INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 33: 2388–2401 (2013) Published online 11 September 2012 in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/joc.3589 Modelling the impact of urbanisation on regional climate in the Greater London Area David Grawe, a,b,c * Heather L. Thompson, a Jennifer A. Salmond, a† Xiao-Ming Cai a and K. Heinke Schl¨ unzen c a School of Geography, Earth and Environmental Sciences, University of Birmingham, Birmingham, UK b National Centre for Atmospheric Science, University of Birmingham, Birmingham, UK c Meteorological Institute, KlimaCampus, University of Hamburg, Hamburg, Germany ABSTRACT: Urban areas have well-documented effects on climate, such as the urban heat island (UHI) effect, reduction of wind speeds, enhanced turbulence and boundary layer heights, and changes in cloud cover and precipitation. The aim of this study is to quantify the impact of the urban area of London on local and regional climate. This is achieved through the coupling of the non-hydrostatic mesoscale model METRAS with the sophisticated urban canopy scheme BEP. The model is configured for case studies of the London region, for typical UHI conditions, and the model results are evaluated using data from meteorological monitoring sites. This study develops a methodology to quantify the regional impact of urbanisation from numerical model results. The urban area, in its current form, is found to affect near surface temperature, the diurnal temperature range, the UHI, and the near surface wind speed and direction. For the selected cases, peak UHI intensities of up to 2.5 K are found during night time hours, with the timing and magnitude of the peak showing good agreement with previous experimental studies for London. The timing of the UHI peak intensity for the current urban land cover for London shows a good agreement with the results of measurements. A significant reduction in wind speed over the urban area was also simulated during both daytime and night time, due to the higher roughness of the city compared to the rural domain. The effect is shown to have a regional character, with both urban and surrounding rural areas demonstrating a significant impact. Thus, the UHI can not only be understood when focussing on local data, but the interaction with the surrounding needs to be considered. KEY WORDS urban climate; urban heat island; UHI; urban parameterisation; BEP; METRAS; regional impact; London Received 27 April 2012; Revised 6 August 2012; Accepted 9 August 2012 1. Introduction Changes in land-use practices may have a significant impact on climate at a variety of scales. The physical characteristics of the earth surface determine the thermal, radiative and aerodynamic properties of the surface which in turn modify atmospheric conditions such as tempera- ture, humidity, wind speed and ultimately weather phe- nomena such as precipitation patterns. Land-use changes are currently taking place at an unprecedented scale and contribute significantly to changing the local and regional climates. Previous studies have even shown that changes in surface landuse may have a significantly larger impact on climate than anthropogenic greenhouse gases (Pielke et al., 2002). Many studies on the effect of land-use change on cli- mate consider modern vegetation compared to natural vegetation (Bonan, 1997), desertification (Charney et al., ∗ Correspondence to: D. Grawe, Meteorologisches Institut, KlimaCam- pus, Universit¨ at Hamburg, Bundesstraße 55, 20146 Hamburg, Ger- many. E-mail: david.grawe@zmaw.de † Present address: School of Environment, The University of Auck- land, Auckland, New Zealand. 1977) or tropical deforestation (in particular the Ama- zonian deforestation) (Henderson-Sellers et al., 1993). However, less is known about the impact of urbanisa- tion, an extreme form of land-use change, on climate at a regional scale. Given the increasing rate of urban- isation at an international scale (it is thought that by 2035 62% of the world’s population will be living in urban areas, compared to 53% in 2011; UN 2012), it is important to understand how urbanisation affects local and regional climates. This is particularly true as the magnitude of some of the reported changes in local cli- mate (especially temperature) are comparable to those projected by future global and regional climate change (Changnon, 1992; Baker et al., 2002). Furthermore, such increases in temperature may exacerbate the effects of summer heat-waves, resulting in a range of consequences including increased mortality (Johnson et al., 2005) and air pollution (Stedman, 2004). The regional scale impact of urbanisation on temperature records also needs to be identified and isolated to improve our understanding of other processes which induce climate change (Kalnay and Cai, 2003). Although urban areas have many impacts on local and regional climates, the urban heat island (UHI), or  2012 Royal Meteorological Society