Cities slow the wind down. Skyscrapers, warehouses, suburbs and shopping centres all create obstacles to airflow and this increase in surface roughness slows the wind down. Across China the rapid growth of cities has resulted in average wind speed decreasing by 11% since the 1980s. But urbanisation also increases surface temperatures within cities, driving more mixing with the air above and potentially increasing wind speed. So which effect is dominant?
Researchers from Nanjing University in China have simulated the impact of the buildings in one of China’s megacities – Shanghai – to understand what impact the cityscape is having on the wind. The research, which is published in the Journal of Advances in Modelling Earth Systems, shows that Shanghai’s built landscape takes a significant amount of energy out of the wind, slowing it down by about 50%. However, during warm periods when the urban heat island effect is particularly strong, the slowing effect of the buildings is outweighed by the energy being added via turbulent mixing with the air above. Under the most extreme circumstances this can result in average urban wind speeds increasing by as much as 30%.
The balance will vary from city to city, but it seems that most of the time buildings slow the wind down. However, in warm weather cities can whip up quite a hoolie.
