China’s Urban Heat Island Problem
How does urban-induced warming in Beijing interact with air temperature in summer?
Beijing has undergone several important urbanization development stages since late 1978. Linked with urbanization, the so-called “urban heat island effect” is a key problem caused by urban land expansion. Such changes in air temperature in Beijing inevitably have an impact on the daily lives of its inhabitants, and is therefore of considerable interest to scientists and the wider public alike.
Dr. Xiaojuan LIU and Associate Professor Guangjin TIAN from the School of Environment, Beijing Normal University, used the mesoscale Weather Research and Forecasting model coupled with a single urban canopy model and high-resolution land cover data to analyze the spatial and temporal patterns of summertime urban warming influenced by three stages of urban land expansion during 1990-2010 across Beijing. They found that urban-induced warming increased with urban land expansion, but the speed of warming declined slightly during 2000-10.
They further examined the impacts of heat waves and high-temperature weather on urban-induced warming and the mechanisms associated with the surface energy balance, and revealed an interrelationship between urban-induced warming and heat waves. Dr. Xiaojuan LIU explains their findings published in Advances in Atmospheric Sciences:
“Heat waves can increase surface air temperatures over both urban and crop areas. Our results indicate that heat waves enhance nocturnal urban warming in summer in Beijing and further cause a change in daily mean warming to some extent. That is, heat waves induce a higher increment of air temperature in urban areas than in crop areas. This implies that urban land expansion may heighten heat waves in urban areas. Therefore, urban-induced warming should be considered as one possible source of heat waves in urban areas.”
Modeling the Warming Impact of Urban Land Expansion on Hot Weather Using the Weather Research and Forecasting Model: A Case Study of Beijing, China
The impacts of three periods of urban land expansion during 1990–2010 on near-surface air temperature in summer in Beijing were simulated in this study, and then the interrelation between heat waves and urban warming was assessed. We ran the sensitivity tests using the mesoscaleWeather Research and Forecasting model coupled with a single urban canopy model, as well as high-resolution land cover data. The warming area expanded approximately at the same scale as the urban land expansion. The average regional warming induced by urban expansion increased but the warming speed declined slightly during 2000–2010. The smallest warming occurred at noon and then increased gradually in the afternoon before peaking at around 2000 LST—the time of sunset. In the daytime, urban warming was primarily caused by the decrease in latent heat flux at the urban surface. Urbanization led to more ground heat flux during the day and then more release at night, which resulted in nocturnal warming. Urban warming at night was higher than that in the day, although the nighttime increment in sensible heat flux was smaller. This was because the shallower planetary boundary layer at night reduced the release efficiency of near-surface heat. The simulated results also suggested that heat waves or high temperature weather enhanced urban warming intensity at night. Heat waves caused more heat to be stored in the surface during the day, greater heat released at night, and thus higher nighttime warming. Our results demonstrate a positive feedback effect between urban warming and heat waves in urban areas.
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