Urban Temperatures and Urban Heat Islands

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1 Urban Temperatures and Urban Heat Islands James Voogt Associate Professor & President International Association for Urban Climate Western University, London ON Canada Scales in the Urban Atmosphere Oke (1997) 1

2 Scales of the Urban Surface Microscale city Neighbourhood (LCZ) element UCL block facet street canyon After Oke (1997) What is the Urban Surface? Complete Roof-top Groundlevel Bird s-eye or plan view Depending on the scale, the active surface may differ Screen-level Zero-plane displacement Voogt and Oke (1997) 2

3 Urban Canyons and Sky View Factor 0 < s < 1 H s W Photo: M. Roth (NUS) Urban Canyons: are a basic building block of the urban surface. They affect radiation receipt, loss and wind and turbulence. SVF ( s ) is defined as the fraction of the overlying hemisphere occupied by sky. Alternatively, the canyon height (H) to width (W) ratio (H/W) is an easier to obtain parameter. Urban Surface Properties Materials (fabric): radiative, thermal, moisture and aerodynamic properties Land cover: impervious, vegetated, built fractions Structure: dimensions and spacing of built (and sometimes natural) elements H/W canyon aspect ratio (building height H, street width W) A c /A p complete aspect ratio (Area A, p = 2d plan, c complete 3d area) Sky view factor Metabolism: anthropogenic emissions of heat, water, pollutants and aerosols 3

4 Photo: J. Voogt Urban climates are (mostly) unintentional modifications to the atmosphere that arise due to changes to the surface and modifications to atmospheric properties Local Climate Zones An objective classification scheme that uses surface properties that affect micro and local scale climates. Variable Land Cover Properties Bare trees Snow cover Dry ground Wet Ground Stewart & Oke (2012) 4

5 LCZ Properties Sky view factor Building height to width ratio Built surface fraction Roughness height Terrain roughness class Albedo Thermal admittance = (k/c) 0.5 Anthropogenic Heat Flux Q F Stewart & Oke (2012) Physical Basis of Urban Modified Climates Energy Water Carbon Altered energy, water and carbon balances Grimmond & Christen (2012) 5

6 Surface and Volumetric EB Representations Plane surface Volume depth at which flux of heat is negligible for time scale of interest Q* = Q H + Q E + Q G This is suitable for application at an urban facet, such as a wall. It may also be used in a meso scale model to represent the urban surface. Q* + Q F = Q H + Q E + ΔQ S Note that here the advection term (ΔQ A ), which is difficult to obtain is ignored. This is appropriate where the surface is homogenous. Oke (1987, 1997) The Urban Energy Balance Q* + Q F = Q H + Q E + ΔQ S + ΔQ A Q* The net radiation flux (incoming minus outgoing short and longwave radiation. Q F Q H Q E ΔQ S ΔQ A The addition of heat and moisture to the atmosphere as a result of human activities such as energy use. The sensible heat flux (the vertical transfer of sensible heat in the atmosphere by turbulent mixing). It is responsible for changes in temperature. The latent heat flux (the vertical transfer of water vapour in the atmosphere by turbulent mixing). It is responsible for changes in humidity. Storage heat flux (the energy stored in a volume represented by its temperature/humidity content The advective heat flux (the transfer of energy in a horizontal direction by average motion, i.e. not turbulence) red increase, blue decrease relative to natural non urbanized surfaces 6

7 Conceptual Radiation Budget Midlatitude Summer Day (clear) 0.2 K 800 K 160 L 350 L 455 Q* = K K + L L ; K*= K K K 760 K 106 L 365 L T R =300 K K* = 640 L* = 105 K* = 654 L* = 138 Q* = 535 W m 2 Midlatitude Summer Night (clear ) L 320 L 393 Q* = 516 W m 2 L 335 L 415 T R =308 K T R =290 K L* = Q* = 73 W m 2 L* = Q* = 80 W m 2 T R =294 K Offsetting of incoming and outgoing fluxes; differences in Q* typically small; Seasonal variations can switch which surface has the largest Q* Oke (1988, 1997) Conceptual Energy Balance Clear Mid-latitude Summer Day (Oke 1988, 1997) Q* 535 Q H 150 Q E 305 Q S 80 Q* 516 Q F 30 Q H 240 Q E 158 Q S 148 Clear Mid-latitude Summer Night Q* -73 Q H Q E Q S Q* -80 Q F 20 Q H Q E 7 13 Q S -80 Important diurnal differences: Positive convective fluxes day and night in urban area Importance of Q S at night Values: Typical midday Units: W m -2 7

