Urban Temperatures and Urban Heat Islands

Transcription

1 Urban Temperatures and Urban Heat Islands James Voogt Associate Professor & President International Association for Urban Climate Western University, London ON Canada Part 2: Urban Heat Islands Beginnings heat island types Conceptual basis Lowry approach. Physical basis for UHIs: Relating UHIs to temperature changes and the energy balance UHIs in the future 1

2 Heat Island Types Boundary Layer Heat Island (BLUHI) Canopy Layer Heat Island (CLUHI) Modified after Oke (1997) Surface Heat Island (SUHI) 3 Urban Heat Islands Types Sub-types UBL (BLUHI) Fixed - tower, sodar Traverse - aircraft, tetroon AIR UCL (CLUHI) Fixed - screen Traverse - automobile Depends on observation methods Standard - grass Non-standard - roof, roadside SURFACE (SUHI) SUBSTRATE True 3-D T - complete surface Bird s-eye 2-D T - aircraft, satellite Depends on definition of surface Zero-plane T - model output Ground T -road After Oke (1995) 4 2

3 Finding Urban Effects: Lowry Conceptual Model M it x = C i t x + L i t x + H i t x M - measured value of a weather element (e.g. temperature) C - background ( flat-plane ) climate L - departure from C due to topography H - departure from C due to human effects (including urban effects) Subscripts: i - weather type t - time period x - station location (see Zones next slide) Lowry (1977) Defining Zones in the Lowry Model u urban area r rural or natural area* e a rural or natural area surrounding the urban area that is influenced by urban effects *rural areas are impacted by human activities such as agriculture; natural areas have none (i.e. H itx = 0) Consider temperature measurements in these zones. r r u u e e Lowry (1977) 3

4 Utility of the Lowry Framework a) shows that measurements (e.g. temperature) in a city have contributions from multiple influences. b) demonstrates that a reference location outside the urban area may not be fully free of human influences c) provides a framework for experimental design and control related to identifying urban effects A difficulty is that the framework has many assumptions that are difficult to fully satisfy What is the Urban Effect? We want to isolate H for an urban site Conceptually: M i0 x = C i 0 x + L i 0 x + 0, so M it x -M i0 x = H i t x But pre-urban measurements are typically not available and C may change over time. An alternative: numerical model for the same site run without the urban area 4

5 Urban Rural Differences M itu -M ite = (C itu -C ite )+ (L itu -L ite )+ (H itu -H ite ) Is M itu -M ite evidence of urban effects? Issues:? What is area e and can it be determined a priori? Is area r a good representation of pre-urban conditions? Note that area e may be large from some climate variables Lowry (1977) The Lowry model and UHI assessment A UHI is an urban effect the Lowry model provides a conceptual basis for identifying urban effects that is important to making UHI measurements (e.g. siting instruments) Lowry model is useful for many other urban effects and is applicable to modeling approaches as well as observations 5

6 Physical Basis for Heat Islands Urban heat islands are created by differences in urban and rural energy balances. Consider the physical basis of the main UHI types Surface Urban Heat Island 35 ~ Agricultural Mixed Rural Warehouse Residential Lt. Indust. Downtown T r ( 癈 ) SN + 2 hrs SS SS + 4 hrs SS + 9 hrs NOAA-11 Fraser River Comm./Res. ~6 ~8 10 ~9 Park Traverse Traverse Distance (km) Land use areas with large amounts of visible impervious surfaces appear hot during the day (Warehouse, Lt industrial) Large diurnal temperature range. At night large and positive SUHI, maximum at end of night. 12 6

7 Diurnal Evolution of Urban Surface Temperature Basel Switzerland Sperrstrasse street canyon: 0700 Jul Jul Surface Temperature Changes Temperature change can be related to the surface energy balance By day we have C is heat capacity of a very thin layer of depth z 7

