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URBAN CLIMATOLOGY
2. Factors controlling urban climate, energy balance, urban boundary layer
Paper to read
Theor Appl Climato] (2009) 95:397-106 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/podzim2024/ZX601/um/67875456/02 Hart Sailor TAC 2009.pdf
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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)
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_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
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Rural     Commercial Downtown agp,, Park |$du*Sai
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)
Shortwave and longwave (thermal) radiation can be transmitted, reflected and scattered
Figure 2, Radiation balance Reflection to soace
The
Sun
\
From Atmosphere
Longwave radiation
Shortwave radiation
From SurfflCQ
5  10 15 20 Wavelength (\un) ft1"1)
sB„«.,„,di„io—4 vL°xr-i
Diffuse__ Direct 4 'USl
radiation radtettw ^ ■
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 (albedo)
V A
Various Urban Environment Albedos _^<=
HiiiMy Reflective „        . Tar El CriMI u'
»**.«,-«*   <^ipwn. ■■■■■
Corrugated fiouf
Allie-t'o valties at urIi furi.ice n
Material
Concrete blacktop/asphalt Brick, red Brick, yellow/buff Brick, white/cream
Roofing shingles Snow, weathered
Tar-gravel roof
Yard (90% lawn. 10% soil!
68.7 25 0
Albedo - ability of active surfaces to reflect shortwave radiation Albedo of urban areas is lower (10-15 %) that that of rural areas
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)
Material
Density p/kg m-3
Specific heat c/J kg'1 K-'
Heat capacity cp/J m~3 K-1
Thermal conductivity X/WrrT1 K-'
Thermal diffusivity a/m2 s"'
Thermal admittance b/J s-°'5 m"2 K-'
Asphalt               2,100             920               2.0-106      " 0. Loamy soil          1,600             900               1.4-106 0. (40 % pore____
^Ratio                 1.3                 1.02              1.4 3. SipluW---,___'__
Loamy soil
One typical day, the Chicago City I lall green roof neascrer almost 80"P (40°C) cooler than the neighboring coneentiona roof.
Emissivity - ability of active surfaces to radiate longwave radiation
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General decrease of wind speed in „strong flow" situations
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
• 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 lower atmosphere.
• Includes three main sources: buildings,transport, and metabolism:
fuel combustion, industry, heating and cooling, 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 t) (Continental)
Typical daily and seasonal variations, direct measurements are replaced with estimates (modelling)
2.2 Energy balance of urban/rural areas
Three forms of energy
•  Radiation energy (shortwave and longwave)
• Latent heat
• Sensible heat
And three modes of energy transfer
• Radiation
• Convection
• Conduction
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 = anthropogenic heat emission (Qfv + Qfh + Qfln) Qe = latent heal flux Qh = sensible heat flux AQs = net heat storage in the city AQa = net advection into or out of the city.
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2.2 Energy balance of urban/rural areas
Latent heat - energy released or absorbed by a body during phase transition.
• Energy stored in a volume, but not sensed by humans
• In atmosphere is transported by convection
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 stored in a volume which is directly linked to its temperature
• Linked to motion of molecules, can be ..sensed" by humans
• Transported via conduction (slow, in solids) and convection (in air) along the temperature gradient
Energy balance of urban/rural areas
Figure 3a: Typical Daily Summer Rural Energy Balance
Q, i	
I ncident	
solar	
radiation	Incoming
7.6	infrared
	5.9
Fteflect&d
solar radiation
Sensible heat 0.8
Outgoing infrared 8.5
Latent
Storage heat
0.3
Figure 3b: Typical Daily Summer Urban Energy Balance
Q,
incident solar radiation
7.6
R		
Q	LJ.	
Incoming		
infrared		
5	9	
Reflected
solar radiation
Sensible heat
1.0
Outgoing    Latent S infrared      heat Storage
Anthropogenic heat
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Day-night differences in energy balance
Incoming Radiation (from sun)
Rural Surface
Ufban Surface
Shortwave
Radiation (from clouds & ground) 24
Heating of Heat Loss    the Ground (from air (From move- canduc-ment) tion)
30
w    Rural Surface
CT
	3	3		1 Ü	1 1 1 2 22
			1/ 1 1		
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 X*^?
—,------- s&-<J-J--cL J—
UBL
Urban "plume"
Mixing layer
lU^llllll. .Surface layer/
Rural
b) Local scale
Jrban
—7"   Rural BL
i
Rural
c) Microscale
Inertial
Surface sublayer layer
^    - A
Roughness ▼ .^...sublayer V.
I.
UCL
Roughness sublayer
c
□ □
n n
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2.4 Another factors controlling urban climate
• role of weather types (radiation dominated vs. advection dominated)
(a) Urban 'dome' No ambient wind
Rural Urban Rural
\
• important role of local geography (relief)
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 Cotivective 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 1-10 degrees
25-90%
1-25%
5-40%
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 Tn 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)
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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?
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