GEOB 401 - Glossary of Terms

Covers also essential prerequisite terms and terms in lectures 1 to 31.

Please see also the list of symbols and units for all reading packages.

A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z |


A

Absorption - the process in which incident radiant energy is retained by an object or a substance (e.g. gas). See prerequisite courses GEOB 204 and 300.

Absorptivity (ζλ) - the relative fraction of the irradiance of a particular wavelength λ reaching a surface that is absorbed. Values for absorptivity range between 0 (no absorption) to 1 (black body). See Lecture 2, slide 5 and prerequisite courses GEOB 204 and 300.

Active surface - the principal plane (height) of climatic activity in a system. It is where the majority of radiation exchange, energy and mass transformation, precipitation interception and drag on airflow occur. See Lecture 2, slide 3.

Advection - A term used to describe predominantly horizontal motion and processes in the atmosphere. See prerequisite course GEOB 300.

Albedo (α) - the ratio of the amount of short-wave radiation reflected by a surface to the short-wave irradiance (amount incident upon it). See Lecture 6, slide 19.

Anisotropy - A characteristic's directional dependence, for example thermal anisotropy. See Lecture 7, slide 10.

Anthropogenic heat flux density (QF) - Energy flux density released by human activity (e.g. combustion processes) from an urban surface expressed in W m-2. Often involving the combustion of fuels. Urban anthropogenic heat fluxes include those due to urban 'metabolism' associated with space heating and cooling of buildings, vehicles, industrial processes and human and animal metabolism. See Lecture 10, slide 3.

Apparent surface temperature (Tr) - the calculated temperature of a surface inferred from its measured long-wave emittance (L↑) by applying the Stefan Boltzmann Law with an emissivity of ε = 1 i.e. assuming that the surface is a black body. See prerequisite course GEOB 300.

Atmospheric boundary layer - same as planetary boundary layer.

Attenuation - any process in which the flux density of a parallel beam of radiative energy decreases with increasing distance from the energy source. See Lecture 5, slide 09.


B

Backing - counter-clockwise change of wind direction; the opposite of veering. In international usage this definition applies in both Hemispheres. See Lecture 21, slide 10.

Black body - a hypothetical body which absorbs all of the radiation striking it, i.e. allows no reflection or transmission. A black body has an emissivity of 1. See prerequisite courses GEOB 204 and 300.

Blending height - height at which flow gradually changes from being influenced by individual roughness elements in the roughness sublayer to being independent of horizontal position in the inertial sublayer. See Lecture 3, slide 13.

Bowen ratio (β) - The ratio of sensible heat flux density to latent heat flux density at land-atmosphere interfaces. See prerequisite courses GEOB 204 and 300.

Boundary layer urban heat island (BLUHI) - characteristic warmth and static stability of the urban boundary layer, approximated by the difference of vertical air temperature profiles in the rural and urban boundary layer. See Lecture 12, slide 4.


C

Canopy - The vertical layer of a land-atmsophere interface where both surface and air co-exist. In the case of a city see urban canopy.

Canopy layer - see urban canopy layer.

Canopy layer urban heat island (CLUHI) - the phenomena of increased screen-level temperatures in the urban canopy layer compared to screen-level temperatures in the rural background. See Lecture 12, slide 4.

Canyon - see urban canyon.

Canyon vortex - see urban canyon vortex.

Cavity zone - A zone immediately before and after an obstacle (e.g. building) where the wind and turbulence are significantly reduced. See Lecture 16, slide 4.

'Cliff' - in the context of the canopy layer urban heat island the region at the edge of a city where horizontal gradients of nocturnal air temperatures are strong. See also 'plateau' and Lecture 13, slide 3.

Cloud condensation nuclei (CCN) - Hygroscopic aerosol particles that can serve as nuclei for cloud droplets. See Lecture 25, slide 15.

Complete aspect ratioC) - ratio of the total three-dimensional surface area of all objects such as buildings and trees to the total plan area. The total 3-D area includes all interfaces between surfaces and the air. See Lecture 2, slide 11.

Computational Fluid Dynamics (CFD) - A general term that summarizes different mumerical methods and algorithms to simulate flow around objects, e.g. around buidlings See Lecture 20, slide 13.

