Final Flashcards
Study for Final Exam
What is the Anthropogenic Heat Flux?
- Qf [W m-2]
- Energy flux density released directly by human activity at the urban-atmosphere interface
- mainly the result of chemical and electrical energy converted to heat and emitted into the atmosphere
What are the components of the Anthropogenic Heat Flux?
Qf = Qfb + Qfv + Qfm
- Qfb: fuel combustion and electricity use in buildings, ~60% of total Qf
- Qfv: fuel combustion in road vehicles, ~30% of total Qf
- Qfm: human and animal metabolism, ~10% of total Qf
How would you estimate Qf using a top-down approach?
Detailed accounting of electricity and fuel consumption stats (utility reports, statistical yearbook, government documents) at larger scales (e.g., megacities)
How would you estimate Qf through a bottom-up approach?
Numerical modelling of individual elements of the urban system (buildings, vehicles, humans)
How would you estimate Qf through an energy balance residual approach?
Estimating the residual of long-term energy balance measurements using Q* + Qf = QH + QE + ΔQs
Where …
- Q* measured with a net-radiometer
- QH + QE measured through eddy covariance
- ΔQs is negligeble over the year
- Qf is therefore the residual value once the EC terms are taken from Q*
What are some typical values of Qf at the city (mesoscale) level?
Large, High density cities
- Annual: 60-160 W m-2
- Winter: 100-300 W m-2
- Summer: > 50 W m-2
Medium density cities
- Annual: 20-60 W m-2
- Winter: 50-100 W m-2
- Summer: 15-50 W m-2
Low density cities
- Annual: 5-20 W m-2
- Winter: 20-50 W m-2
- Summer: < 15 W m-2
What are some typical values of Qf at the neighbourhood (local scale) level?
Large dense, city centre
- Local Climate Zone(s): 1,2
- Hourly Values: 100-1600 W m-2
Medium dense, city centre
- Local Climate Zone(s): 3
- Hourly Values: 30-100 W m-2
Low density, open, low-rise
- Local Climate Zone(s): 6
- Hourly Values: 5-50 W m-2
Heavy Industry
- Local Climate Zone(s): 10
- Hourly Values: 300-650 W m-2
What are controlling factors for Qf values?
Space heating/cooling demand
-> varies based on geography, and related seasons
Urban form and energy efficiency
-> varies based on climate zones, commuting distance, mass transit systems, population density, per capita energy use, “shared walls” theory
Time of Day and Season of Year
-> low-density city in a sub-tropical climate will have similar Qf values throughout the year regardless of TOD and season
-> higher density city in a continental climate will have a greater difference, where summer months see lower Qf values (likely due to control 1)
What is Heat Storage Change (ΔQs)?
- ΔQs [W m-2]
- retention of heat by the urban “volume” (ground, buildings, air vegetation)
- thermal properties of materials determine their ability to transfer and store heat
How would you estimate ΔQs using an energy balance residual model?
Same as with the Qf, except include ΔQs
-> Pros: calculated value of ΔQs is integrated across the entire urban source area of sensors
-> Cons: expensive, technically demanding, and site species; this residual term (ΔQs) contains all errors and uncertainties of the other terms
How would a thermal mass scheme analysis work to model ΔQs?
-> Place multiple heat flux plates within a building-soil-air volume
-> Measure temp. change in a representative set of urban facets and materials
-> Approach is impractical and laborious; requires extensive knowledge of materials and their properties in the study area
How can you numerically simulate ΔQs?
-> Calculation of heat conduction in/out of walls, roofs, and ground is theoretically straightforward, but multi-layered nature of many buildings complicates things
-> Heat transfer is often simulated using a resistance network approach:
—> in series (one pathway, e.g., through a wall) OR in parallel (multiple pathways, e.g., through a building)
-> The Town Energy Balance (TEB) model includes resistance formulations for uptake/release of heat for roofs, roads, and walls
How would Parameterization help in estimation and modelling ΔQs?
-> For solid materials, there is a strong correlation between Q* and the sensible heat conducted into the substrate (QG)
-> However there is an inertial lag in conduction, which results in a characteristic diurnal hysteresis loop
-> Parameterization scheme (or algorithm) is developed based on the known relation between ΔQs and Q* for individual surface types, such as roofs, roads, and lawns
-> The contributions made by the coefficients for each surface type are weighed by area to give an equation that is unique to the site
What are the controlling factors on urban heat storage change?
THERMAL PROPERTIES OF MATERIALS
Natural
-> Clay, Sandy soils (600-2210, 620-2550 mu - thermal admittances when dry and saturated;
-> water (1545 mu);
-> air (390 mu)
Built
-> Asphalt (1205 mu)
-> Concrete (150-1765 mu, when aerated and dense)
SURFACE MOISTURE AVAILABILITY
-> More storage when materials are saturated
-> If surrounding rural areas have extreme values for
URBAN STRUCTURE
-> Due to the trapping of Kin and screening (admittance) of Lout in urban canyons, heat absorption and storage (and thermal admittance) in cities tends to be greater than in the flat surrounding countryside
How would we measure turbulent sensible (QH) and latent (QE) heat fluxes in cities?
Done through eddy covariance systems
-> thermocouples, ultrasonic anemometer-thermometer, open-path infrared gas analyzer
What are the challenges when measuring turbulent exchanges in urban systems?
-> Instruments must be mounted sufficiently high to be in the Inertial Sublayer (e.g., >2 zH), so that measurements represent the local scale and are not directly influenced by turbulence in the Roughness Sublayer
-> Source area of sensors should be reasonably homogenous
-> Equipment is expensive to purchase and requires expert installation on tall masts (often above building rooftops)
How do energy balance terms vary across the day for rural, suburban, and urban source areas?
RURAL
-> Q* peaks midday, and falls in the middle of the night (peak hits ~550 W m-2 around 11:30 AM)
-> All Q terms follow this same pattern, with QH (sensible heat flux) being highest, ΔQs having the largest magnitude, and QE remaining relatively constant
SUBURBAN
-> Q* peaks midday, and falls in the middle of the night (peak hits ~500 W m-2 around 11:30 AM)
-> All Q terms are relatively similar in their diurnal pattern, especially QH and QE which have the same relative values; ΔQs falls a bit more than the others
URBAN
-> Q* peaks midday, and falls in the middle of the night (peak hits ~500 W m-2 around 11:30 AM)
-> All Q terms follow this same patter, but QE has the highest curve and magnitude, while ΔQs and QH are relatively low in comparison
What controls how QH and QE are partitioned?
