The ratio of urban fraction to vegetation fraction within a city influences the size of the urban heat and dry islands. The response to increasing the vegetation fraction decreases the urban heat island but this change is not linear. The degree of non-linearity depends upon the size of the urban area. The boundary layer structure is significantly altered by the presence of an urban fraction. The neutral temperature plume over the city is advected downwind as a lofted plume, with important implications for pollution dispersion. There is also a strong non-linearity in the boundary layer response to an urban fraction. Even small fractions of urban can produce neutral nocturnal boundary layers of a city.
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