to be slightly negative over the entire study period (-0.07 K) as well as during night-time (-0.09 K), and positive during daytime (0.07 K). The CWS show a positive median difference of 0.66 K over the entire study period and 0.98 K during night-time, and a negative difference of -0.18 K during daytime. BCY and LCD both show a distinct pattern of nocturnal intra-urban Tair variability, which is less pronounced in the CWS measurements. Furthermore, the intercomparison of the three approaches across local climate zones reveals that CWS do not well represent forested areas. Whilst the bias sources of the individual approaches require in-depth investigation in future studies (e.g., external heat sources and measurement height for BCY, daytime short-wave radiation errors for LCD, and nocturnal thermal heating by nearby buildings for CWS), we conclude that the combination of the three measurement approaches provides a high observation density in urban environments at low costs.城市气温（Tair）的测量，对于应对日益加剧的城市热应激的适应与减缓政策的有效实施至关重要。然而，城市中的原位气温测量往往稀缺且成本高昂，因此低成本测量方法正日益被用于城市气温研究。此类方法能够以较低成本获取高精度时空分辨率的城市气温观测数据。尽管此类方法具备优势，但普遍存在数据质量不足与潜在测量误差的问题。本研究以瑞士伯尔尼为研究区域，对比了三种低成本测量方法在24小时周期内捕捉城市内部气温变异性的能力：a）79台低成本测量设备（LCD）组成的观测网络；b）搭载于自行车的移动测量断面（BCY）；c）581个Netatmo市民气象站（CWS）。由于BCY传感器采用主动通风设计，因此将其作为与LCD和CWS进行比对的参考基准。相较于BCY，LCD的气温中位数差值在整个研究周期内呈轻微负值（-0.07 K），夜间同样为负值（-0.09 K），日间则为正值（0.07 K）。CWS的气温中位数差值在整个研究周期内为正值（0.66 K），夜间达0.98 K，日间则为-0.18 K。BCY与LCD均呈现出清晰的夜间城市内部气温变异性特征，而CWS的观测结果中该特征则相对不显著。此外，通过在不同局地气候区中对三种方法进行比对，结果显示CWS无法很好地反映林区的气温情况。尽管三种方法各自的偏差来源仍需在未来研究中开展深入探究——例如BCY的外部热源与测量高度问题、LCD的日间短波辐射误差问题，以及CWS受周边建筑夜间热辐射影响的问题——但本研究最终认为，三种测量方法的结合能够以低成本在城市环境中实现高观测密度。