SURFACE MOISTURE
-> availability and spatial arrangement of water is the dominant control (dew, ponds, puddles, rivers, lakes, irrigated lawns, soils, leaf stomate)
-> if impervious surfaces are dry and snow-free, it is usual to assume they are sources only of QH not QE
SURFACE PATCHINESS
-> create local and microscale advection
-> Leading edge effect (microscale) between dry rock and wet grass, leads to an Oasis effect over the grass, where QE jumps up along the edge before plateauing
ATMOSPHERE
-> stability (turbulence), wind speed/direction, thermal and humidity structure of ABL, large-scale advection (local, synoptic)
-> difference between water-vapour deficits at the surface and in the ABL atmosphere drives the exchange of water vapour from the surface (QE)
-> wind and atmospheric instability reduce atmospheric resistance to heat and vapour transfer from the surface
What is the urban energy balance at a facet scale (e.g., roads, roofs, lawns) within a city?
-> Roads are dry most of the time and have large thermal admittance (mu)
-> The EB of roads does not normally contain the latent heat flux term (QE) and the storage term (QG) is large by day
What is the urban energy balance of a dry canyon within a city?
-> Top - integrated effects of roof, walls, and road. Absence of QE in modelled canyon system. QH is positive day and night
-> Bottom - observations from real canyon with gravel floor, small amount of moisture
What is the urban energy balance of a canopy within a city?
-> Top - area is compact with heavy, dense materials (e.g., stone); devoid of vegetation
-> Bottom - daytime QE is negligible and 60% of Q* is stored in the fabric ΔQs; the remaining Q* drives a sensible heat flux
IN SNOW
-> Low surface temp in the snow-covered UCL decreases turbulent heat transfer (QH) from the canopy - latent heat transfer (QE) is negligable
-> Loss of snow exposes roofs and ground to solar heating, which boosts the role of turbulent heat transfer (QH). QE remains negligent
What are the effects on the mean and turbulent flow-fields in the roughness sublayer (RSL)?
-> We can split kinetic energy of flow into a mean kinetic energy (MKE) and turbulent kinetic energy (TKE) per unit mass
-> Mean wind (laminar) vs. Turbulent flow (chaotic)
RATIO OF TKE:MKE AT FIELD SITES
-> Below rooflines (~0.5) turbulence exceeds 1, it at times much greater than MKE, suggesting turbulence is dominance -> shifts to below once you gain height, denoting a more blended, constant flow
What is mechanical turbulence?
-> produced by surface skin drag or obstacle form drag, or else by shear flow, which causes instabilities arising from strong mean velocity gradients
-> mechanically generated eddies are relatively small and they scale with the size of the roughness elements
What is thermal turbulence?
-> produce by differential surface heating, which causes mean velocity gradients between rising thermals (plumes) in the ABL
-> thermally generated eddies scale with height above ground and can be large or small - they are constrained only by the presence of the ground and depth of the ABL
Why is turbulence important?
-> Controls of transfer of sensible and latent heat (QH, QE) in the urban energy and water balance
-> adds additional forces and oscillations that act on building surfaces - the force are difficult to predict, but building engineers cannot rely solely on mean flow
-> turbulence must be included in dispersion models for urban environments (e.g., dispersal of air pollutants)
What is the typical flow patterns and turbulence features of an isolated building?
-> Buildings are impermeable, inflexible, and sharp-edged objects which causes severe local perturbations of the pressure, mean flow, and turbulence fields (MKE -> TKE)
-> What initially looks like chaotic flow is in fact fairly understood, thanks in large part to studies involving physical and numerical models
-> Typical airflow features for an isolated cubical building
Unobstructed flow before the building, then on the sides and above are displacement zones, followed by the wake zone after the building. Immediately, behind the obstruction will be a cavity zone
STAGNATION POINT
-> 2/3 of the way up the centre of the face of the building
How is the vertical wind profile altered by an isolated building?
Upwind: the profile is in equilibrium
Above: acceleration in the displacement zone, deceleration in cavity
Behind: velocity deficit in cavity
Downwind: restoration of wind profile
How does an isolated TALL building influence turbulence?
-> injects momentum to the ground, creating strong or dangerous winds for pedestrians, yet can also help to disperse ground-level air pollutants
-> surface wind speeds can double within a radius equal to tower height
How do arrays of buildings impact turbulence?
-> airflow is controlled by the geometry of the array in the RSL
-> depends on building spacing (H/W) and density (λb, plan area ratio of buildings; λf, frontal aspect ratios)
-> also depend on wind speed and angle of approach relative to the street canyon
HW < 0.35:
-> isolated roughness flow
0.35 < HW < 0.65:
-> wake interference flow
HW > 0.65
-> skimming flow
Isolated tall tower:
-> pollutants circulate down (complex flow in cavity zone)
How do cross-canyons impact turbulence?
-> flow is perpendicular to street canyon axis, controlled by HW ratio, external wind, and roof geometry
-> consider the implications for traffic management and air-pollution control
How do multiple stacked vortices form, and impact turbulence and pollutant dispersion?
-> if HW is large (>2), main vortex develops into secondary cells, one atop of the other
-> Air pollutants re-circulate internally and thus dispersion at ground levels is poor
What is a helical vortex, and how does it impact transport?
-> External wind is at an intermediate angle to the canyon (~45 deg.)
-> transport is carried along as the sum of vectors of cross-canyon vortex and along-axis channelling flow
-> a helical path spirals down the canyon and is very effective at dispersing air pollutants
What is channeling?
-> When the angle of approaching wind is <30 deg., the in-canyon vortex may disappear and flow is channelled along the canyon
-> as it enters the canyon, the flow is constricted and accelerated, causing “jetting”
How do street intersections influence turbulence?
-> Four-way intersections lead to high concentrations of vehicle emissions in a semi-confined space
-> 3D flow structures at street intersections, with wind approaching at 45 deg., helical flows collide at the canyon intersection, creating an upward-tilted “conveyor belt” that vents pollutants out of the canyon
How are vertical wind profiles modelled for the UCL and RSL?
-> Measure horizontally average mean flow in each layer of the system
-> Wind profile follows a roughly logarithmic form post-rooftop, but pre- there is an inflection point where the profile is exponential due to wind shear, turbulence, etc.
-> Exponential law applies only in the mid/upper parts of the UCL -> closer to the floor (e.g., z < 0.15 zH), skin drag produces a more logarithmic decay until the flow is brought to rest at the floor.
What is the vertical wind profile in the ABL above urban and rural surfaces?
MIXED LAYER (ML)
-> Layer of air that is much deeper than the surface layer and is between the ISL and free atmosphere
-> Uniformly mixed by turbulence and is relatively constant in the ISL but not in the RSL
Inertial Sublayer (ISL)
-> Layer of air above RSL and below ML
-> Homogenous in the horizontal and there is no mean vertical wind; sheer-dominated turbulence creates a log wind profile
What is the profile of mean wind in the ISL?
