Urban Surface Characteristics
One of the most distinct characteristics of the urban surface is the amount of vegetated cover relative to the impervious or built cover. Typically this varies across a city, with the largest impervious fraction found in the central business district (CBD), with residential areas more vegetated (Figure A.1). The built fraction consists of paved roads, sidewalks and parking lots as well as actual buildings. The characteristics of buildings also tend to change with distance from the CBD, with the buildings typically becoming shorter. Towards the suburbs, buildings also tend to be of lower density (more widely spaced).
This change in building height (H) and the distance between them (W), impacts wind flow and radiation exchange. In addition, urban building materials have different thermal and radiative characteristics to natural materials which affect heat storage and radiative exchanges.
Taller buildings are also associated with increased anthropogenic heat flux. Additional energy is required for the buildings to operate (e.g. elevators), and the greater density of people results in increased needs for air conditioning, removal of CO2 enriched air, etc. With a focus of activities, such as in the CBD, there are more emissions from transport, which has implications for both energy and air quality.
FIGURE A.1 Variation in plan area fraction by height of trees/shrubs and buildings in a North to South transect of London (OC centre of London, although not the tallest building area of London) (Lindberg and Grimmond, 2011)