8 How Cities Change the Energy Balance Ensemble summer daytime energy balance partitioning Fraction of heat stored Q* + Q F = Q H + Q E + ΔQ S Christen and Vogt (2004) Surface Controls on Urban Climates: Urban Form Surface cover, surface roughness, geometric properties, material properties. These affect the surface energy balance and energy partitioning. Offerle et al. (2006) Christen and Vogt (2004) Photo: J. Voogt 8

9 Types of Temperatures & Their Measurement Surface Kinetic Radiometric (multiple types) Air Canopy Layer Boundary Layer Aerodynamic Equivalent temperature due to loading on a person in the street canyon Temperatures of surfaces in the street canyon Stewart and Oke (2010) Urban Surface Temperature The temperature of every surface depends on its surface energy balance, which is governed by the surface properties: Radiative: geometry (shading), ability to reflect solar and infrared and to emit infrared, transmissivity Surface moisture Thermal properties: ability to conduct, diffuse and store heat Aerodynamic properties: roughness, shelter ESCOMPTE/CLU Marseille These properties form the basis of possible mitigation strategies. 9

10 Kinetic Surface Temperature Measurements perturb the system Many surfaces to sample Photos: J. Voogt Radiometric Temperature Planck s law 2 hc T ) k ( e hc B ( 5 Stefan Boltzmann Law 1 L 2 T 1) ; 10

11 Radiometric or Radiative Source Area FOV off-nadir angle N T 4 L ) Azimuth Radiometric Source Area (Soux et al ) Radiative Source Areas Source areas vary with sensor position and FOV. Relative surface contributions weighted by view factors. Adderley et al. (2015) 11

12 Measurement of urban surface temperature at different scales Hot Satellite Aircraft Tower Vehicle Ground extensive to large; often horizontal surface bias Urban scale Neighbourhoods Neighbourhoods Canyons, Elements, some facets moderate; radial or fixed direction sampling moderate; restricted sampling of surfaces Facets & surfaces limited depending on lens FOV Facets & surfaces Cold Increasing spatial resolution (smaller radiometric source area) Radiometric Temperature Radiometric temperature true (kinetic) surface temperature Viewing a 3d surface with a radiometric sensor yields a biased view of the complete surface. Considerations Surface emissivity Effect of urban geometry on multiple reflections Isotropic vs anisotropic emission (for a particular surface) Geometry (at larger scales) that drives anisotropy Atmosphere-surface coupling 12

13 Effective emissivity Symmetric Canyon p = 0.5 L = 275 W m T 0 = 295 K TUF-2d 0.98 effective emissivity eff L L L L canyon flat _ blackbody H/W As with albedo, the geometry of the surface leads to multiple reflections and trapping an increase in effective emissivity Krayenhoff, pers. comm.; Harman et al. (2004 ) Representing the full 3-d temperature N m Temperature (K) Voogt (2000) 13

14 Temporal Evolution of T c and Components Adderley et al. (2015) Directional variability (anisotropy) Modelled anisotropy of remotely observed surface temperature for different LCZ. Simple unvegetated geometries only. This adds a complication to e.g. SUHI determination Krayenhoff & Voogt (in review) 14

15 Anisotropy Impacts Derived Properties The spatial distribution of apparent thermal inertia over the physical model. Zhan et al. (2012) Spatio-temporal evaluation of surface temperature std dev y Voogt & Christen (2010) Time series analysis allows: extraction of information on the dynamic response of the SEB to atmospheric turbulence, surface thermal admittance, may allow visualization of turbulent motions (Christen et al. 2012) x time 30 15

16 Spatio-temporal assessment Difference from mean image ( C) Christen et al. (2012) Mean temperature ( C) Atmosphere-surface coupling Modified from Oke (1997) Turbulence affects surface temperature patterns Christen & Voogt (2010) 16

17 Atmosphere-surface coupling Important for determining the spatial resolution of future remote thermal sensors. Spatial averaging reduces error from near surface turbulence, but UBL turbulence impacts are filtered (also true for Christen et al. 2012) so requires more work Lagouarde et al. (2013) Surface Temperature Variability Impacted by: Surface form solar access, sky view factor (slow variability in time) Turbulence position in canopy, scale of turbulence; large coherent eddies (30 s to several minutes) control most of the exchange of heat over a rough surface (Christen et al. 2012) Material properties thermal admittance 17

