Urban Climate Chapter 2 - Concepts

What controls climate in the UCL?

the mix of surface properties within a few hundred meters.

What is an urban (roughness) element in the context of airflow?

the primary 3D unit. Buildings/trees/building lots. Made up of facets (smaller units)

What is climatology?

the quantitative description of the climates themselves, the use of meteorological knowledge to explain climate difference, and the use of climate data to solve practical problems.

What dictates the height of the nocturnal boundary layer?

the strength of the heat island and surface roughness.

How does the earth's surface control boundary layer characteristics?

via absorption, reflection, emission of radiation, the transformation of energy and mass, the interception of precipitation and air pollutants, and the deflection of airflow(slowing of the wind).

What is surface cover?

An expression of the plan view area occupies by a cover or element type (A_x) with a ground surface area (A_T) large enough to be representative of an area of interest. Plan area fraction = A_x/A_T

What are local climate zones clustered by?

Approximate ability to modify surface climates due to the four controls (fabric, land cover, structure, metabolism)

How does equilibrium with the surface impact formation of an ISL?

Equilibrium at the surface requires that an internal boundary layer forms having been initiated at the boarder that separates two local climate zone surfaces its takes 1-3 km for an equilibrium layer to develop to the top of the RSL existence of an ISL depends on the distance, in the upwind, to where the first different LCZ boundary is encountered.

What is the sky view factor?

A 3-d measure for a single point on a surface that defines the fraction of the radiative flux leaving the surface at this point that reaches the atmosphere above the urban canaopy.

How to measure pre-/post-urban?

Only possible if there are continuous measurements before and after a site is urbanized. / made not natural. We control for background weather conditions and the difference betforew and after development to find VH

What can the ABL be divided into?

Outer and Inner portions.

What are the climactic properties of a surface?

Radiative (geometry, absorptivity, reflectivity, transmissivity, emissivity) Thermal (Specific heat, heat capacity, thermal conductivity, thermal diffusivity, thermal admittance) Moisture (interception/storage capacity, permeability, stomatal characteristics, chemical nature). Aerodynamic (roughness, zero-place displacement, porosity).

What happens in the ABL during the daytime?

Surface heating creates buoyant thermals, which effectively carry surface influences upward to the top of the ABL. This inversion - at height, zi, is the base of the "free atmosphere"

How do you treat a natural area downstream of a city?

You choose VH = 0 for days where the meteorology inhibits transport to a natural site

What happens at the downwind urban-rural border?

A new rural boundary layer forms so the layer of urban-modified air becomes isolated and aloft this forms the formal "urban plume".

What's the frontal aspect ratio?

A parameter relevant to airflow - indicates the frictional barrier presented by the buildings.

What's more common than wind being absent?

A regional wind pattern and an internal boundary layer that grows upward with distance, starting at the upwind rural-urban border until the whole ABL is filled.

What is the city unit

A set of structures and materials - which contrast to those of the rural environment. This unit has things like "urban pollutant plumes" and the "urban heat island" which crystallize the idea of an urban climate. Have diffuse edges (where does it start and end?) impacts extend into "downstream" areas due to transport by wind.

What is an urban canyon?

A structure formed by the common arrangement of a street and its flanking buildings. These are most clearly defined in the dense areas of a city. They are described by their two-dimensional ration - H/W where H is the height of the buildings adjacent to the street and W is the width. this is known as the "canyon aspect ratio"

What is the neighborhood unit?

A unit that includes land use zones used in urban planning (industrial / residential / commercial) These can be subdivided

What is the Entrainment zone?

A zone below the free atmosphere where buoyant thermals bombard the underside of the inversion. Some penetrate into the free atmosphere through inertia. When these penetrating thermals settle back down - they bring cleaner, warmer, dry air to the ABL.

H vs Z_H

Building height for one building vs mean height of all building elements in a larger area.

How are facets distinguished?

By their directional aspect (compass direction, vertical sloped or horizontal)

What other changes might an urban canyon experience?

Changes in airflow / chagnes in building materials / changes in heating/cooling demand, and decrease in vegetated area.

Why is fabric important?

Each material has its own set of radiative, roughness, thermal, and moisture properties. At larger scales (neighborhood/city) it is acceptable and necessary to average these properties.

what are the exterior/interior parts of the UCL?

Exterior - outdoor area Interior - indoor living spaces controlled by heating/cooling

What are the four classes of urban surface properties?

Fabric (materials) Surface Cover Urban Structure Urban metabolism.

What are the four climactically relevant controls on urban climates?

