Spatiotemporal Linkage and Transmission of Urban Heat Islands in the Yangtze River Delta Urban Agglomeration: The Role of Urban Heat Advection

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 about 0.3 °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 0.6 °C (9–42 %) and BUHII by up to about 0.35 °C (19–141 %), with detectable canopy‑level warming extending beyond 100 km downwind. These results demonstrate that cross‑city UHA superposition, strongly regulated by boundary‑layer dynamics, is a key physical process coupling UHIs within urban agglomerations, requiring explicit consideration in regional climate assessments.