9/26/2022 URBAN CLIMATOLOGY 2. Factors controlling urban climate, energy balance, urban boundary layer Paper to read Theor Appl Climato] (2009) 95:397^»06 DOI 10.1007/SÜ0704-008-0017-5 ORIGINAL PAPER Quantifying the influence of land-use and surface characteristics on spatial variability in the urban heat island Melissa A. Hart ■ David J. Sailor https://is.muni.cz/auth/el/sci/podzim2022/ZX601/um/67875456/02 Hart Sailor TAC 2009.pdf 1 2.1 Factors controlling urban climate Climate categories (scales) • Urban climate is a typical example of the local climate. However, it can be studied on different scales from mesoclimate to microclimate (see further for urban climate scales) MierrtiirtMltt-IMi-M,) Mi -Gomlield - Foresr disarm^ M, - Hilf s(öJHr - lc* field t*\ - Cf«i no.™* LrtjIdiffHSL+isili-LJ Lq ■ Croplands \U. plain (q ■ fores! Mi, - MounlSHil ^ I Maunrjm-ftivironmtiir _Source: Climatology, Oliver and Hidore, P.163. 2.1 Factors controlling urban climate For local climate category it is typical that processes in lower layers of the atmosphere are primarily formed by radiative, thermal, aerodynamic, and moisture properties of active surfaces Active surface (layer) is the surface or layer at which energy is redistributed (e.g. reflected) or transformed to another type of energy 1 Rural Commerce Downtown R^"a, Park In broader sense active surface controls the exchange of energy, mass and momentum 2.1 Factors controlling urban climate 1) Thermal and radiation properties of active surfaces, which are decisive for the intensity of absorption and reflection of short-wave electromagnetic radiation and emission of long-wave radiation 2) Surface geometry of active surfaces, which increases their total area, contributes to a significant proportion of surfaces with vertical orientation, to the creation of so-called street canyons and to high roughness 3) Waterproofing of active surfaces forming increased runoff of precipitation, reducing evapotranspiration and air humidity 4) Atmospheric pollution related to the occurrence of pollutants in the air and increased occurrence of condensation nuclei 5) Anthropogenic heat Thermal properties of the surface materials (radiation balance) Thermal properties of typical urban surfaces cause accumulation of thermal energy during the day and its release during the night Thermal properties of the surface materials Comparison of selected thermal characteristics for typical urban and rural surfaces (modified after Oke, 1987 and Zmarsly et al. 2002) Specific Heat Thermal Thermal Thermal Material Density heat capacity conductivity diffusivity admittance p/kg m"3 c/J kg'1 cp/J m~3 X/Wnr1 a/m2 s-1 b/J s-°'5 K-' K-' K-' m"2 K-' Asphalt 2,100 920 2.0 10s 0.75 0.4 106 1.200 Loamy soil 1,600 900 1.4 -106 0.25 0.2 106 600 (40 % pore space; dry) Ratio 13 1.02 1.4 3.0 2.0 2.0 Asphalt/ Loamy soil Thermal properties of the surface materials (albedo) Various Urban Environment Albedos ^iwe ±111111. wiiite tjoi / Roofing shingles 25.0 Snow. «55.0 Stone 31.7 Tar-gravel roof 13.5 Yard (90% lawn, 10% soil) 24.0 Albedo of urban areas is lower (10-15 %) that that of rural areas 9/26/2022 • Height to Width Ratio (H/W) • Sky View Factor (SVF) 5 Surface roughness «0« - UriAH Ali* SUIUMS lEVfl COUNTtr soo GtASIINI HINP -»4-7 .400 -»y GIADIEM WIND ■ ~]00 X 9 "W -»y CtADIINT HIND X 100 ~**i "**7 j L "*/ ~"7 100 Slif "**/ t^fe ____ -1»J source: http://www.mfe.govt.nz/ General decrease of wind speed in „strong flow" situations Surface roughness Rural 5uburban Pond Warehouse Urban Downtown Urban Park Suburban Rural or Industrial Residential Residential source:https://www.quora.com/ Higher turbulence due to higher air instability (strong local winds in „UHI situations") Surface waterproofing Higher proportion of impervious surfaces is responsible for direct changes in moisture conditions and changes in water balance and indirect changes in temperature conditions 40% evapotransplration 30% evapotransp I ration 25% deep infiltration Lower soil moisture and higher drought danger Lower evapotranspiration causes higher air temperatures High and fast surface runoff Polluted surface runoff Air pollution Average air pollution concentration over an urban area Factors increasing urban air pollution: • Higher concentration of sources (vehicles, industry, heating) • Lower wind speed due to surface roughness • Role of relief (basins, concave shapes of urban relief) • Higher stability of urban