Urban areas modify the atmosphere above them in a number of ways. There are many reasons for this, but broadly speaking, it is because they are rough and grey… We think of cities as being ‘rough’ because of all the obstacles that the air has to pass over as it traverses them (houses, offices etc.). On average, this means that a city will slow the air down as it passes over it, much like driving a car into sand. The ‘greyness’ of an urban area means that it can absorb more solar radiation than surrounding green areas. This can make the air warmer above a city, a phenomenon known as the ‘urban heat island’.
Some people think that the urban heat island could cause a convection cell above a city… If the city is hotter than the surrounding area, air will tend to rise above it because it will become less dense, drawing in air from the surrounding countryside. So, even if the wind speed in the wider region is zero, we would expect a non-zero wind speed within the city. However, this effect can be quite hard to measure using wind speed measurements, for example, because it varies significantly in space and time.
In the Atmospheric Environment paper “London air pollution climatology: Indirect evidence for urban boundary layer height and wind speed enhancement“, we try to identify the presence of an ‘urban heat island circulation’ over London using air pollution observations. The idea was that, if there is a heat island circulation, the wind speed (and boundary layer height) should be prevented from reaching very small values in London even if they are small outside the city, and therefore, pollutant concentrations shouldn’t be allowed to reach the very high values within the city. By comparing pollutant concentration to regional wind speed and boundary layer height, we find that pollutant concentrations are more accurately predicted if we incorporate a minimum wind speed and boundary layer depth into a simple model of urban pollutant transport. This is consistent with the presence of an urban heat island circulation in London.