10/16/2024 URBAN CLIMATOLOGY 4. Urban heat Island, UHI types, atmospheric UHI, UHI intensity Paper to read https://is.muni.cz/auth/el/sci/podzim2022/ZX601/um/67875456/04_UHI_BasicsCompendium.pdf 1 4.1 Urban Heat Island (UHI) concept Table U2 Urban climate effects for a mid-latitude city with about 1 million inhabitants lvalues tor summer unless otherwise noted) t'h.mge Magnitude/comments Turbulence intensity Wind speed Wind direction UV radiation Solar radiation Infrared input Visibility Evaporation Corrective heat flux Air temperature Greater Dec re a sed Increased Altered Much less Less Greater Ri'dlkvd Les-. G rcater ligr^ W, hi no]1 10-50% 5-30% at 10 m in strong flow In weak flow with heat island 1-10 degrees 25-90% 1-25% 5-40% Humidity Cloud Fog Precipitation Snow Total Drier More moist More haze More cloud More or less Lc\s More? More About 200% 1-3CC per 100 years; 1-3°C annual mean up to 12°C hourly mean Summer daytime Summer niglit. all day winter In and downwind of city Especially in lee of city Depends nn aerosol and surroundings Some turns to rain To the lee of rather than in city Urban Heat Island Profile Suburban Residential cial Urban Suburban Residential Residential Dai'.1 ntoivn Park http://weather. msfc. nasa. gov/urban/urban_heat_island. html Simplified model because numerous natural and anthropogenic factors cause considerable spatial and temporal variability in the UHI Urban Heat Island (UHI) concept • UHI - closed isotherms indicating an area of the surface that is relatively warm; most commonly associated areas of human disturbance such as towns and cities. • The physiographic analogy derives from the similarity between the pattern of isotherms and height contours of an island on a topographic map. • The annual mean temperature of a large city (say 106 inhabitants) may be 1°-2°C warmer than before development, and on individual calm, clear nights may be up to 12°C warmer. • This difference is called the UHI intensity (UHI magnitude) • The warmth extends vertically to form an urban heat dome in near calm, and an urban heat plume in more windy conditions. AMS Glossary of Meteorology, 2nd Edition (2000) Mesoscale UBL - Urban Boundary Layer nni Wind^> Urban Heat Island processes . _____s^-<£2)-JL Ii- / Rojgh urban surface / stows winds -r Addition of heat from roofs and tops, of urban street jg|^ canyons Warmer, more polluted urban boundary layer Urban "plume" il'wind^ .■llllllllli.l- Addition of anthropogenic _ heat from chimneys and . vents. ^ >"*i / _/_I Rural BL Rural- Urban Rural Microscale Absorption of solar radiation by low reflectance surfaces and trapping by reflections Insulated surfaces lead to high daytime surface temperatures Obstructed view of sky: trapping of radiation heat by surfaces UCL Impermeable surfaces -reduced surface moisture and evapotranspiration UCL - Urban Canopy Layer -—i—r—7 Warmer air I \ temperature _lu; Increase of stored heat by thermal properties of urban materials and increased surface area Source: J. Vooqt EPA presentation A Conceptual Model for Urban Climate Measurements Mitx = Citx + Litx + Uitx M - measured value of a weather element C - background ("flat-plane") climate L - departure from C due to topography U - departure from C due to urban effects Subscripts: i - weather type t - time period x - station location (urban, environs, rural) Recognizes that a measured element is impacted by influences at a number of different scales - the trick is to try and isolate urban influences. Lowry (1977) 4.2 Types of UHI Q Q 1. Atmospheric Boundary layer Urban Heat Island 2. Atmospheric Canopy Layer Urban Heat Island 3. Surface Urban Heat Island 4. Subsurface Urban Heat Island What variables are measured? (Oke et al., 2017) UHI profiles Surface Temperature [Day} AirTemperature {Day) Surface Temperature (Night) AirTemperature [Night) Q. £ □jJj^'rAtLLL -I ftt ^r- Rural Suburban Pond Warehouse Urban or Industrial Residents