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The Diurnal Evolution of the Urban Heat Island of Paris EGU Journal Logos (RGB) Open Access Open Access Open Access Advances in Annales Nonlinear Processes Geosciences Geophysicae in Geophysics Open Access Open Access Natural Hazards Natural Hazards and Earth System and Earth System Sciences Sciences Discussions Open Access Open Access Atmos. Chem. Phys., 13, 8525–8541, 2013 Atmospheric Atmospheric www.atmos-chem-phys.net/13/8525/2013/ doi:10.5194/acp-13-8525-2013 Chemistry Chemistry © Author(s) 2013. CC Attribution 3.0 License. and Physics and Physics Discussions Open Access Open Access Atmospheric Atmospheric Measurement Measurement Techniques Techniques The diurnal evolution of the urban heat island of Paris: a Discussions Open Access model-based case study during Summer 2006 Open Access Biogeosciences 1,2 1 2 1 Biogeosciences2 H. Wouters , K. De Ridder , M. Demuzere , D. Lauwaet , and N. P. M. van Lipzig Discussions 1VITO, Flemish Institute for Technological Research, Department of Environmental and Atmospheric Modelling, Mol, Belgium Open Access 2KU Leuven, Department of Earth and Environmental Sciences, Leuven, Belgium Open Access Climate Climate Correspondence to: H. Wouters ([email protected]) of the Past of the Past Received: 11 June 2012 – Published in Atmos. Chem. Phys. Discuss.: 2 October 2012 Discussions Revised: 2 July 2013 – Accepted: 22 July 2013 – Published: 2 September 2013 Open Access Open Access Earth System Earth System Abstract. The urban heat island (UHI) over Paris during 1 Introduction Dynamics summer 2006 was simulated using the Advanced Regional Dynamics Prediction System (ARPS) updated with a simple urban Discussions Temperature tends to be higher in cities than in its nat- parametrization at a horizontal resolution of 1 km. Two in- ural surroundings, especially during the night (Landsberg, Open Access tegrations were performed, one with the urban land cover of Open Access 1981; Oke, 1987; ArnfieldGeoscientific, 2003). This is known as the Geoscientific Paris and another in which Paris was replaced by cropland. urban heat island (UHI) effect. Urban surfaces distinguish The focus is on a five-day clear-sky period, for which the Instrumentation Instrumentation themselves from their natural surroundings by particular ur- UHI intensity reaches its maximum. The diurnal evolution of Methods and ban surface characteristics like increased thermal inertia (Cai Methods and the UHI intensity was found to be adequately simulated for et al., 2008), a lowered vegetationData Systems cover and impervious land Data Systems this five day period. The maximum difference at night in 2 m cover which reduces the evapotranspiration (Grimmond and Discussions Open Access temperature between urban and rural areas stemming from Open Access Oke, 1999), a different albedo, emissivity and aerodynamic the urban heating is reproduced with a relative error of less characteristics due to presence of streets and buildings. To- Geoscientific than 10 %. The UHI has an ellipsoidal shape and stretches Geoscientific gether with the release of anthropogenic heat, this leads to along the prevailing wind direction. The maximum UHI in- Model Development the UHI effectModel reaching Development its maximum during the night. It tensity of 6.1 K occurs at 23:00 UTC located 6 km down- Discussions is especially favoured by high solar irradiation (clear sky) stream of the city centre and this largely remains during the during the preceding daytime period, no precipitation, low Open Access whole night. An idealized one-column model study demon- Open Access wind speeds and stable stratification. Under these conditions, strates that the nocturnal differential sensible heat flux, even Hydrology and Hydrology and a large amount of solar radiation reaches the surface, which is though much smaller than its daytime value, is mainly re- better transformed to heat,Earth and subsequently System retained as stor- Earth System sponsible for the maximum UHI intensity. The reason for age heat (Grimmond et al., 1999) for a longer time in urban this nighttime maximum is that additional heat is only af- Sciences Sciences areas compared to rural areas. fecting a shallow layer of 150 m. An air uplift is explained Discussions Open Access Accounting for the UHI is important whenOpen Access studying the by the synoptic east wind and a ramp upwind of the city cen- effect of land-use change (e.g. forest to croplands, or vege- tre, which leads to a considerable nocturnal adiabatic cooling tation cover to urban land) on climate on the mesoscale and Ocean Science over cropland. The idealized study demonstrates that the re- global scale or to test mitigationOcean strategies Science (e.g. Bowler et al., duced vertical adiabatic cooling over the city compared to Discussions 2010; Dimoudi, 2003; Alexandri and Jones, 2007). In addi- cropland induces an additional UHI build-up of 25 %. The tion, the inclusion of urban effects can also serve to improve UHI and its vertical extent is affected by the boundary-layer Open Access weather forecasts as shown in Hamdi et al. (2012Open Access ). In this re- stability, nocturnal low-level jet as well as radiative