-> We can disregard complexity in the flow around 3D elements of the UCL and think of the urban surface rough plane
-> zd (zero-plane displacement) defines the height at which the mean velocity is zero due to large obstacles such as buildings/canopy
-> z0 (roughness length) is defined as the height at which the mean velocity is zero due to substrate roughness
What is the logarithmic profile of mean wind in the lower atmosphere?
-> At 50m above an urban surface, wind speed is reduced by ~50% from the gradient wind speed in the free atmosphere
-> Ocean has 30% reduction, and crops surface has 20%
-> Wind accelerates faster on smoother surfaces like the ocean, and the effects are diluted on rougher surfaces
How does mean wind change in the lower atmosphere across an urban-rural gradient?
-> Wind reaches greater height on city surfaces compared to countryside/canopies
-> With step-increase in urbanization (surface roughness), wind speed is reduced
-> Effects on wind speed for rural surfaces are diluted
What are practical applications of the wind profile equation?
-> The log wind profile is a valuable model that can link sites where measurements are made to others where they are not
How would you estimate the micrometeorological roughness parameters zd and z0?
-> Use field measurements of wind or turbulence to solve for the parameters using the eddy covariance system
-> however the set-up is not accessible to everyone due to expensive equipment and need for tall masts
How would you estimate the morphometric roughness parameters zd and z0?
-> use functional relations between roughness parameters and easily measures of urban structure (e.g., height and density of elements)
-> rule of thumb: z0 ~0.1 zH and zd ~0.7 zH but using zH ignored the effects of building density of airflow
-> a more preferred way of measuring roughness layer for most people, but it is only based on height so it can be too one dimensional
How do roughness parameters react when buildings are close together?
-> zd (zero-plane displacement) levels off when the buildings are very close together (not common in real cities) but the zd/zH increases as closeness increases as well
What are typical values of roughness parameters?
RURAL
-> Bare soil, cut grass: 0.01-0.02 mm
-> Grass, stubble field: 0.03-0.06 mm
-> Forest: 0.8-2 m
URBAN
-> Low height and density: 0.3-0.8 m
-> Medium height and density: 0.7-1.5 m
-> Tall and high density: 0.8 - 2 m
-> High-rise: > 2 m
How is air flow characterized in the Mixed Layer (ML)?
Surface Roughness
-> creates drag on the mean flow and increases mechanical turbulence
Urban Heat
-> excess warmth of the city changes spatial patterns of air pressure
With strong regional winds, roughness effects are dominant, but with weak winds thermal effects are more dominant
What are the influences of roughness on wind speed and direction?
Wind speed and direction, vector forces producing geostrophic winds in the free atmosphere are resultant winds in boundary layer
How is wind affected across an isolated area of greater roughness?
Rough-to-smooth transition could be urban-to-rural, land-to-water, or forest-to-grassland
-> Veering - clockwise change of wind direction
-> Backing - counter-clockwise change of wind direction
How is wind thermally influenced?
-> In calm and clear conditions, heat from city is sufficient to generate a local circulation cell called the country breeze/urban heat island circulation
-> Factories should be sited i) downwind of the city relative to prevailing winds and ii) beyond the limits of the country breeze
–> country breeze brings factory plume into the suburban/urban areas
When combined, how do roughness and thermal influences impact the boundary layer flow?
When there is strong flow, with weak UHI, roughness dominates = deceleration and convergence of winds across the city (urban plume created)
When there is weak flow, strong UHI, thermal effects dominant = divergence and acceleration of winds into the city, followed by convergence and deceleration out of the city (creating an urban plume)
When it is calm, and UHI is present, thermal effects dominate = a convergence to the centre of the city (creating an urban dome)
What is the qualitative definition of urban heat island?
-> term refers to the relative warmth of urban areas compared to their non-urbanized (rural or natural) surroundings
-> the warmth of the UHI is measured in the air but also at the surface and substrate
-> UHI is the clearest expression of inadvertent climate modification by humans
What is the morphological features of heat islands?
-> UHI magnitude is highest over the city centre, sharp increase in temperature over cliffs, but dips at the plateau
-> City core retains more heat in the nighttime compared to rural areas
What are the main causes of UHI in the UCL and surface?
Heat islands are caused by the process of urbanization that alters the surface energy balance -> alterations lead to marked differences in urban and rural cooling and warming rates at the surface, in the substrate, and in the air
- Greater absorption of solar radiation due to multiple reflection and radiation trapping by building walls and verticals surfaces in the city
- Greater retention of infrared radiation in street canyons due to restricted view of the radiatively “cold” sky hemisphere
- Greater uptake and delayed release of heat by buildings and paved surfaces in the city
- Greater portion of absorbed solar radiation at the surface is converted to sensible rather than latent heat forms
- Greater release of sensible and latent heat from the combustion of fuels for urban transport, industrial processing, and domestic space heating/cooling
What is the quantitative definition of urban heat island?
UHI = ΔTu-r = Tu - Tr
-> Heat island magnitude is useful for making comparisons of climate impact among cities
-> Difference between the maximum urban temperature and a representative temperature of the surrounding rural area over a specified period
-> The definition is vague, non-standard, and not indicative of local climatic differences within cities
UHI = ΔT lczx-y = Tlczx - Tlczy
-> it is preferable to use LCZs for defining UHI magnitude (air and surface types)
-> LCZs are more explicit than terms urban and rural because they account for geometric, radiative, thermal, moisture, and aerodynamic properties of surfaces
-> LCZ classes can quantify urban-rural and intra-urban temperature differences in a standardized manner
How do the geometry of the surface influence energy balances and temperature?
The sun’s position will dictate which urban faces will retain heat and be the warmest throughout the day
How do the radiative qualities of the surface influence energy balances and temperature?
Albedo along can greatly influence surface temperature, a white surface can be 5-10 degrees celsius cooler than the surrounding darker area
How do the thermal qualities of the surface influence diurnal energy balances and temperature?
Daytime Roofs
-> Hot due to large solar absorption, poor conduction (roof insulation), and no water
-> Strong radiative heating and sensible heat flux into atmosphere
Nighttime Roofs
-> Cold due to large sky view, poor conduction, and good convection
-> Strong radiative cooling
How do thermal controls on Tsfc cycles and SUHI magnitudes?
Urban surfaces with high thermal admittance (i.e., concrete) vs. rural surfaces with low thermal admittance (i.e., dry soil)
-> larger temperature magnitudes in rural setting, with lower temps overnight, and higher peak temps
-> SUHI magnitude dropping at solar noon (Urban Cool Island)
-> Rural areas can be warmer in the daytime compared to the city, but will cool faster at night
Urban surfaces with thermal admittance vs. rural surfaces with medium-high thermal admittance (i.e., wet soil)
-> Urban temp is always higher than the rural, and has higher magnitudes
-> SUHI magnitude drops (staying positive), and peaks after solar noon (Urban Heat Island)
How does surface energy balance simulations differ depending on LCZ zone in humid subtropical city?
