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
