Interacting urban heat island circulations as affected by weak background wind

This paper presents a numerical study of two interacting urban heat island circulations (UHICs) in the presence of weak background wind. The Weather Research and Forecasting (WRF) model was used with ideal settings without the Earth's rotation effect. UHICs were simulated over circular cities of the same size arranged in tandem or side by side with respect to the background wind direction, at different spacing distances. The study analyzed the flow structures and city ventilation arising from thermally driven UHICs and background wind. The arrangements of the cities have a significant modifying effect on the interaction between UHICs and winds. If side-by-side UHICs are sufficiently close (spacing distance less than half of the city diameter), they have a blockage effect similar to that of a single larger UHIC, and only one counter-rotating vortex pair structure exists around the cities. As the spacing distance increases, the city ventilation improves slightly. In the tandem arrangement, a chain flow is formed between UHICs at a spacing distance of between 0.5 and 1.5 times the city diameter. An upstream UHIC blocks the incoming wind flow to the downstream city, which results in a ventilation deficiency. Due to lateral entrainment, which partly compensates for the deficiency in the incoming flow, the ventilation of the downstream city is slightly improved by an increase in spacing distance.