Heating/cooling rate is greatest (greatest magnitude in SUHI) for LCZ 8 (industry)
Heating/cooling rate is smallest (smallest magnitude for SUHI) for LCZ 2 (compact mid-rise)
SUHI is large during the day, small at night
How does surface energy balance simulations differ depending on LCZ zone in hot desert climates?
Heating/cooling rate is greatest for LCZ C (Bush, Shrub) -> surface temp high during day due to arid conditions and cools very fast
Heating/cooling rate is smallest for LCZ 6 (open low-rise)
SUHI magnitude is small during the night, and negative during the day (cool island)
What are methods to observe the “Surface” UHI (SUHI)?
Ground-based sensors
-> radiometers on masts or ground vehicles, or with thermal infrared (TIR) hand-held sensors
-> capture important microscale details of surface heat, but the “field of view” (FOV) is narrow and unable to sample “complete” surface temperatures of LCZs or whole cities
Hand Held Sensors
-> FLIR C2 camera used to produce thermal images of surfaces in a city
Fixed Platforms
What are the advantages and disadvantages of Satellite Remote Sensing of Surface Heat Islands?
Satellite Thermal Infrared (TIR) imagery is by far the most popular medium for SUHI analysis
Advantages:
-> spatially continuous thermal imagery for the entire city
Disadvantages:
-> Anisotropy effects: where they only analyse a specific fact side and rooftops
-> Rooftop bias by satellite, where they only get vertical representations
-> temporal resolution is poor, and limited to the time of satellite overpass and cloud-free conditions
What are the problems with apparent surface temperatures?
-> All satellite thermal sensors detect radiation emitted and reflected by the surface, rather than temperature directly -> output is then apparent surface temperature, not the true surface temperature
-> transmission of radiation to the sensor is reduced by the intervening atmosphere -> can be corrected
-> thermal images “assume” that all surfaces are black bodies -> surface emissivity corrections
-> Preferential view of horizontal surfaces means that we can’t see the vertical walls -> anisotropy corrections
Satellite images do not:
-> Air temperature (both UCL or UBL)
-> Heat load on building walls
-> Surface temperature beneath tree canopy
What is the spatial and temporal variability of SUHI?
-> News media use satellite thermal imagery because urban areas are brilliantly coloured as hot zones, both day and night
-> Often used to mistakenly justify heat mitigation strategies in cities and neighbourhoods
How does weather influence SUHI magnitudes?
Wind
-> strong wind reduces the SUHI magnitude
Rain/Fog
-> reduce SUHI magnitude
What is the spatial morphology of Canopy-Layer Heat Islands (CUHI)?
-> Idealised cross-section of nighttime CUHI (mid-latitude city, flat terrain, clear skies, no wind)
Temperature changes
–> rural-suburban boundary causes a cliff;
–> plateaus over suburban areas;
–> valleys over parks;
–> hill at suburban-urban boundary;
–> peaks over urban centre
What are the temporal and spatial changes of CUHI and SUHI?
CUHI dominates at nightime, while SUHI dominates during the day
They change positions as well, where SUHI has in higher temperatures than CUHI during the day, but at night CUHI has higher temps
What are the changes in CUHI with urban topography?
As the city gets more built up the temperature changes become more pronounced
What are the diurnal variations of CUHI magnitude?
-> amplitude of daily rural Tair cycle is greater than the urban tair cycle
-> in late afternoon, rural cooling rate greatly exceeds urban, due to its large Sky View Factor (SVF), small thermal admittance (if soils are dry) and small heat store
-> CUHI is essentially a nocturnal phenomenon
What are the seasonal variations of CUHI magnitude?
In Mid-Latitude Cities
-> we see the greatest magnitudes over the summer months, lowest over the winter
In Humid Tropical Climates
-> monsoon effects impact magnitudes, where the summer months have the highest magnitudes, while late fall into winter, have low magnitudes
In Hot, Semi-Arid Climates
-> depends mainly on precipitation, where CUHI is lowest in high precipitation months, with the highest magnitudes in low-precipitation
In the Winter Season at High Latitudes
-> CUHI was greatest regardless of LCZ class when the ground was covered in snow
How do physical and functional properties impact CUHI magnitudes?
-> Higher Sky View Factors reduce maximum CUHI magnitudes
-> Higher Canyon Aspect Rations (H/W - i.e., taller buildings) increases CUHI magnitudes
How does wind speed/direction influence CUHI magnitudes?
-> Is a surrogate measure of atmospheric transport and mixing, which limits horizontal and vertical temperature differences
-> To isolate the influence of wind speed on CUHI, restrict data analysis or data collection to clear-sky days and to the time of max. magnitude (1-3 hours after sunset)
-> Magnitude decreases with higher wind speeds
How does cloud cover alter CUHI magnitudes?
-> Amount reduces solar receipt and “traps” longwave radiation at the surface, diminishing urban-rural Tair differences
CLOUD TYPE indicates opacity, base height, and therefore temperature of the cloud, also affects radiation receipt and trapping at the surface
-> CIRRO (high cloud, >5km, cold cloud) -> 25% reduction in outgoing longwave radiation
-> ALTO (middle cloud, 2-8km) -> 75% reduction
-> STRATO (low cloud, <2km, warm cloud) -> 90% reduction
To isolate the influence of cloud on CUHI, restrict analysis to calm days:
-> conduct mobile surveys of CUHI during nights with completely overcast skies (10/10 cover) and calm or light winds
What is the combined influence of wind and clouds on CUHI magnitude?
With complete cloud cover and high winds, the magnitude is at its lowest, as you decrease cover and speed, it become higher
How do passing air masses and fronts impact CUHI magnitudes?
When air masses and fronts cross an urban area that can overwhelm or destroy the CUHI
They can also create spurious heat (or cool) islands if the air mass or front is present at one station of an urban-rural pair and not the other
How does soil moisture impact CUHI magnitudes?
-> When soil thermal admittance is highest (under saturation), the CUHI magnitude is lowest
What is the definition of a Boundary-Layer Heat Island?
-> Difference between air temperature in the UBL (above the UCL) and the ABL (above rural area)
-> Urban and rural air temperature measurements in the boundary layer must be made at similar height, relative to the BL top
What are the causes of BUHI?
Canyon Geometry
-> increases surface area for absorption and multiple reflection; reduction of SVF; reduction of wind speed
Construction Materials
-> increased thermal admittance and “waterproofing” of the surface
Air Pollution
-> greater absorption and re-emission of radiation
Anthropogenic activity
-> building and traffic heat/moisture emissions from fuel combustion and electricity use
What controls the BUHI?
