Turbulent Flow at 190 m Height Above London During 2006–2008: A Climatology and the Applicability of Similarity Theory

Abstract

Flow and turbulence above urban terrain is more complex than above rural terrain, due to the different momentum and heat transfer characteristics that are affected by the presence of buildings (e.g. pressure variations around buildings). The applicability of similarity theory (as developed over rural terrain) is tested using observations of flow from a sonic anemometer located at 190.3 m height in London, U.K. using about 6500 h of data. Turbulence statistics-dimensionless wind speed and temperature, standard deviations and correlation coefficients for momentum and heat transfer-were analysed in three ways. First, turbulence statistics were plotted as a function only of a local stability parameter z/ (where is the local Obukhov length and z is the height above ground); the σ i /u * values (i = u, v, w) for neutral conditions are 2.3, 1.85 and 1.35 respectively, similar to canonical values. Second, analysis of urban mixed-layer formulations during daytime convective conditions over London was undertaken, showing that atmospheric turbulence at high altitude over large cities might not behave dissimilarly from that over rural terrain. Third, correlation coefficients for heat and momentum were analyzed with respect to local stability. The results give confidence in using the framework of local similarity for turbulence measured over London, and perhaps other cities. However, the following caveats for our data are worth noting: (i) the terrain is reasonably flat, (ii) building heights vary little over a large area, and (iii) the sensor height is above the mean roughness sublayer depth.

Similarity Scaling Over a Steep Alpine Slope

“…We should note that using local scaling over the city of London, Wood et al (2010) could obtain similar near-neutral limits of σ i /u * l in accordance with those reported over flat and homogeneous terrain where MOST applies. In Table 2, we see that σ u /u * l and σ v /u * l are slightly above the range of values previously reported in the literature, whereas σ w /u * l is much smaller.…”

“…For instance, Wood et al (2010) recently detailed a successful use of local scaling with turbulence measurements collected at 190 m above the city of London, UK, similar to what Al-Jiboori et al (2002) had shown with observations from a 325-m meteorological mast in Beijing, China. Local scaling has also been successfully demonstrated with airborne measurements over coastal zones (see Shao and Hacker 1990;Brooks and Rogers 2000).…”

“…Wood et al 2010). We argue that the traditional concept of a surface layer is not valid here, perhaps as a result of advection from thermal alpine circulations.…”

Boundary layer dynamics over London, UK, as observed using Doppler lidar during REPARTEE-II

“…51 • 31 17.31 N, lon. 0 • 8 20.12 W), as first reported in Wood et al (2010). The Tower is located approximately 1.2 km to the east of the lidar site as shown in Fig.…”

“…Various studies have used tall towers and found broad agreement with locally scaled similarity relationships, particularly for complex terrain (Al-jiboori (2002), Beijing; Vesala et al (2008), Helsinki). Wood et al (2010) found similar behaviour, but also estimated boundary layer height using turbulence measurements on the 190m BT Tower in London to test mixed layer scaling. Han et al (2009) used a 250 m Tower in Tianjin to relate nocturnal pollutant levels to mixing height (MH) dynamics.…”