Daily Archives: May 4, 2018

Urban heat waves linked to urban heat island effect

The urban heat island effect is a significant confounding problem for global surface temperature measurements. Heat waves in a particular location may not be driven by external atmospheric changes but locally from surface development. Consequently, an increase in surface temperatures may not be driven by changes in climate but rather by changes on the earth’s surface. More on this paper here at WUWT.

Xiaojuan Liu, Guangjin Tian, Jinming Feng, Bingran Ma, Jun Wang, Lingqiang Kong. Modeling the Warming Impact of Urban Land Expansion on Hot Weather Using the Weather Research and Forecasting Model: A Case Study of Beijing, China. Advances in Atmospheric Sciences, June 2018, Volume 35, Issue 6, pp 723–736.

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 mesoscale Weather 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.