Fabric / land cover / structure / metabolism

What leads to more vigorous mixing in the mixed layer over cities and entrainment at the top of the UBl which makes it deeper than the rural ABL?

Heating is typically stronger in cities compared to rural surroundings?

What challenges does the spatial heterogeneity of land surface types in an urban setting present?

How do you identify a site where representative measurements can be taken for an urban environment.

Taylor - in your study of Houston what was lacking from a "surface" sense. What perspective were you using.

I was using the perspective of an observer who considers the surface synonymous with the ground - I was ignoring the role of upstanding elements in the urban setting.

How does horizontal averaging simplify the urban system?

In application - it simplifies the urban system to one effective dimension.

What is the implication of the make-up of the surface layer?

In city centers, there may not even be an intertial sublayer. For one to exist, the ISL must be above the effects of individual roughness elements and its properties must be in equilibrium with a homogeneous surface.

What are the parts of the surface layer?

Inertial sublayer and roughness sublayer.

How does UHI typically change with wind?

It increases with decreased wind.

In the outer layer what is the daytime situation?

It is referred to as the mixed layer (the top 90% of the ABL - excluding the surface layer) This layer homogenizes atmospheric properties so that the vertical profiles of potential temperature and water vapour, wind speed, and direction are almost uniform with height.

What happens to the ABL at night?

It shrinks as cooling at the surface creates a stagnant layer near the ground that is about 200 - 400 meters deep. This layer inhibits vertical mixing. (nocturnal boundary layer). Above this, to the height of the daytime ABLS, is a layer with properties from the prior day. Called the residual layer. This is mixed, but there is little active mixing

What is the urban block unit?

Its a unit formed by the road network which is usually comprised of a number of adjacent street canyons with similar structure. block units are large enough that they can produce localized breeze systems or clouds.

How does oversimplification of an urban model introduce difficulty?

Its hard to assign values to "surface properties" such as roughness - thermal admittance - or moisture.

What does a lidar system observe & how?

Lidar's work by emitting laser towards the surface or atmosphere that are extremely sensitive and record the light scattered back from suspended particulate matter (aerosols) in the atmosphere)

What is so modified about the nocturnal boundary layer in cities versus rural areas.

Mixing caused by surface heat , in cities, and the greater urban roughness causes the atmosphere over a city to be better mixed at night. In dense areas, the near-surface atmosphere experiences weak mixing and patchy elevated inversions at 50-300 meters.

Where is the greatest variability in microclimates?

Near the surface (greatest day->night ranges of values).

What are the three major types of atmospheric emissions?

Sensible heat (from combustion of fuels in transportation and industry and from air conditioning of internal spaces) Water vapour release during combustion, from industrial processing, and afrom air conditioning and garden irrigation Particulate and gaseous air pollutants are emitted into the air from vehicles, buildings, waste management and industrial processes

For weekend weekday differences?

Since there's no reason VB or VL varies between weekday and weekend, the difference is taken to represent the urban effect. (VH)

How do upwind-downwind differences take root?

Take difference of VM at upwind and downwind stations - the difference ends up being the estimate of the urban signal.

What is the free atmosphere

The atmosphere, which starts at the ABL, where influences of Earth's surfaces are negligible.

What is the ABL when wind is absent?

The climate influences of the city are relegated to a self-contained urban dome

What is the urban boundary layer?

The envelope of air modified by the presence of an urban area. 1-2 km deep by midday but shrinks at night

What is the roughness sublayer?

The layer below the inertial sublayer. Flow here responds to the nature of hte individual roughness elements themselves. Lots of flow, deflection, up and downflows, vortices and plumes arising from facets based on their temperature, moisture, cleanliness. (this needs to be approached in a full -3D context).

What is the atmospheric boundary layer?

The lowest parto f the earth's atmosphere that's in direct contact with the earth's surface (100-3000 m depth)

What controls boundary layer characteristics?

The nature of the Earth's surface at a location.

What are climatopes?

The spatial units which describe areas with similar near-surface atmospheric properties.

What is urban climatology?

The study of the statisically preferred states of urban weather

What is the subsurface layer?

The thermal and moisture regimes underneath the city - urban soils / ground below (tunnels and pipes) exchange heat, water, adn air pollution with the UCL above.

What is in the urban plume?

The thermal, moisture, and kenmatic effects of the city for tens of kilometres downwind. several cities, if nearby, can combine to form a megalopitan plume. (brown clouds from the eastern sesaboard of north america and south asian and east asia, detectible thousands of kilometres downstream.)

What is the complete aspect ration?

The total 3d external surface area of all elements related to the total plan area they occupy.

