A quantification of classic but unquantified positive feedback effects in the urban-building-energy-climate system

The interaction between urban air temperature (T) and building cooling energy demand (E) generates a wellknown positive feedback (PFB), which is mediated by sensible anthropogenic heat (Q(fs)) and named Q(fs)-T-E PFB in this study. This PFB could induce self-reinforced warming in urban areas, but its effects have not been completely quantified. Hence, this study aimed to clarify these effects by targeting Osaka, a Japanese major city. Focusing on the from-weekends-to-weekdays increase in urban energy consumption including E increase as an observable trigger of the PFB, its induced T rise due to growth in Q(fs) was estimated with the fed-back additional E gain on weekdays based on observed ground-level T and district-wise electric power consumption during summer. The result indicated that the weekdays-weekends contrast in energy consumption over Osaka could induce the Q(fs)-T-E PFB effects, which resulted in fed-back E gain reaching 10% on weekdays. Such observational PFB impact on E was found to be roughly reproducible by the proposed urban meteorological model, named WRFCM-BEM. Thus, the validated model was applied to the quantification of the climatological PFB impact on T based on feedback gain (g(A)) which means a percentage of T variation caused by the PFB. An attempt was made to quantify g(A) through the two-cases simulations of the weekdays-run and holidays-run for the months of August in 10 years, focusing again on the weekdays-weekends contrast in urban energy consumption. The simulations provided estimates on g(A), whose daytime averages reached nearly 10% in the downtown commercial areas and 20% in the leeward-located residential areas, suggesting the influence of sea breeze heat advection of downtown Q(fs). Such estimated impacts on T were roughly in the same order of magnitude compared to those in a few earlier studies that were not based on observational validations and seemed to be non-negligible, considering the feedback impacts on global surface warming estimated with g(A) of approximately 50% by the Intergovernmental Panel on Climate Change.