8 SUHI: daytime surface heat exchanges Roofs Surfaces - unobstructed, dark, dry, insulated if winds weak, strong heating roofs very hot Q H T roof K K T interior Q G Q G T wall 1 Q H,, T wall 2 Q H, Q H Q G Canyons Surfaces - walls & floor part shaded, large area, dry, sheltered, good conduction good heat storage warm not hot walls & floor unless open & sunlit T floor Q G Surfaces - T roof >> T walls > T floor > T interior SUHI - large positive: T surban > T srural Oke et al. Urban Climates forthcoming, Cambridge University Press 15 SUHI: nighttime surface heat exchanges Roofs Surfaces - large sky view, dry, insulated if winds weak strong radiative cooling very cold Q H T roof Q G T interior T wall 1 T wall 2,, Q G Q H, Q H Q G T floor Canyons Surfaces - dry, small sky view, near calm, good conduction weak radiative and convective drain, heat from storage and interior warm walls & floor Surfaces - T interior > T walls > T floor >> T roof SUHI - positive: T surban > T srural Oke et al. Urban Climates forthcoming, Cambridge University Press 8

9 Urban & Rural Surface Temperature Changes SUHI (K) 2 K 0 SR Roof SS Urban T/ t (K h 1 ) 0 Canyon SN Rural H/W 1 Time of day Night time SUHI: Important controls Δμ u r Oke et al. (1991) SVF s : sky view factor : thermal admittance Largest SUHI occur with small SVF, small rural, large urban 9

10 SUHI Temporal Variability Imhoff et al. (2010) F Forest, G Grassland, D Desert, M Mediterranean Forest Canopy Layer Heat Islands Canopy Layer: UHI is a maximum at night, under calm and clear conditions (max 12 C, annual average 1-2 C). May be small or even negative during the day. Oke (1982) Data from Oke in Aguado and Burt (2004) 20 10

11 Canopy Layer UHI: Spatial Structure ~-1 Vancouver BC Canada Clear August (summer) day. T air ( 癈 ) Tunnel ~5 ~9 ~10 Mixed rural Residential Lt. Industrial SN + 2 hrs SS SS + 4 hrs SS + 9 hrs Park 5 Agricultural Warehouse Comm./Res. Downtown Traverse Distance (km) Voogt and Oke (unpublished) Unlike SUHI afternoon CLUHI is negative By sunset it is growing fast Maximum at end of night Rural has cooled 16 but city core by only 6. Clearly CLUHI is due to thermal inertia - difficulty in warming and cooling 21 Differences in urban & rural cooling underly the CLUHI Air Temperature Sunset Urban (a) T U R 2 C Sunrise Heating/Cooling Rate Rural TIME (h) Oke (1982, 2011) Important differences in the early morning and near sunset 11

12 CLUHI temporal intensity AIR TEMPERATURE Sunset (a) Urban Rural 2 C Sunrise HEAT ISLAND INTENSITY (b) T u-r = CLUHI TIME (h) Oke (1982, 2011) Temperature Change in a Layer 1 1 Convergence = div Divergence (div) Oke (1987) Simplified situation: large, flat, homogeneous surface No net horizontal transfers. No latent heat release. By day, when atmosphere well mixed, divq* z 0 12

13 Temperature Change in a Volume 1 for both radiation and sensible heat flux consider the volumetric divergence e.g. Stull (1988; 2015) CLUHI: daytime heat exchanges Roofs Air - contact & convective cooling hot layer strong convection that feeds BLUHI K Q H T roof K Q G T wall 1 T wall 2 T interior Q G Morning: divq HV in urban canyon is weaker than for rural Q H,, T floor Q H, Q H Q G Canyons Air - warm surfaces heat air but not more than rural (less evaporation more storage) thus small or negative CLUHI unless sunlit & exposed Q G Air - T a roof > T a canyon CLUHI - small: T a urban ~ T a rural Oke et al. Urban Climates; forthcoming, Cambridge University Press 13

14 CLUHI: heat exchanges at night Roofs Air - contact & convective cooling cold layer, occasional katabatic slumps into canyons Q H T roof Q G T interior T wall 1 T wall 2 Canyons Air - warm surfaces convect & radiate to air & other surfaces,, hence slow cooling CLUHI, & weak plumes feed BLUHI Q G Q H, Q H Q G T floor Air - T a roof < T a canyon CLUHI - large, positive: T a urban > T a rural Oke et al. Urban Climates; forthcoming, Cambridge University Press Volume Processes: Night Clear and Calm Urban Weak divq* y, radiation from canyon walls; stronger divq* z (but not as strong as in rural area due to sky view factor obstruction) Weak divq Hy : heat from canyon walls Net result: radiative cooling that is weaker than for the rural case Rural Strong radiative divergence div Q* z under stable night time conditions Weak sensible heat flux convergence divq Hz from above Net result, strong radiative cooling 14