Represents a convective accumulation of warmer air (2, 3) with that entrained from above (4) and internal radiative effects (1)
- Polluted boundary layer - increased absorption of longwave and shortwave radiation by aerosols and gases
- Anthropogenic Heat - heat injected upward into UBL from chimneys and factor smokestacks
- Sensible heat flux - turbulent, upward mixing of warmer canopy-layer air
- Entrainment - injection of warmer drier air from above the capping inversion, down into the UBL
What is the spatial structure of the BUHI?
-> Isotherm patterns of the CUHI are consistent with those at the base of the UBL
-> BUHI shifts with wind direction, forming a plume downwind of city
-> With calm or weak winds, an urban dome forms above city (causing a UHI circulation)
How does urban surface warming, influence subsurface heat islands?
-> SUHI extends down into the soil, bedrock, and groundwater beneath the city
-> Measurements of deep soil temperature at 10-100m below city surface
-> Thermal effects of surface paving and underground infrastructure (tunnels, sewers, foundations)
-> Spatial pattern of the subsurface heat island corresponds with the built form of the city, higher magnitude corresponds to a large source of industrial heat; also affected by surface water drainage
What are some societal impacts of Urban Heat?
Inverse relationship between heat exposure and income level
-> Affluent neighbourhoods are cooler than poorer neighbourhoods
-> Low income populations are disproportionately affected by urban heat, partly due to lack of blue/green infrastructure (trees, parks, ponds, streams, lakes) in poor neighbourhoods
Human Mortality
-> increase in mortality is almost exponential above the heat-health threshold of 22 deg.
UHI increases building energy use and photochemical smog formation
UHI enhances rainfall and flooding in large cities
UHI can create economic opportunities (e.g., in urban planning, in hazard mitigation)
Is UHI detrimental, beneficial, or neutral to cities?
Depends on the city:
In Singapore (tropical rainforest climate, daily max 32 deg., no distinct seasons)
-> Hot humid conditions make it mainly detrimental from a human health, energy use, and hydro-meteorological (Monsoons) sense
In Vancouver (Marine west coast climate; warm, dry summers and cool, wet winters, distinct seasons)
-> Mostly detrimental due to urban aspects
What are the terms of the Urban Water Balance Equation?
P + I + F = E + ΔS + G + ΔR + ΔA
INFLOWS
-> P = Precipitation
-> I = Piped water supply
-> F = Anthropogenic Water Vapour
OUTFLOWS AND STORAGE CHANGE
-> E = Evaporation
-> ΔS = Storage change in buildings and ground
-> G = Groundwater
-> ΔR = Net Runoff
-> ΔA = Advection
What are the three layers of urban-water balance processes?
- Surface, where precipitation (P), evaporation (E), anthropogenic injection (F), irrigation (I), and advection (ΔA) all occur
- At surface, and just below, we see surface water and piped water dominating, sewers and surface water influencing runoff (ΔR), and pipe leaks and percolations moving down to the third layer
- Unsaturated Zone + Groundwater, where the remnants of the hydrologic cycle that has permeated through the surface end up (G), influencing water table depths, and creating cones of depression where wells pull water for use
How do we measure the precipitation?
-> Difficult to measure in cities due to spatial variability, low sampling density, air turbulence and rain gauge inadequacies
-> Evidence suggests that cities modify (increase) precipitation above and downwind of urban areas
MEASUREMENTS
-> Use rain and snow gauges at ground level and away from the microscale turbulence effects of trees and buildings
-> BAD PLACEMENT: at rooftop sites, or close to trees/buildings
-> Can also translate observations from nearby rural climate stations, but not ideal because of spatial variability in precipitation amounts
What are other considerations in Precipitation measurements?
RAINFALL INTERCEPTIONS
-> A large portion of P in urban settings is interception by building roofs, walls, or tree leaves - a small portion is retained on the intercepting surfaces and evaporates (E), while the remainder finds its way to the ground (G) and into the soil (ΔS)
-> Intercepted rainfall (P) by urban trees and vegetation plays a role in reduction and retention of storm-water runoff (ΔR) in cities
–> helps to reduce the intensity of precipitation and subsequent erosion at the surface
SNOWFALL INTERCEPTION
-> may settle on urban surfaces before melting (ΔR, ΔS) - the amount of water temporarily stored in the snowpack (ΔS) depends on its density and depth of accumulation
How does surface influence interceptions?
-> Green roofs and pacement can be installed to store more water (ΔS) and reduce runoff (ΔR) - the amount stored depends on the slope of the green surface and the technology used
-> Impervious roofs shed water (P) down to the ground level through gutters, where the water becomes runoff (ΔR) or is stored into the soil (ΔS) - roofs themselves provide little storage capacity (ΔS), a small portion of rainfall is retained on the roof before it evaporates (E)
What is Pipe Water Supply (I)?
-> Water used for buildings, garden, irrigation, and industrial processes, and that “lost” leaky deteriorating pipes
-> Per capita water use depends on the availability of water, the maturity of the economy, the affluence of the population, the background climate, and water-pricing schemes
What Influences Pipe Water Supply (I)?
BUILDINGS
-> Piped water used in residential, commercial, and institutional buildings for drinking, washing, cooking, etc. is eventually channelled back into the sewer system (ΔR)
GARDEN IRRIGATION
-> Piped water used for irrigation has immediate effects on surface and soil water availability (ΔS), and is a critical control on evaporation (E) -> can greatly change the local and microscale climates of neighbourhoods
Leakage from Piped Water Supply
-> Pipe water will leak from old, deteriorating water and sewer systems, damaged pipes from earth movement also leak water
-> Contributes to soil moisture (ΔS), and groundwater recharge (G), which can raise the water table
What are Anthropogenic Influences on Water Balance Inflows?
Anthropogenic Water Vapour (F)
-> released into the urban atmospher (BL) from human activities (e.g., vehicles, heating systems, industrial processing, cooling pods, human respiration)
Combustion
-> In the water balance, F refers only to newly formed water as a result of the combustion of gasoline, wood, natural gas, and other fuels in vehicles, heating systems, and industry
What are the effects of urbanization on Evaporation?
-> E is smaller in urban than rural environments due to systematic replacement of vegetation and soils with paving and building materials (except in irrigated desert cities)
-> Responds to water conditions and land cover, during wet periods, E is much smaller than P and ΔR
What are the effects of urbanization on Net Runoff (ΔR)?
-> Urban environments are much less permeable than rural environments, so water is easily repelled and enhances runoff that increases the risk of flooding
-> Magnitude of ΔR increases in urban areas, and so does the ratio of R/E -> controlled mainly by impervious surface cover fraction
How is water partitioned differently under increased urban development?
-> Evaporation decreases;
-> Infiltration/percolation decreases;
-> Runoff increases
What are the effects of urbanization of streamflow (R)?
- Greater volume of runoff (R) is generated following a storm event
- Shorter time for water input (P, I) to appear in gutters, drains, streams, etc.