18 Air Temperatures: Link to Scales T a Boundary Layer T a Canopy Layer Modified after Oke (1997) Tsfc 35 Air Temperature Measurement Consideration Instrument siting - for local scale urban temperature measurement: Place sensors in UCL at same height as rural stations (ensure homogeneity of LCZ) assumes well mixed conditions well mixed atmosphere some scope for height variations Place sensors in ISL (tall tower) to obtain blended values that can be extrapolated down to UCL Instrument exposure: radiation shielding, aspiration, avoid rooftops, avoid anomalous patches Oke (2006); WMO (2008) 18

19 Source Areas for Scalars Scenario 1b: Profile Method Scenario 1c: Bowen Ratio z 0 = 10-2 m z m = 1 m L = m z m = 2 m v /u * = 1.5 z m = 5 m 100 m Mast 50% source area max. Source Weight Schmid (1997) Source Area Weights Schmid and Oke (1990) 19

20 Hypothetical Canopy Layer Source Areas Stewart and Oke (2012) Internal boundary layer and fetch requirement Each local scale surface type generates an internal boundary layer (IBL) within which the flow is adapted to that particular surface type. IBL growth rate (height : fetch ratio) is about 1:100 for neutral atmosphere. z (exaggerated) Sensor z IBL Suburban 800 m Urban z IBL - height of equilibrium IBL; z d - displacement height; z r - height of RSL; z H roughness z r z H z d z F 04 Industrial - mean height of x Fetch = 100 (z r - z d ): E.g. z H = 10 m, z r = 15 m, z d = 7 m Fetch = 800 m Following fetch requirements it is possible to place sensors in the inertial sublayer where standard boundary layer theory applies (e.g. log wind profile) and measurements are representative of the local (neighborhood) scale. 20

21 Consider Differences in Measurement Source Areas White outline: radiative source area of a thermal remote sensor (but not all surfaces are seen within the circular outline) Yellow ellipse: source area for a thermometer measuring air temperature Voogt and Oke (2003) 41 Source Area Application: Urban Surface Temperatures and Heat Fluxes max 50 C Source Area Weight Surface Temperature 0 15 C Matched radiative and turbulent source areas Voogt and Grimmond (2000) 21

22 Modelling Surface Temperatures Use a SEB approach Scales represented: patches, facets, elements, canopy, neighbourhoods When derived in a model, T 0 is an equilibrium surface temperature; because the other terms can represent temperatures in different forms (Lagouarde et al. 2012). Krayenhoff & Voogt (2007) Surface Temperature (1400 LAT, June 26; latitude 47.6º; view from SW) Typical UEB Computational Scheme Horizontal Irradiances Canyon Windfield Canyon Panel Radiation Budget Canyon Panel Energy Balance Control Volume Air Temperature Canyon Top Energy Balance H Urban Canyon CV panel facet Mills (1993) 22

23 Matching a surface to the application Surface Urban Heat Island SUHI based on only canyon surfaces Urban GEM SURF (Leroyer et al. 2011) model output. Dyce & Voogt 2015 report to Middlesex London Health Unit Aerodynamic temperature wind air temperature T 0 = aerodynamic temperature z 0T = roughness length for heat z 0 = aerodynamic roughness length d = displacement length Malhi (2006) Extrapolated (dashed) profile of air temperature yields the aerodynamic temperature that is linked to Q H 23

24 Aerodynamic temperatures are not equivalent to radiometric temperatures Voogt and Grimmond (2000) Aerodynamic Radiometric Challenges: Surface Temperature Scale of the observation vs scale of the desired data development of remote sensing techniques Temporal frequency of observation how to get observations for the time needed? Matching the observed temperature to the type of temperature needed for the application Impact of atmospheric turbulence on remotely observed temperatures Directional effects: structural and surface properties Linking the observation to the surface of interest Time series applications Developing general methods - links to urban EB model development Emissivity and non isotropic surface characteristics 24

25 Challenges: Air Temperatures UCL temperatures Adherence to best measurement practices (WMO 2008) Longevity of city-specific networks? Regularization of urban measurements by national agencies? Integration of temperatures from digital devices (methods needed?) UBL temperatures Infrequently observed limited access Summary A hierarchy of scales describe the urban surface and atmosphere LCZ classification is one relevant scheme for linking urban surface properties to thermal conditions Urban thermal conditions are driven by the surface energy balance Urban thermal conditions may be characterized by surface and air temperatures these have subtypes that depend on the method and/or location of observation Measurements of temperatures of all types should consider the relevant source area in order to match the observation to the relevant surface. 25

26 End, Thank you James Voogt Department of Geography Western University, London ON Canada N6A 5C2 Tel: x Website: gt_james.html 26