15 Maximum UHI vs Surface Controls Maximum Urban Heat Island (K) Sky view factor, s Canyon aspect H/W ratio, H/W Oke (1981), and H/W Oke (1987) Seasonal Variation of Rural Soil Thermal Admittance UHI UCL Pot = UHI UCL Obs / [u -½ (1 - kn 2 )] Runnalls and Oke (2000) 15

16 Temporal Evolution of Canopy Layer Heat Island 10 MONTH Łódź, Poland TIME OF DAY Note: Daily pattern Seasonal pattern with daylength Seasonal effects of prevailing weather and rural surface conditions Plotted by T.R. Oke with data from Klysik and Fortuniak 31 When is the canopy layer heat island best developed? 1 Normalized heat island intensity Sun Set Sun Rise Normalized time of day Runnalls and Oke (2000) Heat island intensities for Vancouver from

17 Weather Controls on the CLUHI CLUHI Wind speed: T u-r = u -0.5 Cloud Cover T u r = T u r (1 k n 2 ) CLUHI calm u windy Cloud cover relation n = fraction of cloud cover k = coefficient based on cloud type k larger for thick low clouds Combined Effects: Weather Factor w w = u 0.5 (1 kn 2 ) w how much will UHI be reduced from its maximum potential value; 0 w 1. (Runnalls and Oke 2000) overcast clear 33 Surface UHI and 3-D City structure Temperature ( 癈 ) Tair Tsfc (3-D corr) Tsfc (nadir) Agricultural Mixed Rural Warehouse Residential water Day: 1530 LDT; Solar Noon + 2 hrs Night: 0530 LDT; Sunset + 9 hrs Res./Comm. Lt. Indust. Downtown Park Traverse Distance (km) Voogt and Oke (2003) 34 17

18 UBL Heat Islands Boundary Layer Heat Island Increased absorption of K by air pollutants. Anthropogenic Heat (Q F ) heat production by space heating, particularly stacks. Increased Q H from below (heat flux from roof tops and canopy layer). Increased Q H from above (increased roughness leads to increased turbulent entrainment) Modified after Oke (1997) 35 Boundary Layer Urban Heat Islands Day Night Oke (1982) 36 18

19 Control of Urban Form on the Heat Island Urban Form Characteristic Surface geometry Surface reflectance Surface thermal properties Greenspace Irrigated areas Impact on Heat Island Low SVF promotes high night time CLUHI. Shading reduces daytime SUHI and CLUHI. High reflectance reduces all daytime HI, with some effects continuing to the nighttime HI Materials that are good storers of heat will contribute to night time CLUHI, BLUHI and potentially to negative daytime cool islands. Some materials that are well insulated (e.g. roofs) may contribute to daytime SUHI and BLUHI due to their high temperatures. Reduces all daytime heat island and possibly night time heat island depending on the exact nature of the vegetation used (e.g. trees vs grass). Strong reduction of daytime SUHI, CLUHI with some transfer to BLUHI Based on Oke (1982) Heatwaves and Cities European Heat Wave: 55,000 deaths Russian Heat Wave: 70,000 deaths The urban heat island can magnify heat wave effects Heat is the deadliest of weather hazards (US Data) 19

20 Future Changes to UHI With large scale climate change, CLUHIs both increase and decrease McCarthy et al. (2010) Factors Affecting Heat Islands Geographic Location climate topography rural surrounds Time day season Synoptic Weather (Limits UHI) wind & cloud stability manageable City Size fetch distance density of use UHI City Function energy use water use pollution Mitigation Measures City Form materials (fabric) structure cover Oke (pers. comm.) 40 20

21 Summary Lowry conceptual model is useful for thinking about how to measure and interpret urban heat islands and all urban measurements. Urban actions to address heat islands should recognize the different types and their specific processes. Physical basis of the urban heat island is rooted in energy balance differences that differentially affect urban and rural heating and cooling rates at the surface and in the UCL or UBL air volume. Cities of the future are likely to be hotter but the UHI may not necessarily be larger depending on how rural cooling rates are affected. End, Thank you James Voogt Department of Geography Western University, London ON Canada N6A 5C2 Tel: x Website: gt_james.html 21