A new US National Center for Atmospheric Research (NCAR) urbanisation versus wind-driven pollution study does not make much sense -on a global scale- to us, mere science writers, who however by accident were raised by a couple of wild meteorologists in the deep taiga – so therefore [really] can’t help to have some intuitive understanding of high and low air pressure, and how these always tend to work towards some balance.

Dark and dry paved areas create a lot of heat when the Sun shines over them. That’s why urbanisation may influence the diurnal cycle. The natural situation in the above image shows high pressure over the (cool) forested land surface and low pressure over the (warm) water surface, so an offshore wind. The city case shows temperatures over land and water to be more in balance, so a smaller pressure difference at the surface, and less wind. Problem with this image is it shows only one half of the diurnal cycle. Late afternoon, on hot summer days, one would expect the city to be not equally warm, but a lot warmer than the sea [in most geographical locations] and the air conditioner over city areas to switch to high gear – and compensate. (The air cycle would be clockwise then, but that should not matter much for getting rid of the pollutants.) Infographic provided by UCAR.

Warmer city -> less circulation

The NCAR research, based on atmospheric measurements and model simulations, suggests – in the case of Houston – further increases of paved areas will diminish the night-time circulation over the coast and therefore decrease air quality.

Whether this would apply for other cities depends not only on geographical setting (for instance cold Atlantic on US NE Coast or warm Gulf of Mexico in Houston), also on sea surface temperature anomalies – and most of all on season and time of day.

Warmer city -> more circulation?

To keep things simple: bad for circulation is equal temperature over land and sea. Good for circulation is a large temperature difference, but it doesn’t matter whether it’s the sea (at night) or the land (during daytime) that is the warmest.

For cities like New York, Liverpool or Sydney, having excessive build-up of heat when the Sun rises over dark, paved areas, is the best guarantee to have a refreshing onshore ocean breeze during the late afternoon. And this will likely do just as much good to lower the concentration of air pollutants over populated areas as offshore winds at night.

The NCAR scientists’ publication was accepted for the Journal of Geophysical Research-Atmospheres on 18 March 2011 and is listed as upcoming. More detailed information about the methodology, findings and authors’ conclusions can be found in the NCAR | UCAR News Center.

City climate management

The research does raise a universal point about city climate management. Lowering albedo and moisture content over the vast populated areas is, in the face of climate change and increased risk of damaging heat waves, not a wise thing anyway. On the UCAR site the authors advise to better consider the incorporation of parks and other substantial green zones in city planning, which are not only lighter than asphalt parking lots, but also convert incoming solar radiation to evaporation – instead of (direct) heat. As this will lower the heat peaks during the afternoon that is important advice in itself.

The researchers also add one other urbanisation concern, hindering air circulation and thereby contributing to stagnant air pollution: buildings. That should not hurt anyone’s intuition.

© Rolf Schuttenhelm | www.bitsofscience.org

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