Structural Uncertainty in the Sensitivity of Urban Temperatures to Anthropogenic Heat Flux

One key source of uncertainty for weather and climate models is structural uncertainty arising from the fact that these models must simplify or approximate complex physical, chemical, and biological processes that occur in the real world. However, structural uncertainty is rarely examined in the context of simulated effects of anthropogenic heat flux in cities. Using the Weather Research and Forecasting (WRF) model coupled with a single-layer urban canopy model, it is found that the sensitivity of urban canopy air temperature to anthropogenic heat flux can differ by an order of magnitude depending on how anthropogenic heat flux is released to the urban environment. Moreover, varying model structures through changing the treatment of roof-air interaction and the parameterization of convective heat transfer between the canopy air and the atmosphere can affect the sensitivity of urban canopy air temperature by a factor of 4. Urban surface temperature and 2-m air temperature are less sensitive to the methods of anthropogenic heat flux release and the examined model structural variants than urban canopy air temperature, but their sensitivities to anthropogenic heat flux can still vary by as much as a factor of 4 for surface temperature and 2 for 2-m air temperature. Our study recommends using temperature sensitivity instead of temperature response to understand how various physical processes (and their representations in numerical models) modulate the simulated effects of anthropogenic heat flux. Numerical models are often used to simulate the effects of anthropogenic heat flux, which is generated by human activities such as building energy consumption, transportation, etc. These models inevitably have structural uncertainties because they simplify the real world; hence, the simulated effects of anthropogenic heat flux also possess structural uncertainties. Yet, the structural uncertainties associated with the simulated effects of anthropogenic heat flux have not been studied before. In this study, we quantify such structural uncertainties using a suite of simulations conducted with the Weather Research and Forecasting (WRF) model. Using the sensitivity of urban temperatures to anthropogenic heat flux as the metric, we find that different urban temperatures show different sensitivities to anthropogenic heat flux and these sensitivities can vary by as large as an order of magnitude depending on the anthropogenic heat flux release methods and key model structural choices. Model choices can affect the sensitivities of urban temperatures to anthropogenic heat flux by as much as an order of magnitude The substantial structural uncertainty highlights the challenges associated with simplifying complex urban environments in numerical models The sensitivities of urban temperatures to anthropogenic heat flux seem to converge at large anthropogenic heat flux