Vertical Profiles Over Flat Terrain - Wind Energy Meteorology: Atmospheric Physics for Wind Power Generation - page 62

generation would avoid the transport of large amounts of electrical energy from
offshore wind farms and desert solar energy plants to the heavily populated cities
of the world.
3.7.1 Characteristics of Urban Boundary Layers
Urban agglomerations have recently received special interest in atmospheric
boundary layer studies. Nowadays, more than half of mankind is living in cities
and the number of megacities with more than 10 million inhabitants is steadily
growing. Cities are large pollution sources and because the temperature is already
higher than in their surroundings they are especially prone to the effects of a
warming climate. All these aspects have fostered studies on the structure of the
urban boundary layer (UBL). UBL meteorology has become a special subject in
boundary layer meteorology. One aspect of UBL studies is the analysis of wind
profiles and thermally driven secondary circulations over cities (urban heat
islands). See, e.g., Kanda ( 2007 ) and Hidalgo et al. ( 2008 ) for an overview of
urban meteorology and of urban heat islands. The urban heat island brings about a
secondary circulation with winds towards the urban centre near the ground,
uprising motion over the urban centre and compensating outflow towards the
surrounding rural areas aloft (Shreffler 1978 , 1979 ).
Urban surfaces are characterized by large roughness elements, wide-spread
sealed areas, reduced moisture availability at the surface, and increased possibil-
ities for heat storage. This leads to higher turbulence intensities in the urban
boundary layer (UBL) and to stronger sensible heat fluxes from the urban surface
into the UBL. Both facts induce a greater depth of the boundary layer (see the
urban dome in Fig. 3.23 ). During daytime the reduced moisture availability leads
to smaller latent and thus larger sensible heat fluxes at the urban surface compared
to rural surfaces. The reduced radiative cooling of the urban surface or even the
persisting upward heat fluxes (Velasco et al. 2007 ) at night prevents the formation
of a stable nocturnal boundary layer. Both the increased sensible heat flux during
the day and the reduced cooling during the night cause higher temperatures in the
UBL compared to the surrounding rural boundary layer. This effect is known as the
urban heat island (Atkinson 2003 ; Chow and Roth 2006 ). The urban heat island is
enhanced by human energy production (Crutzen 2004 ; Kanda 2007 ), which with
20-70 Wm -2 can be 5-10 % of the energy input by solar irradiation.
In a horizontal flow, the presence of the city results in a change in surface
properties. Towns are often isolated islands featuring these special surface prop-
erties surrounded by rural terrain so that the flow above them is not in equilibrium
with the urban surface. Following Sect. 3.1.1.1 , this leads to the formation of
internal boundary layers (Fig. 3.23 ). The internal layer formed by the properties of
the urban surface is often called an urban plume.
Following Plate ( 1995 ), Roth ( 2000 ) and Piringer et al. ( 2007 ), the urban
boundary-layer (UBL) is usually divided into four layers in the vertical (Fig. 3.24 ):