Observation and simulation of low-level jet impacts on 3-D urban heat islands in Beijing: a case study

Abstract

In this study, we focused on the impacts of the planetary boundary layer (PBL) low-level jet (LLJ) on the horizontal distribution, vertical development, and 3-D structure of urban heat island (UHI). Observational datasets were collected from 224 automatic weather stations (AWSs), and an intensive sounding experiment was conducted in Beijing from 28 August to 2 September 2016. 3-D simulations were operated by the Weather Research and Forecasting (WRF) model. The results show the following: Ri was smaller than 0.25 at both urban and suburban stations near the surface when LLJ was present. Through turbulent mixing, the LLJ extended the horizontal distribution of the canopy UHI downwind and increased the total UHI area by approximately 1 × 103 km2. The temperature lapse rate in the urban area was 0.7°C/100 m with LLJ, twice that in the absence of an LLJ. The jet enhanced the vertical mixing above the urban area, accompanied by a near-surface TKE up to 0.52 m2 s-2, elevating the vertical UHI development height to 200 m. The LLJ is capable of increasing the temperature of the downwind urban area by a maximum of 8.5°C h-1 through warm advection. The temperature advection in the upper air caused by LLJ also tilted the 3-D UHI structure as a plume. Results reproduced the process by which LLJ affect the 3-D UHI structure through turbulence and advection, and could also provide ideas regarding the influence of LLJ in other PBL processes.