Orographic influence on urban climate
Urban induced meteorological phenomena are normally restricted to time and length scales between a single roughness element or heat source (micro-scale β) and a large metropolitan area (meso-scale α). For this reason, orographically induced phenomena pertaining to these scales exert the most important influence upon the four relevant budgets (energy, water, air contaminants and momentum) of an urban air volume. A comparison of both orographically induced meteorological phenomena and urban budgets leads to four processes, or process groups, being important for the urban climate: Air deflection and channelling in broad valleys. Strong downslope winds on the leeside foothills of high mountain chains. Orographically induced precipitation in high slope areas and on mountain tops, and finally, the most important phenomena, the thermotopographic wind systems. These winds interact in a complex manner with the geostrophic wind and the various thermal and mechanical effects caused by the urban land surface.