atmosphere (temperature inversions) Anthropogenic heat • Results mainly from electrical and chemical energy that are converted to heat and released in UBL. • Includes three main sources: buildings,transport, and metabolism: fuel combustion in and industry, heating and cooling of, many processes of everyday life lightning, heating of water, etc. • Depends on population density, but very regionally specific (climate and geography, economy, transport modes, cultural habits etc.) □ Australasia O f) (Continental) • Typical daily and seasonal variations, direct measurements are replaced with estimates (modelling) 2.2 Energy balance of urban/rural areas The city energy balance can be simplified to: Q* + Qf= Qe + Qh + AQs + AQa where: Q* = net all-wave radiation = K* + L* (net shortwave and longwave radiation) Qf = anthropogenie heat emission (Qjv + Qfh + Qfiri) Qe = latent heat flux Qh = sensible heat flux AQs = net heat storage in the city AQa = net advection into or out of the city. Latent heat - energy released or absorbed by a body during phase transition. Evapotranspiration (from liquid water to water vapour) - consumption of latent heat Condensation (from water vapour to liquid water) - release of latent heat Sensible heat - energy transported to atmosphere via turbulent exchange Energy balance of urban/rural areas Figure 3a: Typical Daily Summer Rural Energy Balance Q, i Incident solar radiation Incoming 7.6 infrared 5.9 Reflected solar radiation Outgoing infrared 8.5 Latent heat Sensible heat 0.8 °s Storage heat 0.3 Figure 3b: Typical Daily Summer Urban Energy Balance Q, QR Incident solar QL4. radiation Incoming 7.6 infrared 5.9 Reflected solar radiation Anthropogenic heat Sensible heat 1.0 Outgoing Latent 3 infrared heat Storage mn 1.0 heat Day-night differences in energy balance Rural Surface Incoming Radiation (from sun) 100 Shortwave Radiation (from clouds & ground) 24 Heat Loss (from evaporation) 24 Longwave Radiation (from clouds) 35 Longwave Radiation (from ground) 54 Heating of Heat Loss the Ground (from air (from move- conduc-ment) tion) 8 30 i r Long- Long- Heat Loss wave wave Heat Loss (from Radiation Radiation (from air Heat Loss evapora- {from (from move of the tion) clouds) ground) ment) Ground Rural Surface* ^ 33 44 11 Urhan Surfarp 1 1 33 t 50 Í 22 1 I Í i Relative differences in surface energy balance (SEB) of urban and rural surfaces during day (positive SEB) and night (negative SEB) (Oke et al. 1998; Alberti 2008) 2.3 Stratification of lower urban atmosphere Urban boundary layer (Rural boundary layer, Planetary boundary layer) Urban canopy layer (UCL) a) Mesoscale r~v-i rT*' W —,-------^^-<£_3-^L_3-- PBL UBL Urban "plume" Mixing layer Rural liiilin-i. _ Jrban Surface layery -"í ■y' Rural BL ^^^■^^ Rural b) Local scale Surface layer Inertial sublayer Roughness _T.^...5ii_NayerV ,,. c) Microscale I. UCL Roughness sublayer □ □ □ □ 2.4 Another factors controlling urban climate • role of weather types (radiation dominated vs. advection dominated) (a) Urban 'dome' No ambient wind O .''""'S (b) Urban 'plume' Ambient wind important role of local geography (relief) 9/26/2022 Typical features of urban climate Table U2 Urban climate effects for a mid-latitude city with about i million inhabitants (values for summer unless otherwise noted) Variable Char Magnitude/comments Turbulence intensity Wind speed Wind direction UV radiation Solar radiation Infrared input Visibility Evaporation Couvective heat flux Heat storage Air temperature Humidity Cloud Fog Precipitation Snow Total Thunderstorms Greater Decreased Increased Altered Much less Less Greater Reduced Less Greater Greater Warmer Drier More moist More haze More cloud More or less Less More"? More 10-50% 5-30% at 10 m in strong flow In weak flow with heal island I—10 degrees 25-90% 1-25% 5^H)% About 50% About 50% About 200% 1-3°C per 100 years; 1-3T annual mean up to 12°C hourly mean Summer daytime Summer night, all day winter In and downwind of city Especially in lee of city Depends on aerosol and surroundings Some turns to rain To the lee of rather than in city (Landsberg 1981) 2.4 Final remarks and questions 1. What are the main factors controlling urban climate? 2. What are the main terms of urban climate energy balance? 3. How we can define urban climate scales? 4. What are the main features of vertical stratification of the atmosphere in urban environment? 5. What other factors form typical urban climates? 11