- Lower base-flow (stream flow that is not runoff) due to less water infiltrating the substrate (ΔS)
How is surface water quality changed under urbanization?
-> Urban runoff (R) is contaminated by surface pollutants (salt, dust, refuse, metals, particulates), which are carried into rivers and aquatic ecosystems downstream -> “toxic shock”
-> Urban runoff is relatively warm due to the SUHI, which raises the temperature of large rivers flowing through cities -> “thermal shock”
What is Water-sensitive urban designs main goal?
-> To offset adverse effects of urban development on runoff quantity and quality (ΔR)
-> A key measure of WSUD performance is the proportion of precipitation that is diverted from runoff (ΔR) to storage (ΔS) following a rainfall event
How is storage change (ΔS) impacted by urbanization?
-> Represents temporal changes in water mass in the air (humidity) on the surface (pools, ponds), and in the soil and substrate
-> Small component of urban water balance and decreases with urban development (surface sealing), as does E
-> Makes soil water available to plants and is a major control on E; also useful for flood forecasting in cities
SOIL AND SURFACE MOISTURE
-> Measuring soil and surface moisture is impractical because the landscape is highly fragmented
-> This is due to localized irrigation, patchiness of urban surfaces, and disturbance to natural soils and drainage patterns
How is Groundwater (G) impacted by urbanization?
-> Water that infiltrates through the near-surface soil layer will percolate down toward the water table (G)
-> Surface sealing in urban areas reduces infiltration and groundwater recharge (G)
-> Water withdrawn by wells further decreases the water table, but with increased piped water supply (I) in cities the extraction of groundwater is slowed; irrigation and leaks in pipes can help restore the water table
How is Advection (ΔA) changed by urbanization?
-> Describes the horizontal motion of the atmosphere due to wind and transport of turbulent fluxes (heat, mass, momentum) - if there are discontinuities across the urban hydrological unit under study, advection must be included in the water balance equation
-> Is significant in cities where moisture transition zones occur (coastline, urban-rural boundary, LCZ boundary)
What is the total water content of the UBL governed by?
-> Evaporation and Condensation at the surface
-> Net moisture and entrainment at the top of UBL
-> Injection of anthropogenic vapour
-> Net moisture advection in the UBL
What is the moisture budget of the UBL?
Change in absolute humidity of air volume over time, which is described through advection by mean wind, direct water vapour emissions, and the difference in evaporation between surface and top of ML
What is advection like within a city?
-> There are many dry and wet surfaces located side by side, with sharp boundaries
-> Promotes advection and boosts evaporation from wet surfaces
-> Leading to the Oasis Effect
What is the Urban Dry Island (UDI)?
-> During the day, cities are often warmer and drier than the surrounding countryside
-> At night, urban air is often farther from its saturation point because of the UHI, so relative humidity is low compared to countryside
What is Urban Moisture Excess (UME)?
-> At night, the UCL in cities are often moister than the surrounding countryside
HYPOTHESIZED CAUSES:
-> At night, evaporation rates weaken considerably in rural and urban areas, especially in winter
-> In cities, anthropogenic emissions of water vapour continue through the night, while in the countryside, there are no such emissions
-> Reduced dewfall in cities due to higher surface temps at night (SUHI) leads to less water vapour removed from the surface-layer in the form of dewfall
What are exceptions to UME?
-> Desert cities can be more moist than the countryside both day and night due to urban irrigation and fuel combustion
-> Tropical cities can be drier both day and night due to the abundance of vegetation and strong, sustained evapotranspiration in rural areas
What is the diurnal pattern of moisture in the UCL in a mid-latitude city?
Daytime Moisture Deficit: Summer
-> Less evaporation in city (small λv, large λi)
-> greater mixing with dry, above roog air
Nightime Moisture Surplus: Summer
-> Absent, reduced, or delayed dewfall (SUHI)
-> Anthropogenic Water Vapour Emissions (F)
Daytime and Nighttime Surplus: Winter
-> Anthropogenic Water Vapour Emissions (F)
What are the moisture effects in the UBL?
-> Limited observations
-> Moisture sources, such as vegetation, vehicles, vents, and chimneys from the UCL are diffused upwards into the UBL
-> Moisture is directly injected into UBL from tall chimneys and stacks that bypass the UCL
-> Enhances entrainment (both day and night) between turbulent UBL and drier (or moister) free atmosphere
What is the occurrence of dewfall in cities compared to the countryside?
Two Hypotheses for dewfall removing less moisture in urban areas
1. Less nighttime cooling of the urban surface (SUHI) reduces the probability that T0 drops to Td
2. Greater shelter within the UCL compared to rural areas limits the supply of water vapour to the surface from above roof level
What is the importance of Urban Dew?
-> Source of water for plants, animals, and insects
-> Aids in deposition of air pollutants and particulates
-> Promotes growth of surface mildew and mould
-> Corrodes + reduces the energy efficiency of PV panels (Solar Panels)
What is Fog and its impacts?
-> Clouds at the ground level
-> Reduces visibility at the ground level to at least <1km
What is smog?
Term “smog” was introduced to describe the mixture of smoke and fog, but is now also used to describe photochemical smog, which actually contains few to no fog droplets
What is Radiation Fog?
-> Forms overnight as the air near the ground cools and stabilizes, reaching its saturation point
-> Usually dissipates quickly after the sun comes out
-> Forms only in clear, windless skies
What is Advection fog?
-> Forms as warm, moist air flows over cooler ground
-> The warm, moist air is cooled down to its saturation point
-> Unlike radiation fog, can form with cloudy skies and moderate wind
What is upslope fog?
-> Orographic Effect
-> Can sometimes just be considered a stratus cloud
-> Often will start at the top of mountains and build downward into the valley
-> Can be maintained at relatively high wind speeds
What are the two opposing Urban Fog Theories?
- Fog is less common because cities are driver
-> warm cities produce “urban clear islands”
-> decreased densities of “dense fog” due to the UHI effect
-> Historical trends in most cities show fog frequency decreasing with de-industrialization (and stronger UHI) - Fog is more common because pollution acts as condensation nuclei
-> Increased frequency of “light fog” due to urban aerosols
-> Fog was a daily occurrence in many European cities during the industrial revolution
What are some methodological challenges with observing cloud and precipitation in urban settings?
-> Spatial distribution of precipitation is uneven
-> Standard meteorological networks are too sparse
-> Non-urban effects on precipitation exist in all cities
-> Precipitation gauge is too poor of a sampling device
What scientific methods has Lowry advocated greater attention for?
-> Experimental control (use of pre-urban data is best)
-> Sample size (sufficiently large to eliminate sampling uncertainties)
-> Replication (conduct similar experiments in multiple cities)
-> Stratification (sort data into meaningful groups for statistical analysis)
What is the “La Porte Anomaly”?