cooling. spect, it is indispensable to fully understand all the boundary- Therefore, improvements of representing these boundary- layer processes that contribute to the UHI. Solid Earth layer features in atmospheric models are important for UHI Solid Earth The aim of this paper is to investigate the dominant heat- Discussions studies. ing terms that contribute to the maximum UHI intensity, and Open Access Open Access Published by Copernicus Publications on behalf of the European Geosciences Union. The Cryosphere The Cryosphere Discussions 8526 H. Wouters et al.: The diurnal evolution of the urban heat island of Paris their interactions with boundary-layer processes and circu- vection model (Sect. 3.4) for an in-depth analysis of the evo- lations. Therefore, two model runs are performed with the lution of the boundary layer over the city during the night. state-of-the-art atmospheric model ARPS (Xue et al., 2000, Hereby, the sensitivity of the nocturnal UHI build-up to the 2001) updated with a simple urban surface parametrization. different boundary-layer processes and stability are investi- The first run accounts for the current land use including the gated. Our conclusions are formulated in Sect. 4. urbanization of Paris. For the second run, the urban land cover is replaced by cropland. By looking at the differen- tial output between the two runs, the impact of urbaniza- 2 Method tion on the surface energy balance, the consequential UHI and its vertical extent are analysed. Even though the surface 2.1 Mesoscale model description and set-up energy balance of the urban surface and the consequential boundary-layer UHI has already been addressed in earlier Meteorological fields are simulated using the Advanced studies, such as Lemonsu and Masson (2002), their direct Regional Prediction System (ARPS), a non-hydrostatic interaction with the nocturnal boundary layer have not been mesoscale atmospheric model developed at the University investigated in detail. It is known that the mixed-layer height of Oklahoma (Xue et al., 2000, 2001). The turbulent fluxes is at least eight to ten times smaller during the night than of momentum and sensible heat are calculated by account- during the day so that urban heat release is mixed over a sig- ing for stability effects and the roughness sublayer (Gar- nificantly smaller depth (Bohnenstengel et al., 2011). Yet, the ratt, 1992). Therein, flux-profile relationships for wind speed direct link between the urban surface heating and nocturnal and temperature from Cheng and Brutsaert (2005), Businger increase in boundary-layer UHI intensity has not been inves- (1966) or Dyer (1967), and De Ridder (2010) are used. These tigated. The interaction of urban surface heating with noctur- flux-profile functions are solved for ζ = z/L iteratively (z is nal boundary-layer stability, near-surface radiative cooling, the lowest model layer height above the displacement height low-level circulations such as the low-level jet and the verti- and L the Obukhov Length) by establishing a relation involv- cal motion induced by orography have not been addressed as ing the bulk Richardson number RiB (Arya, 2001), and us- well. Moreover, the representation of these boundary-layer ing Ridders (1979) root finding scheme as described in Press features during the night remains a challenge in state-of-the- et al. (1992). The land-surface scheme used in these simula- art atmospheric models. It is not known to what extent these tions is that of De Ridder and Schayes (1997), which contains uncertainties affect the representation of the UHI at that time. advanced parametrizations of plant transpiration. Tempera- In order to address these questions, an idealized advection ture and water content of the vegetation canopy and of five model is developed in which a moving air column is advected soil layers are calculated using prognostic conservation equa- over the city during the night. The model is used to study the tions, and water flow in the soil is calculated using Richard’s underlying mechanisms that favour a maximum boundary- equation (Garratt, 1992). Soil moisture and temperature were layer UHI build-up at night even though the urban surface initialized using data from the Global Land Data Assimila- heating is relatively low at that time. In addition, it is inves- tion System (GLDAS see Rodell et al., 2004). For reasons of tigated how the UHI extends in the vertical under nocturnal consistency, the soil texture taken was the same as that used boundary conditions. The idealized advection model also al- in GLDAS. From the soil texture information that comes with lows to determine the interaction of urban surface heating the GLDAS data, we estimated domain-mean fractions of with boundary-layer features including orographical effects, clay, sand, and silt, with contributions of 25, 35, and 40%, the low-level jet, radiation cooling and stability. Therefore, it respectively. The corresponding soil textural parameters of provides insight to which extent uncertainties of the noctur- the Clapp and Hornberger (1978) relations used in our model nal boundary layer affects the representation of the UHI.
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