What is the inertial sublayer? (ISL)

The upper surface-layer. Here, the atmosphere responds to the integral effects of urban neighborhoods. Can also be termed the constant flux layer because the variation of turbulent flux with height is small.

In a city, what is the lower part of the RSL?

The urban canopy layer - which is the height of the main urban elements. This is the site of intense human activity and exchange and transformation of energy, momentum, and waater. The top of this is defines as the height of the urban elements (buildings / trees) Unusually tall elements (very high isolated buildings) play a significant role in the generation of turbulence and roughness.

What is the outer ABL?

Thermal effects of the Earth's surface dominate

Why are dimensions of urban elements important?

They impact radiation and airflow in the urban canopy

How do you choose the type of surface to observe?

This depends on the entity involved, and the sensing system available. The accuracy required and the scale of interest.

Remember urban mixing is strong in hte mixed layer

This is responsible for the uniform murkiness of the polluted atmosphere over the city during the day

Lowry's framework in: Urban Rural Natural

Urban - VH is likely major Rural - VH is relevant Natural - VH = 0

Are changes in surface and atmospheric properties greater in vertical or horizontal?

Vertical.

What is lowry's framework?

Vm( measured value of a weather variable) = Vb(background value of a variable) + Vl(departure from Vb due to landscape or local climate) + Vh(departer from VB due to human activities)

Taylor - in your own research what surface type is the pandora? how might the street canyon results reflect this?

We use a roof-top view - basically we ignore the complexity below the roof-top which represents us leaving out a lot of the NO2 dynamics below the roof and treats it as a black box.

Roof level vs -canopy?

at roof level - wind shear is intense and mixing. The canopy however provides shelter from wind speed. Also it disrupts radiation because of the restricted view of sun and sky.

What is the primary process that merges urban features?

atmospheric turbulence

What dominates the morphometry of the urban core?

buildings and street patterns

What can the plan area fraction represent?

buildings, Vegetation, impervious ground

What is the form of the urban boundary layer at the mesoscale in calm and regional airflow

calm - urban dome where low-lying rural air circulates towards the urban plume and rotation occurs (clockwise on the right, anticlockwise on the left) as air moves towards the city from both sides. Regional - follows the airflow and the urban plume moves downwind.

What are intrinsic features in an urban climate?

created by the city's builders - retain the power to alter them.

What is the timing of these three varieties of waste?

daily and weekly cycles that are dictated by human activity. (Work commutes / schedules / shifts / cultural practice / vacations / sleep ) these are called anthropogenic.

What controls the subsurface?

heat conduction / diffusion.

What will these street-canyon changes impact?

heat storage, evapotranspiration, and heat released by the burning of fuelds.

How might an urban canyon height to width ratio change a setting?

it alters the receipt and loss of solar radiation as well as the longwave radiation exchange and the state of airflow.

Why is the canyon aspect ratio important?

it is relevant to many features of the urban climates. This includes radiation access, shade and trapping, wind effects, thermal comfort, and the dispersion of pollution If there aren't roads - it will describe the height and spacing of whatever is the dominant element.

What are extrinsic features in an urban climate?

latitude/ altitude / proximity to water/ biophysical character of the surrounding area.

What is the surface layer?

lowest 10% of UBL - where effects of the city are most profound

Where is the bottom of the ISL - how does this impact its formation?

minimum 10-20 but can be 100 or more in height - suggests the blending height (zr) could be hundreds of meters in hte core of large cities this means an ISL is unlikely to form over such chaotic clusters.

In a low-roughness setting - what is the make-up of the surface layer?

mostly Inertial sublayer.

Over a rough surface what is the make-up of the surface layer?

mostly roughness sublayer.

What does the sky view factor depend on?

position and orientation of a surface relative to the amount of sky obstruction overhead. On a roof = unity in a canyon = less calculating it requires consideration of spherical geometry.

What controls the atmospheric boundary layer?

roughness, thermal mixing, and hte injections of moisture and air pollution.

What are Lowry's means to find a pre-urban value?

run a specific numerical/physical model without the urban area included. Use observations from non-urban stations whose Vb and Vl values are expected to be a close analogue for the urban site Use stats to extrapolate backwards in time, combined with appropriate historical information about urbanization.

What dominates the morphometry of low-density urban areas?

street buildings and trees contribute equally.

What is hte upper boundary of the roughness sublayer?

the blending height

What is the inner ABL?

the lowest 10% - called the surface layer (SL) which is dominated by friction.

What is the surface layer?

the lowest 10% of the atmospheric boundary layer. Surface influence (heating / cooling / roughness) dominate here and frictional force creates most mixing.