INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. (2014) Published online in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/joc.4087 Modelling the inluence of urbanization on the 20th century temperature record of weather station De Bilt (The Netherlands) S. Koopmans, N. E. Theeuwes, G. J. Steeneveld * and A. A. M. Holtslag Meteorology and Air Quality, Wageningen University, The Netherlands ABSTRACT: Many cities have expanded during the 20th century, and consequently some weather stations are currently located closer to cities than before. Due to the urban heat island (UHI) effect, those weather stations may show a positive bias in their 2-m temperature record. In this study, we estimate the impact of urbanization on the temperature record of WMO station De Bilt (The Netherlands). This station has a long historical record, but the nearby city of Utrecht and its suburbs expanded during the 20th century. The temperature rise due to urbanization is estimated by conducting representative mesoscale model simulations for the land-use situation for the years 1900 and 2000. This is performed for 14 different episodes of a week, each representing a typical large-scale low regime (Grosswettertypes) in both the winter and the summer season. Frequency distributions of these low regimes are used to estimate an average temperature rise. We ind that the model results with two rather different atmospheric boundary-layer schemes, robustly indicate that the urbanization during the 20th century has resulted in a temperature rise of 0.22 ± 0.06 K. This is more than a factor of 2 higher than a previously estimated temperature trend by using observed temperature records of stations close to De Bilt. KEY WORDS urbanization; temperature trend; climate change; WRF; mesoscale modelling; The Netherlands; land-use change; urban heat island; anthropogenic heat Received 20 September 2012; Revised 15 May 2014; Accepted 24 May 2014 1. Introduction Many observational studies of climate change report a substantial rise in the globally averaged 2-m tempera- ture in the last century (e.g. Jones et al., 1986a, 1986b; Hansen and Lebedeff, 1987; Brohan et al., 2006; Smith et al., 2008). Based on this research, the Intergovern- mental Panel on Climate Change (IPCC) documented a global combined land and ocean temperature increase of ∼0.89 ∘ C over the period 1901–2012 (Stocker et al., 2013). Observational temperature records close to urban areas, however, might be inluenced by nearby city expan- sion and consequent heat advection from the city towards the observing stations (Oke, 1982; Karl et al., 1988; Schaefer and Domroes, 2009; Fall et al., 2010). Although on the global scale, urbanization contributes only for 0.02 K to the global temperature trend in the 20th century (Jones et al., 1990; Trenberth et al., 2007), the local trend for urbanized areas might be substantially higher, i.e. ∼0.3 K (Karl et al., 1988; Balling et al., 1998; Kalnay and Cai, 2003; Fujibe et al., 2007). A numerical study by Adachi et al. (2012) showed for Tokyo that urbanization leads to higher temperatures on top of the enhanced greenhouse effect. Van der Schrier et al. (2013) report urbanization as one of the key aspects * Correspondence to: G. J. Steeneveld, Meteorology and Air Qual- ity, Wageningen University, PO box 47, 6700 AA Wageningen, The Netherlands. E-mail: Gert-Jan.Steeneveld@wur.nl of which its contribution to the northwest European tem- perature rise is not completely understood. Urbanization may lead to elevated temperatures in the surroundings due to heat advection from the city (Oke, 1982), and can affect the temperature of a nearby weather station in two ways. Firstly, the urban heat island (UHI) intensity (in the urban boundary layer) may change when the urban thermal properties, the urban vegetation fraction or population density change for a constant city area (Oke, 1973; Sailor and Lu, 2004; Steeneveld et al., 2011a). Secondly, the inluence of the city increases when the city area expands and the distance between the weather station and city margin decreases (Oke, 1982). In brief, the elevated urban temperatures occur because of the modiied surface energy budget compared with the countryside. Typically, the urban street canyon structure triggers an eficient entrapment of solar radiation and thus provides a rather small albedo (Kanda, 2007). Overall, up to 30% of the incoming solar radiation is stored during daytime and is subsequently released at night (Oke, 1982). Also, street canyons limit the sky view for outgoing long-wave radiation. Moreover, the rather high fraction of impervious surface initiates a relatively high Bowen ratio (ratio of surface sensible and surface latent heat lux). Anthropogenic heat (AH) production is mainly important during winter (Sailor and Lu, 2004). Overall, the largest and most intense UHI occurs in the evening, because of the large heat capacity of the urban fabric, and the reduced turbulent diffusion in the countryside. © 2014 Royal Meteorological Society