Effects of heat storage on urban thermal environment and surface energy balance by scaled outdoor experiments

Abstract

Heat storage ($\Delta Q_S$) is a crucial component of the urban surface energy balance (USEB) and urban thermal environment. However, the estimation of $\Delta Q_S$ in real urban environments is associated with significant uncertainty. As a novelty, we conducted a scaled outdoor experiment to investigate the impact of $\Delta Q_S$ on the urban thermal environment and USEB under different sky conditions. The experiment performed a comparative analysis using Case Water (water models with high thermal mass) and Case Hollow (hollow models with low thermal mass). Compared with Case Hollow, the daily heat storage in Case Water increases by 0.26 MJm⁻² day⁻¹, resulting in a lower daily temperature range (DTR). The difference in DTR between Case Water and Case Hollow is 3.6 °C (clear sky), 2.8 °C (partially cloudy sky), and 1.4 °C (cloudy sky). As the cloud cover increases, the effect of increased $\Delta Q_S$ in reducing DTR becomes less significant. The impact of sky conditions on $\Delta Q_S$/$Q^{*}$ (heat storage/net radiation) is minimal (37.5 %, 37.8 %, 35.6 % for clear sky, partially cloudy sky and cloudy sky, respectively). Furthermore, $\Delta Q_S$/$Q^{*}$ shows a clear negative correlation with friction velocity (u_*), with the influence of u_* becoming more pronounced as the cloud cover increases. u_* explains approximately 30 % of the variation in $\Delta Q_S$/$Q^{*}$. This study enhances our understanding of how $\Delta Q_S$ influences the urban thermal environment, while also providing high-quality data to support numerical simulations and theoretical models.