Downtown Urban Park Suburban Rural Residential Reducing Urban Heat Islands: Compendium of Strategies and (Voogt 2000) UHI types characteristic Table 1: Basic Characteristics of Surface and Atmospheric Urban Heat Islands (UHls)4 Feature Surface UHI Atmospheric UHI Temporal Development • Present at all times of the day and night • Most intense during the day and in the summer ■ May be small or non-existent during the day ■ Most intense at night or predawn and n the winter Peak Intensity (Most intense UHI conditions) • More spatial and temporal variation: ■ Day: 18 to 27°F (10 to 1 S°C) ■ Night: 9tol8°F(5to 10°C) • Less variation: ■ Day: -1.8 to 5.4°F (-1 to3°C) ■ Night: 12.6 to 21.6°F (7 to 12°Q Typical Identification Method • Indirect measurement: ■ Remote sensing • Direct measurement: ■ Fixed weather stations ■ Mobile traverses Typical Depiction • Thermal image • Isotherm map • Temperature graph Reducing Urban Heat Islands: Compendium of Strategies and (Voogt 2000) Atmospheric Boundary layer UHI Source: Oke (1997) • Forms either a dome or a plume in UBL • Measured by temperature sensors mounted on tall towers in urban and rural regions, • Use of balloons, radiosondes, aircraft flown across the area • Maintained by enhanced heat flux from city structures • Higher turbulence especially during day - higher air temperatures compared to rural BL at all levels Atmospheric Canopy layer UHI Roughness _ sublayer^ Canopy Layer Heat Island Source: Oke (1997) Surface Heat Island Measured at networks of sensors at standard (2 m) or screen-level (~1.3 m); sensors must be adequately shaded and ventilated Sensor location is critical in urban environments: • What are the measurements trying to represent? • What is the sensor source area - the surface type upwind of the site? • Are there multiple rural - non-urban types surrounding the city? Maximum at night, under calm and clear conditions, may be small or even negative during the day. Surface UHI Average Land Surface Temperature in Paris, France over the heat wave event of August 4 to 13, 2003 (bousset et al. 2007) • Uses thermal remote sensing technology from different platforms (see next lecture) • Surface temperature is different from air temperature • Weather type limitation, atmospheric effects, viewing geometry, • Mostly used as a relative surface temperature differences • Very spatially variable especially during the day as the surface temperature of every facet depends on numerous factors (see lecture 2) • SUHI is well expressed both day and night Surface Urban Heat Island Temperature (K) Generalized form of sub surface Isotherms representing sub surface UHI at Cologne, Germany temperature profiles at ^ m depth and in Winnipeg, Canada in 20 m depth Oke eta I., 2017, Urban Climates © Cambridge University Press 2017 • Measured in bore holes • Forms due to transfer of sensible heat from the urban surface to the ground • Accumulates for a long time period • Responds to natural (climatic) and anthropogenic processes on long time scale (monthly to decadal) UHI intensity (ATu„r) i i i i i i i Iii jf—V Sunset Sunrise *~\v! ! topography • rural surrounds City Size 4 linked to form * and function / / / / i i • energy use i • water use \ • pollution Synoptic Weather [Limits UHI) wind cloud City Form^ • materials \ • geometry 1 • greenspace / / City Function Mitigation Measures / Source: J. Vooot EPA presentation Daily and seasonal variation of the canopy UHI intensity |an Feb Mar Apr May |un Jul Aug Sep Oct Nov Dec Time of year i i i i i i ii A7„_s 0 0.5 1 1.5 2 2.5 (K) The canopy UHI intensity for Basel (Switzerland) calculated as a difference between urban and rural station (average over the 10 year period, SR and SS are times of the sunrise and sunset respectively. Oke eta I., 2017, Urban Climates _© Cambridge University Press 2017 UHI intensity and weather types Nocturnal UHI intensity in London during the radiation-driven (a) and