Heat, water and carbon exchanges in the tall megacity of Shanghai: challenges and results

摘要

The observed surface energy balance fluxes are essential to improve model forecasting ability but such data are scarce for subtropical cities and urban areas with tall buildings. One year of eddy covariance flux data for an area within Shanghai with a wide range of building heights (8-150m, mean by direction) are analysed by wind direction. Consideration is given to how to distinguish between local- and micro-scale fluxes. At the local-scale, median daily peaks of sensible heat flux occur in the early afternoon (winter: 87Wm(-2), spring: 205Wm(-2), summer: 292Wm(-2), autumn: 135Wm(-2)). The latent heat flux is small in winter (median daily maxima 21Wm(-2)) and slightly larger in spring, summer and autumn (49, 65, 49Wm(-2), respectively). The monthly mean daytime Bowen ratio under all-sky conditions ranges from 2 to 4.7. At this site, with predominately impervious surface (85% buildings/pavement, 14% vegetation), the enhancement of evaporation following rainfall usually lasts for about 12h. Consistently larger Bowen ratios at the micro-scale than the local-scale are attributed to roughness effects and the impact of extensive areas of dry walls. The daily median CO2 flux is dominated by traffic emissions, with two peaks associated with morning and evening rush hours. The data provide insights into urban surface controls on momentum, energy and carbon dynamics, with implications for urban planning strategies in the context of rapid global urbanization and climate change.