Interactions of urban heat island circulations in multi-city configurations under calm conditions

Urban heat island circulations (UHICs) play a critical role in shaping local airflow, pollutant dispersion, and thermal environments within city clusters. While previous studies have primarily focused on single-city or two-city interactions, the influence of city size disparity, spatial arrangement, and spacing distance on inter-city UHIC coupling remains insufficiently understood. This study employs idealized Weather Research and Forecasting (WRF) simulations to investigate UHIC interactions across multiple urban configurations, including two-city, and triangular three-city arrangements. Two definitions of inter-city spacing—center distance (CD) and edge distance (ED)—are adopted to assess their roles in modulating circulations. Results show that under calm background wind conditions, the chain flow mechanism—the linkage of outflow from one UHIC to the inflow of another—emerges when differences in mixed/reversal heights exist. For two-city cases, stronger chain flows occur when size disparity is large, while excessive spacing weakens direct coupling and shifts linkage downstream. In triangular arrangements, the addition of a third city introduces new dynamical features, including the shifting and the rotation of convergent center over small city, which reorganize inter-city circulations, highlight that greater size contrast between neighboring cities strengthens layer mixing and enhances chain-flow robustness. These findings underscore that urban circulation is not governed solely by inter-city distance but also by relative city size and city arrangement, emphasizing the requirements to consider entire city clusters in urban climate studies and sustainable planning strategies. • Chain-flow dynamics emerge when outflow of one UHIC links with inflow of another. • Stratification compresses UHICs vertically, shifting convergence centers downstream. • City arrangement strongly modulates chain-flow strength and stagnation zone formation.