Urban heat island circulations over the Beijing-Tianjin region under calm and fair conditions

Abstract

The Beijing-Tianjin-Hebei region has experienced severe urban heat island (UHI) effects in recent decades. Using the Weather Research and Forecasting (WRF) model and the new World Urban Database and Access Portal Tools (WUDAPT) land-use dataset, the urban heat island circulations (UHICs) and other local atmospheric circulations over Beijing and Tianjin were studied during a severe UHI event in 2012. It was shown that in the daytime, various UHICs over Beijing and Tianjin developed separately under a southerly background wind. Two of these UHICs formed over Beijing due to multiple hot regions, involving a chain flow mechanism in which the outflows from one UHIC over nearby mountains were connected to the inflows to the other UHICs in a west-to-east direction. At night, more robust UHICs occurred and a chain flow mechanism again formed over the Beijing-Tianjin region, as the upper-level outflows from the UHIC over Tianjin were connected to the inflows of the UHIC over Beijing. Moreover, the rotary air movements formed by the flows over the mountain and the down-slope winds formed a convergent region in the northwest of Beijing. In particular, sensitivity experiments replacing either Beijing or Tianjin with cropland grids revealed that the presence of either of those urban areas modified the penetration of the sea-breeze circulation. The city shape was found to affect the UHIC over Beijing at night, as the convergent zones were distributed diagonally and the wind speeds were higher in the diagonal direction.