Biophysical Drivers of Seasonal Hysteresis of Urban Heat Islands Across Climates and Urban Landscapes

The phenomenon of seasonal hysteresis between urban heat islands (UHIs) and background surface temperature was reported in many studies, but a unifying explanation for the causes of its magnitude, shape and looping direction remained less clear, and the impacts of urban landscapes and background climate on such hysteresis were also the subject of enquiry. Here we performed a systematic study based on a long-term offline simulation for surface energy budgets, surface air and skin temperature of the urban-biosphere system in East China. It was carried out with an up-to-date land surface model - the CLM5-LCZs (the Community Land Model version 5 (CLM5) coupled with a newly developed urban canopy model representing diverse urban landscapes with the local climate zones classification scheme). To isolate the causes of UHIs hysteresis, a simplified framework was constructed by combining a mathematical representation of grid/subgrid model outputs, the two-resistance mechanism method and the time-lagged cross correlation analysis. The results exhibited distinct hysteresis patterns of UHIs such as twisted, concave-down, concave-up and convex-up curve across diverse background climates and urban landscapes. The basic shapes of hysteresis depict a gradual transition from concave/convex-up to concave/convex-down by characterized by a larger winter UHIs from a wetter and warmer climate to a drier and colder climate. Magnitude and seasonality of surface UHIs are separately dominated by the urban-rural contrast of evapotranspiration and heat storage at daytime and nighttime, whereas across-climate variations of daytime and nighttime surface UHIs hysteresis are regulated by the phase shift of convection efficiency and anthropogenic heat. Urban landscapes affect the hysteresis mainly by altering the amplitude rather than the shape of looping curves. Our work could provide a fuller picture of how to mitigate urban warming considering inter-seasonal tradeoffs over a broader region. The seasonality of urban heat islands (UHIs) has been investigated toward its characteristics and controls, of which analyses were conducted on several cities and left intra-urban variations out of account. It was essential to understand whether the local seasonality-arguments can be extended to broader geographic regions or lots of cities with different landscapes. In this study, an updated urban climate model describing intra-urban heterogeneity better was utilized to simulate surface energy fluxes and UHIs. Based on the subgrid outputs of this model and the hypothesis of phase shift mechanism, we presented a simplified framework to isolate the contribution of time lags of different biophysical factors to the magnitude, shape and looping direction of UHIs hysteresis. These hysteretic characteristics are varied with background climates and urban landscapes. The time lags of evaporative cooling and heat storage corresponding to background surface temperature between urban and rural areas account for different hysteresis patterns at daytime and nighttime. Hysteretic typology for different urban classes or cities in diverse background climates is regulated by the time lead/lag of other secondary biophysical factors. The results could serve as a valuable reference for coping with the growing heat risk in cities. The seasonality of surface and canopy urban heat island exhibited hysteresis-like curves, whose looping patterns vary across background climate and urban features A simplified conceptual model to explain hysteretic behaviors was constructed based on a second-order Fourier representation, the attribution method and a newly developed land surface model The urban-biosphere time lags of evaporative cooling rates and storage heat, dominated such hysteresis at daytime and nighttime respectively