-> Study conducted in the 60s and 70s, which reported an increase in precipitation downwind of Chicago in La Porte, IL using data from 35-65
-> Hypothesized that the anomaly was due to water, heat, and aerosol emissions from iron and steel mills in nearby Chicago
-> Changnon (1968) reported huge 15 year increases in precipitation, thunderstorms, and hail in La Porte, which spawned a lot of controversy
-> Niyogi and Schmid (2014) revisited the anomaly using data from rain gauges and numerical models, and reported that the anomaly is real and caused by heat and aerosol effects from Chicago
What is METROMEX?
-> The most detailed study ever conducted on urban clouds and precip. in St. Louis (chosen due to its flat terrain and the presence of industry -> urban effects easily isolated)
-> Huge output from weather stations, aircraft, and ground-based radars
-> However, has never been replicated, undermining a cardinal rule of science
What did the METROMEX study find?
-> Increased cloudiness (+10%): clouds from earlier over city; clouds are higher
-> Increased summer rainfall (+30%): afternoon rainfall maximum on downwind side of city
-> Increased severe storm activity (+100%): urban clouds grow deeper and more active
-> Topography is not a significant control on cloud and precipitation formation
What causes did METROMEX attribute to their findings of increased precipitation and cloud effects?
Heat Island Circulation
-> Main cause
-> Can be initiated when conditions are favourable for heat island formation
-> Associated with confluence of winds, convergence, and upward motion over the city
Roughness of Urban Surface
-> Secondary cause
-> Modelling studies show that thermal effects tend to dominate over surface roughness
Ingestion of small and large urban aerosols
-> Modifies droplet size distribution over cities and alters precipitation patters
What are the basic cloud formation requirements?
-> Cooling and condensation of air (by uplift)
-> Aerosols acting as cloud condensation nuclei (CCN)
-> Droplet growth by collision and coalescence
Why are urban areas preferred for cloud and precipitation formation?
-> Urban atmosphere are rich in aerosols that enhance cloud formation
-> Cities are warm with rough surfaces that enhance cloud formation
-> Cities are warm with rough surfaces that enhance convergence and uplift in the UBL
What is the hypothesis for why cities enhance the formation of clouds and precipitation?
-> On first impression, the formation of “fumulus” clouds developing over industrial facilities supports this hypothesis
-> Industrial emissions contribute to downwind development of fog, cloud, rain, and snow
What are the microphysical differences between rural and urban air?
PRISTINE AIR (RURAL)
-> Low aerosol concentration
-> Fewer CCN
-> Fewer cloud droplets form, condensing more water
-> Efficient collision-coalescence process
-> Few large drops
POLLUTED AIR (URBAN)
-> High aerosol concentration
-> More CCN
-> More cloud droplets from condensing less water
-> Inefficient, delayed collision-coalescence process
-> Many small drops
How does drop size affect vertical growth of clouds?
-> In urban-affected cumulus, delays in collision-coalescence result in more liquid water moved to the top of the cloud where evaporation of smaller droplets leads to cooling; destabilized the atmosphere and promotes the growth of deeper clouds
-> Precipitation is delayed but cloud development is enhanced and leads to taller clouds with more water, stronger updrafts, and conversion to ice-phase precipitation in the upper part of the cloud (glaciation)
-> Vapour pressure is lower than water, it rushes towards the ice particles and deposition occurs, releasing latent heat and further destabilizing the atmosphere and promoting deeper clouds
What are some general facts about cities currently?
-> Cities cover only 2-3% of the Earth’s surface
-> Cities generate >70% of total GHG emissions
-> Cities are often located in places of high risk to projected climate change effects (e.g., floodplains, coastlines)
-> Less developed regions are growing in population much faster than developed regions
What are some drivers of global climate change?
Urbanization, land demand, population growth, energy consumption, industrialization, technology development
What are the impacts from main drivers of global climate change?
Higher air temps, warmer oceans, rising sea level, reduced ice cover, reduced snow cover, variability in precipitation
What is radiative forcing?
The change in net all-wave radiation (Q*) at the top of the troposphere after a perturbation, relative to the undisturbed value pre-industrial revolution
What are the forcing agents of radiative forcing?
Anthropogenic (from highest forcing to least)
CO2 (~1.5 W m-2);
Other well mixed GHG’s (~1 W m-2);
Ozone (~0.9 W m-2);
Stratospheric water vapour from CH4 (~0.1 W m-2);
Aerosol-radiation interactions (~ -0.5 W m-2);
Aerosol-cloud interactions (~ -0.5 W m-2)
TOTAL ANTHROPOGENIC FORCING = (~2.25 W m-2)
Natural
Solar irradiance (~ 0.1 W m-2)
Which gases have the highest global warming potential?
- Halogenated gases, CFCs -> >5000
- Nitrous Oxide (N2O) -> 265
- Methane (CH4) -> 28
- Carbon Dioxide (CO2) -> 1
What are the diurnal and seasonal patterns in GHG fluxes in cities?
Diurnal
-> More emissions in the morning and at night (rush hour, cooking, lights on, etc.)
Seasonal
-> In winter, emissions are often higher because of heating (in mid-latitude cities)
What are the controls on GHG emissions in cities?
LCZ
-> In more open LCZ, the larger vegetation cover percentage allows photosynthesis to decrease some of the CO2 emissions
-> E.g., LCZ 6 sees net uptakes in the spring, summer, and fall months before reaching net zero over the summer; in contrast LCZ 5 sees net emissions across summer months before stabilizing over the winter
What is the urban carbon budget?
FCO2 + ΔS = C + R - P
-> FCO2: Vertical fluxes refer to the exchange of carbon between the urban surface and atmosphere
-> C: Combustion; many developed cities with smaller population have huge GHG emissions per capita (related to lifestyles, built form, economy, background climate, heating/AC)
-> R: Respiration; reverse of P; rate of R in urban areas is seperated into soil Rs, above-ground plants Ra, and humans and animals Rm; controls on Rs include Tsoil, water content, and supply of organic matter (litter, clippings)
-> P: Photosynthesis; requires photosynthetically active radiation (PAR); urban-rural differences in P depend upon Tair, water availability, and access to PAR
How would you measure CO2 fluxes at the microscale?
-> Using mobile surveys or bottom-up inventories
-> Can map plumes of emitters in the UCL or determine emissions from individual urban elements
How would you measure CO2 fluxes at the local scale?
-> Using flux measurements from towers in Inertial Sublayer (ISL)
-> Determine emission profiles for neighbourhoods
How would you measure CO2 fluxes at City/Regional scales?
-> Use satellite-based systems or airborne platforms in the mixed or urban boundary layers (ML or UBL)
-> Can interpolate emissions for an entire city
What are the objectives of a well-planned city?