Conduction - the transfer of energy in a substance by means of molecular motions without any macroscale motion. See prerequisite courses GEOB 204 and 300.

Convection - mass motions within a fluid resulting in transport and mixing of properties (e.g. energy and mass). We distinguish between forced convection and free convection See prerequisite courses GEOB 204 and 300.

Country breeze - Near surface winds blowing into a city as part of a thermal circulation. Formed as a consequence of the boundary layer urban heat island. See Lecture 22, slide 3.

Correlation coefficient (rab, where a and b - Describes the degree of correlation between a and b as a number between 0 (no correlation) and (-)1 describing a perfect (negative) correlation. See Lecture 19, slide 12.

Corkscrew flow - A specific flow pattern in an urban street canyon where air parcels are spiralling along the canyon's axis. See Lecture 18, slide 16.


D

Davenport classification - A classification of surface roughness into eight classes (1 - smooth to 8 - rough) often used in urban modelling. See Lecture 17, slide 13.

Dewfall - The process when atmospheric water vapour condenses onto objects near the ground. Typically happens due to radiative cooling during night. See Lecture 25, slide 11.

Diffuse short-wave radiation (D) - short-wave radiation reaching the Earth's surface after having been scattered from the direct-beam by molecules, aerosols or cloud-droplets in the atmosphere. See prerequisite courses GEOB 204 and 300.

Direct-beam short-wave radiation (S) - that portion of short-wave radiation received in a parallel beam 'directly' from the Sun. See prerequisite courses GEOB 204 and 300.

Displacement zone - A zone around an obstacle (e.g. building) where the air is displaced upward and accelerated as it moves over the obstacle. See Lecture 16, slide 4.


E

Eddies - Coherent parts in a turbulent flow that have roughly same temperature, vorticity, momentum, and/or mass concentrations. See prerequisite courses GEOB 204 and 300.

Eddy covariance method - An approach to directly measure flux densities by tracking the vertical movement of eddies and the properties they carry. This is practically implemented with fast responding ultrasonic anemometers and potentially gas analyzers. See prerequisite courses GEOB 204 and 300.

Ejection - an event in a turbulent flow that transports momentum deficit from a low velocity region to a high velocity region (upwards). Opposite is a sweep. See Lecture 19, slide 11.

Emission - the process in which an object or surface actively releases radiant energy. See prerequisite courses GEOB 204 and 300.

Emittance - the radiant flux density in W m-2 a surface releases by emission.

Emissivity (ε) - the ratio of the total radiant energy emitted per unit time per unit area of a surface at a specified wavelength and temperature to that of a black body (emissivity = 1) under the same conditions. See Lecture 7, slide 7.

Energy flux - rate of flow of any energy in Joules per second (J s-1). Examples are heat fluxes or radiant fluxes.

Energy flux density - rate of flow of any energy per unit surface area in Joules per second and square meter (J s-1 m-2 = W m-2). Examples are heat flux densities or radiant flux densities.

Entrainment - process of mixing of fluid across a density interface bounding a region of turbulent flow. Relatively quiescent fluid is engulfed by eddies penetrating across the interface. Practically this is an important process in the entrainment zone of the inversion capping the planetary boundary layer. See Lecture 13, slide 16.

Eulerian model - A numerical modelling approach where all flow equations are separately solved for stationary grid-cells (volumes). See Lecture 20, slide 4.

Evaporation - the process by which liquid water is transformed into water vapour without physiological control. See also transpiration and prerequisite courses GEOB 204 and 300.

Evapotranspiration - the sum of evaporation and transpiration from a given vegetated surface. See prerequisite courses and Lecture 23, slide 3.


F

Fetch - distance measured in the upwind direction to the next change in local-scale surface properties (e.g. change in UCZ) . See Lecture 27, slide 18.

Field of view (FOV) - see instantaneous field of view.

Flux - rate of flow of some quantity (examples: energy flux, mass flux). See prerequisite courses GEOB 204 and 300.

Flux density (Q)- the flux of any entity through a unit surface area (examples: energy flux density or mass flux density). See prerequisite courses GEOB 204 and 300.