advection-driven (b) weather (h) Urban 'plume' UHI intensity for heat wave"1 days (HW) and non-HW days (NHW) during the summer months (JJA season) of the 2011-2020 period at Brno BISK station. Vertical lines are mean UHIIs, and their differences express the heat magnitude (HM). (a) UrtMii 'dome' No ambient wind Ambient wind Oke etal., 2017, Urban Climates © Cambridge University Press 2017 UHI intensity and city size (parametrs) STU-R I max ) •c 14- 12-'0 -8 - 4 - 2 - 1000 10000 100000 1M 10M POPULATION Relationship of the maximum heat island intensity with urban population in European, North American and tropical cities (Escourrou, 1991) are different. The size of the city can be characterized via number of inhabitants There is a relation between maximum UHI intensity (UHImax) and number of dwellers (P) (van Hove et al. 2011): UHImax = 2,93 log P-11,95 For Brno (P = 380 ths.) UHImax = 4,4 °C What other city parameters can influence UHI intensity? Problems with UHI definition and UHI intensity evaluation 1NTPRNATIONAL JOURNAL OF CI.IMATOmOY ha. ). CUmaM 31: 200 217 (2011) Pjhlislied online 15 Ap-ril 2010 in Wiley Online Library (wilcyonlinclitMarv.com) DOl: 10.l002)joc.214l Ruy h ftufcif (further ueqetatiun umu Temperature deviation [°C] -1 -0,5 D 0,5 1 1,5 2 2,5 _ hranice katastrálního území města Brna T=f(NDVI) Spatial distribution of mean daily air temperature (Tavg), temperature minimum (Tmin) and intensity of UHI (AT) in Brno area during clear and calm days in summer; air temperatures are expressed as deviations from mean temperature of the study area (c) AUHI - mobile measurements Design of measuring routes (a), air temperature profile along measurement routes (b) and typical spatial distribution of air temperature in central part of Brno in early night hours; air temperature is expressed as deviation (dT) from mean value of the study area and is typical for clear and calm weather during summer (c) Corrections required for temperature changes during the time of the traverse AUHI - from mobile measurements to maps SUHI - remote sensing Spatial variability of LST values within the Brno region; LST derived from Landsat thermal imagery acquired on 15 June 2006. 2001 — mean :ooe .....Im ......................... ---1--- 1 b T Intensity of surface UHI in Brno region defined as the difference between urban (M) and rural areas (P) 10/16/2024 4.4 UHI consequences • UHI impacts may be direct and indirect, negative effects prevail in general • Diurnal Temperature Range is smaller in cities • Increased temperatures during summer in cities amplify energy demand for air conditioning. • Peak electricity demand increases 1.5 to 2% for every 1°C increase in summer temperatures. • Higher surface temperatures can heat storm water runoff with negative effect of various water ecosystems (thermal pollution) • Impacts to plants through changes in phenology may be ambiguous (beginning and end of individual phases of the growing cycle UHI consequences Higher air pollution reduce nighttime cooling, both factors increase a discomfort for city dwellers Higher temperatures enhance urban ozone formation. Higher temperatures increase evaporative emissions, adding volatile organic compounds (VOCs) to the air. Higher daytime and nighttime temperatures affect human health, including general discomfort, respiratory difficulties, heat cramps, heat strokes, and heat related mortality. Urban heat islands make extended heat waves more damaging, particularly to sensitive populations such as children and older adults. How to Reduce UHIs negative impacts? (see special lecture on adapatation and mitigation measures) 14 10/16/2024 4.5 Final remarks and questions • How do Urban Heat Islands form? • What are the UHI types? • How we can estimate UHI intensity depending on available data? • What are the main problems related to UHI? • What is a relation between heat waves and UHI? • Can be there any benefits of UHI? 15