As the fraction of the World's population that live in cities continues to grow, it is becoming increasingly important to be able to provide accurate forecasts for these environments. The development of the boundary layer over a city can also initiate mesoscale flows between the urban and surrounding rural areas, as well as having important effects for pollution dispersion both inside and downwind of the city. To be able to model these effects and provide accurate forecasts for a city, urban phenomena such as the urban heat island must be represented within numerical weather prediction models.
Two schemes will be presented for the surface exchange from an urban area. The first is the traditional scheme which represents an urban surface in the same way as a bare soil surface, but with the surface properties appropriate for the urban environment. The second scheme considers the urban area as a canopy of concrete above the underlying soil. It will be shown that although the
traditional scheme can produce an urban heat island, the magnitude of this heat island is smaller than that which can be obtained from the canopy scheme. Also, the traditional scheme fails to maintain a neutral temperature profile into the night, whereas the canopy scheme keeps the neutral profile to depths approaching those observed.