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