- Efficient use of resources (land, energy, water, sun) to minimize global and regional impacts (GHGs, air pollutants)
- Improved/maintained microclimates around buildings and their environments
- Protection from extreme weather (for people and infrastructure) by considering future climate variability and extremes
What are the two primary reasons for studying UCL in the context of urban planning/design?
- Plan new communities that are clean, energy efficient, and comfortable
- Address current climatic problems in existing communities
How is climate data useful in planning/design?
-> Very important for urban planners and designers
-> Urban Climate Maps (UCMaps) present climate data alongside environmental and land use/cover info -> proven methodology for transferring local climate info to city planners and practitioners
What are the guiding principles for influencing decisions about Climate Sensitive Urban Design (CSUD)
- Background Climate Matters: Thermal Comfort Assessment
-> How much climate intervention is needed?
-> Thermal comfort is the condition of mind when an individual is satisfied with the thermal environment (feels neither too warm nor too cold, no desire to alter clothing/activity levels or change environment)
-> Thermal comfort (and the human energy balance in general) is governed by 6 variables: radiation, air temp, humidity, wind speed, clothing, human metabolism - Consider Climate Issues ar the Earliest Opportunity
-> Once land has been developed, much of the urban climate response is “locked in” - opportunities for change are very limited
-> Priority of this principle is to protect natural resources as much as possible through land zoning and management - No single design can meet all climate objectives
-> we need to compromise or find a priority
-> shade v. sun; ventilation v. shelter
-> Rule of thumb: H/W values of 0.4-0.6 satisfy the need for wind, shelter, solar access, and shade
How can CSUD be applied at a city scale?
-> Global-scale impact is greatest with sprawling cities
-> Goals to regulate the whole city, not its individual parts
-> Retain the natural terrestrial, hydrological, and atmospheric resources through:
—> renewable resources
—> emissions reduction
—> public transportation
—> compact development (reduced GHG emissions, more natural environment surrounding city, reduced solar access, fewer trees and vegetation, stronger heat island, degraded environment)
How can CSUD be applied at a neighbourhood scale?
-> Define neighbourhoods according to LCZ
-> LCZ properties indicate climate
-> Microscale impact greatest in compact LCZs (heat and air pollution need to be intervened at this scale)
-> Ventilation corridors and wind paths
—> arrange streets, buildings, and planting structures parallel to wind direction to decrease wall effects
—> need to remember the need for wind shelter though
—> beneficial cities and seasons with low wind speed
—> creates a “breathing city” that can inhale cool, fresh air, and exhale warm, polluted air
-> Daylighting Streams
—> Reduces air and surface temp. around river
—> Reduces aerosols and pollution
—> Increases habitat connectivity and ecosystem services
How can CSUD be applied at a park/garden scale?
-> Relief from paved and polluted urban environment
-> Cool, clean, and moist air: oasis effect
-> Microclimatic diversity: shelter, wind, shade, sunshine
-> Park surfaces cool quickly after hot, sunny days
-> Fresh air from the park penetrates the surrounding city (multiple small, evenly distributed parks are better than one large)
-> Parks should be built on higher ground so katabatic winds can drain the cool, clean air into the urban areas
-> Nighttime cooling
—> In low-density, residential areas (LCZ 5/6), parks with open-grassy spaces for radiation heat loss
-> Daytime Cooling
—> In high-density working areas (LCZ 1/2), parks with extensive tree canopy for shading
How can CSUD be applied to streets and blocks?
-> The form and function of street blocks exert considerable influence on the formation of microclimates, especially in compact areas
-> Street geometry regulates airflow near ground level by shaping interactions with above roof air
-> Street orientation and H/W are critical street parameters that govern radiation exchanges
-> Tall buildings block sunlight from surrounding areas
How can CSUD be applied to buildings?
-> Primary role is to ensure outdoor climate suits the needs of the occupants (manage energy gains and losses across the envelope)
-> Tall buildings deserve extra attention due to shadowing and airflow effects
-> “Permeable” buildings that allow airflow through create breezeways
—> Can use separations between buildings, elevated bases, small footprints, and urban windows
—> helps to avoid wall effects
-> Wall Effects
—> clean fresh air from the countryside is blocked and cannot ventilate the city (decreases heat and pollution dispersion)
—> Most common in waterfront cities (sea-breeze is blocked)
—> Also can occur in mountain/valley cities (Katabatic winds at night are blocked)
How can trees and vegetation be useful in CSUD?
-> Most versatile element used to modify city climate
-> Can be used for: radiation exchange, wind and airflow, pollution concentration, temperature control, humidity control
-> Most commonly used to provide shade, cool temperatures, and shelter from wind on streets for pedestrians
-> They can help remove air pollutants
—> HOWEVER, if trees are too dense, they may reduce airflow and trap dust and pollutants, especially if there is heavy vehicle traffic in the area
-> Design features that mimic trees
How can CSUD be applied to buildings?
Increasing Albedo
-> White roofs: must be cleaned often to keep their high reflection capacity
-> Reflective pacement: must be cleaned often
Green
-> Green roofs: provides other ecosystem services and habitat for animals; helps with air pollution; can be heavy
-> Green corridor: provides ecosystem services and habitat for animals; cools the corridor region
-> Green facades, urban gardens
Previous pavements
-> Can also reduce the likelihood of flooding
Water
-> Decreases surface and air temperature near the water
-> Fountains/ponds are often integrated into parks and courtyards where people sit to eat, stroll, and relax
-> Water walls and roofs
-> Wetting of streets can help mitigate SUHI - as well as keep away dust
What are the basics of Water Sensitive Urban Design (WSUD)?
-> Primary goal: mimicking the hydrological cycle and offsetting adverse effects of urbanization on runoff (ΔR) quality
-> A key measure of WSUD performance is the proportion of precipitation (P) that is diverted from runoff (ΔR) to storage (ΔS) following a rainfall event
What are the trade-offs for CSUD strategies?
-> All heat mitigation measures have secondary impacts, both positive and negative
-> In mid-latitude cities, impacts are closely tied with changing seasons
-> Trade-offs create serious challenges when deciding the “right” mitigation measure for city or neighbourhood
E.g., Reflective Pavement
-> Positive: reduce solar heat gain, reduce surface and air temps, magnitude of CUHI and SUHI is reduced
-> Negative: increased reflective solar radiation, increase daytime heat load on pedestrians and building walls, and decreases human thermal comfort
E.g., Street trees
-> Negative: green canyons trapping pollutants
E.g., Lakes and ponds
-> Negative: higher thermal capacity, retaining heat during the day and releasing at night
What is a well-planned city?
-> The well-planned city takes both WSUD and CSUD into account
-> Urban climate design guidelines should accommodate both warm and cold seasons
—> Aim to provide a diversity of microclimates to protect citizens from extreme heat and cold
-> Winter design guidelines in Edmonton are really good; strategies which block wind and maximize solar exposure during winter