Flux density divergence (divQ) - in a one-diomensional view: the change of any flux density with distance.

Forced convection (mechanical convection) - Turbulent motion induced by instabilities in the flow due to mechanical forces such as deflection or friction. See Lecture 19, slide 4.

Free convection (thermal convection) - Turbulent motion caused only by density differences in a fluid (e.g. due to different surface heating). See Lecture 19, slide 4.

Friction velocity (u*) - A charateristic velocity in m s-1 valid for the whole planetary bounardy layer that expresses the total momentum flux density directed towards the surface. See Lecture 18, slide 3.

Frontal aspect ratio - ratio of the total area of buildings and trees projected in the direction of the mean wind to the total plan area. See Lecture 2, slide 11.


G

Gap effect - flow accelleration through passageways at the base of a building resulting in wind discomfort. See Lecture 29, slide 4.

Gaussian plume model - A numerical dispersion model that describes the lateral and vertical spread of a pollutant plume based on a few simple statisticsl parameters. See Lecture 20, slide 4.

Grey body - As opposed to a black body a body with an emissivity < 1. See prerequisite courses GEOB 204 and 300.


H

Haze - The situatiuon when aerosols and small water droplets are suspended in air that are reducing visibility. See Lecture 25, slide 16.

Heat capacity (C) - the amount of energy absorbed (or released) by unit volume of a system for a corresponding temperature rise (or fall) of 1 Kelvin in J m-3 K-1. See Lecture 2, slide 5 and prerequisite courses GEOB 204 and 300.

Heat flux - rate of flow of sensible or latent heat in Joules per second (J s-1). See prerequisite courses GEOB 204 and 300.

Heat flux density - A particular form of energy flux density. Rate of flow of sensible or latent heat per unit surface area in Joules per second and square meter (J s-1 m-2 = W m-2). See prerequisite courses GEOB 204 and 300.

Hysteresis - describes the behaviour of a system. Current conditions are depending on past processes and can not be solely explained by the current state.


I

Inertial sublayer (ISL) - the part of the surface layer above the roughness subalyer where shear-dominated turbulence creates a logarithmic velocity profile and properties are independent from horizontal position. See Lecture 3, slide 14.

Instantaneous field of view (IFOV) - the solid angle that can be simultaneously seen by any imaging system such as a camera or thermal scanner. See Lecture 12, slide 13.

Internal boundary layer - The layer of air above a particular surface where air is well mixed with the surface layer and therefore in equilibrium with it. See Lecture 27, slide 18.

Irradiance - total radiant flux density received by a surface in W m-2 in a given wavelength band. See prerequisite courses GEOB 204 and 300.

IRT - see thermal infrared thermometer

Isolated roughness flow - Flow pattern over roughness elements (e.g. buidlings) that are widely spaced, so that in-between the elements flow can readjust to the ground surface. See Lecture 17, slide 8.

Isotherm - line of same temperature on a map or graph. See for example Lecture 13, slide 7.


J

Jetting - A specific flow pattern in an urban street canyon where flow is channelled along the canyon's axis. See Lecture 16, slide 16.


L

La Porte anomaly - A significant increase in convective precipitation and thunderstorms between 1930 and 1945 in La Porte, IN following the installation of steel mills. The La Porte anomaly was the basis for METROMEX See Lecture 25, slide 4.

Lagrangian dispersion model - A dispersion model that tracks numerically the trajectories of a huge number individual particles released based on a randomization approach. See Lecture 20, slide 4.

Latent heat - the energy released or taken up per unit mass by a system in changing phase. See prerequisite courses GEOB 204 and 300.

Latent heat flux density (QE) - The heat flux density of latent heat. A term of the surface energy balance. See prerequisite courses GEOB 204 and 300.

LiDAR (Light Detection and Ranging) - Any remote sensing technique that uses laser beams. It might be a technology that employs an airborne scanning laser rangefinder to produce detailed and accurate surveys of e.g. urban structure. See Lecture 2, slide 8 or a ground-based system sending laser beams to the atmopshere and using the backscatter to identify aerosols and clouds. See Lecture 27, slide 16.

