Exploring the impacts of surface heterogeneity on urban heat storage under heat waves via the Analytical Objective Hysteresis Model

Heat waves (HWs) are projected to occur more frequently under the future climate, which may exacerbate conditions in cities where temperatures tend to be higher due to the urban heat island (UHI) effect. Understanding surface energy balance (SEB) responses to HWs is crucial to appreciating the synergies between UHIs and HWs. Among the urban SEB components, heat storage is of particular importance due to its contribution to UHI generation, however, the flux is particularly difficult to model. Based on estimates of storage heat flux (ΔQS) from net all-wave radiation (Q*) following the Objective Hysteresis Model (OHM), we develop an analytical solution to the heat conduction-advection equation by explicitly accounting for surface properties. The proposed Analytical OHM (AnOHM) is evaluated using SEB observations in four urban areas in Asia and Europe (Beijing, Łódź, London and Swindon). The results indicate good performance in reproducing the multi-year heat storage series. AnOHM is used with the 35-year WFDEI dataset (i.e., an ERA-Interim based land surface model forcing dataset) to examine the impacts of surface heterogeneity on heat storage under HWs for multiple cities. The simulation results suggest that more heat is stored in cities where lower wind speeds are observed, a common condition during HWs. Cities with a considerable proportion of green space are able to access moisture reserves in the soil, which tend to offset adverse UHI effects.