Spatiotemporal linkage and transmission of urban heat islands in the Yangtze River Delta urban agglomeration: the role of urban heat advection

Abstract

Abstract. Urban heat islands (UHIs) substantially modify the urban thermal environment, yet the contribution of non-local processes such as urban heat advection (UHA) in dense urban agglomerations remains poorly quantified. Using five years of high-density automatic weather station data and Weather Research and Forecasting (WRF) simulations, we investigate how UHA links canopy-layer UHI (CUHI) and boundary-layer UHI (BUHI) across the Suzhou-Wuxi-Changzhou metropolitan area in the Yangtze River Delta, China. UHA exhibits pronounced spatiotemporal variability, systematically transporting heat from upwind to downwind cities along the prevailing winds. Under northwesterly flow, daily-mean UHA intensities increase from negative values in upwind regions to 0.32 °C downstream, with nocturnal UHA during peak hours reaching roughly 0.6 °C. Observations show that nighttime UHA is nonlinearly modulated by wind speed and planetary boundary-layer height (PBLH), with maximum downstream warming under moderate winds and intermediate PBLH, whereas deep daytime convective boundary layers (PBLH ≥ 800 m) dilute urban heat plumes and can reverse UHA to a net cooling effect. WRF experiments further indicate that urbanization in the upstream city of Changzhou enhances CUHII in the adjacent downstream Wuxi by up to about 0.68 °C (8.1 %–60.4 %) and BUHII by up to about 0.40 °C (4.7 %–41.7 %), with detectable canopy-level warming extending beyond 100 km downwind. These results demonstrate that cross-city UHA superposition, strongly regulated by boundary-layer dynamics and thermodynamics, is a key physical process coupling UHIs within urban agglomerations, requiring explicit consideration in regional climate assessments.