The urban heat island circulation in the presence of the background wind

Abstract

The urban heat island (UHI) effect is increasing in both single cities and city clusters worldwide, due to the increase of urbanization. This aim of this thesis was to investigate urban heat island circulations (UHICs) in the presence of background wind by idealized simulations, and with a particular focus on the real case study in the Beijing-Tianjin-Hebei region, as it has experienced frequent heavy haze pollution in recent decades. The Weather Research and Forecasting (WRF) model was used to study the interactions between UHIC and background wind. First, the flow regimes of UHIC were found to depend on the buoyancy velocity ratio (R) (the ratio of the horizontal convective scale velocity (UD) to the background velocity (U0)) in both two-dimensional and three-dimensional simulations. Three flow patterns could occur as R varied. For R > 2.0, the strong UHIC flow pattern was characterized by upstream back-currents and returned inflows on the leeside, and a vortex ring was present. For 0.5 < R < 2.0, the distorted urban plume was characterized by the disappearance of the upstream back-currents and the occurrence of the returned inflows on the leeside. The coherent asymmetric vortex rings were destroyed and a counter-rotating vortex pair (CVP) formed. For R < 0.5, upstream back-currents and the convergence of regions of the UHIC both disappeared. Second, the effects of various city shapes and the Coriolis force on a single UHIC were investigated. The recirculation zones were found to be highly correlated to the aspect ratios (ARs) (where AR = L/D, the ratio of the span-wise dimension L to the stream-wise dimension D of an urban area), in that the deceleration of oncoming airflow increased with the increase of AR. In addition, in the presence of wind, the Coriolis effect was also ultimately responsible for the collapse of the CVP on the leeside of the UHIC, resulting in the redistribution of the temperature fields. Moreover, two UHICs interacting in the presence of weak wind were simulated over circular twin cities, where these UHICs were arranged in tandem or side-by-side with respect to the background wind, and at different spacing distances. It was found that sufficiently close side-by-side UHICs had a blockage effect similar to that of a single larger UHIC, such that only one CVP existed around the cities having these. In the tandem UHIC arrangement, a chain flow was formed between UHICs, which altered the pollutant distribution between twin cities. Finally, using the new World Urban Database and Access Portal Tools (WUDAPT) land-use dataset, the UHICs that existed over Beijing and Tianjin during an extreme UHI event in 2012 were studied. It was shown that a chain-flow mechanism existed over the Beijing-Tianjin region at night, as the upper-level outflows from the UHIC over Tianjin became connected to the inflow to the UHIC over Beijing. In addition, sensitivity experiments showed that the city shape affected the UHIC over Beijing at night, as the convergent zones were distributed diagonally and the wind speed was higher in the diagonal direction. (498 words)