Local scale - class of atmospheric phenomena with horizontal dimensions from 100 m to 50 km, with lifetimes of less than one day. Usually associated with topographic features, e.g. land/sea, and mountain/valley winds and urban heat islands. See Lecture 1, slide 14.

Logarithmic wind law A law that desribes the vertical change of horizontal wind in the spatial average. See Lecture 18, slide 3.

Long-wave radiation (=far infrared radiation or thermal infrared radiation) - radiation with a wavelength between 3 µm and 100 µm. Long-wave radiation in the climate system originates from emission of objects on Earth (surface, atmosphere, clouds etc.). See prerequisite courses GEOB 204 and 300.

LUMPS (Local-scale urban meteorological preprocessor scheme) - an empirical model that handles the surface-atmosphere exchange in a city by using a few input parameters and coefficients derived from field measurements. See Lecture 28, slide 5.


M

Macroscale - class of atmospheric phenomena with horizontal dimensions of hundreds to thousands of kilometres and a life time of weeks to years. Continental to global size features such as jet streams, weather systems, monsoons, global climate distributions. See Lecture 1, slide 14.

Mass flux - rate of flow of mass in kilogram per second (kg s-1). See prerequisite courses GEOB 204 and 300.

Mass flux density - rate of flow of mass per unit surface area in kilogram per second and square meter (kg s-1 m-2). See prerequisite courses GEOB 204 and 300.

Mesoscale - class of atmospheric phenomena with horizontal dimensions of 20 to 200 km such as tornadoes, squall lines and thunderstorms. See Lecture 1, slide 14.

METROMEX (Metropolitan Meteorological Experiment) - massive multi-institutional urban study in St. Louis between 1971 - 1975, with many urban climate findings but focus on urban precipitation enhancement. See Lecture 25, slide 4.

Microscale - class of atmospheric phenomena with horizontal dimensions from millimetres to 1 km and lifetimes of minutes to hours. Examples include climates of leaves, animals, gardens, buildings, fields and rivers. See Lecture 1, slide 14.

Microwave wind profiler - Remote sensing technique using radiation in selected microwave wave bands to determine wind speed and wind direction in the troposphere including the planetary boundary layer. See Lecture 27, slide 16.


N

Net all-wave radiative flux density (Q*, also 'net all-wave radiation') - The net radiant flux density in W m-2 when calculating the radiation balance, i.e. short-wave irradiance minus short-wave reflectance plus long-wave irradiance minus long-wave surface emittance / reflectance. See prerequisite courses GEOB 204 and 300.


O

Objective hysteresis model (OHM) - An empirical approach to describe the hysteresis 'loop' of the storage heat flux density based on measured or modelled net all-wave radiative flux densities and a set of empirical constants. See Lecture 9, slide 13.

Off-nadir angle (ONA) - the angle any imaging system such as a camera or thermal scanner is tilted away from the normal when looking at a surface. See Lecture 12, slide 13.

Outer layer - the major part of the planetary boundary layer above the surface layer where properties are stirred and uniformly mixed by turbulence. See Lecture 1, slide 4.


P

Pilot balloon - a helium-filled balloon that is tracked using theodolites or RADAR. It may carry a mini-sonde with sensors transmitting the signals to a base-station on ground via radio. See Lecture 27, slide 15.

Plan area ratio - ratio of plan area of a particular surface cover type to the total plan area (viewed vertically). Typical examples include built, vegetated, roof, irrigated plan area ratios. See Lecture 2, slide 7.

Planetary boundary layer (PBL; also: the atmospheric boundary layer or Ekman layer) - the layer of the atmosphere from the surface to the level where the frictional influence of the surface is absent. See Lecture 3, slide 3.

'Plateau' - in the context of the canopy layer urban heat island the region within the city where nocturnal air temperature is significantly elevated compared to the rural background but horizontal gradients are weak. See also 'cliff' and Lecture 13, slide 3.


R

RADAR (radio detection and ranging) - Remote sensing technique based on electromagnetic waves to detect water droplets or artificially released objects (e.g. balloons) in the atmosphere. See Lecture 27, slide 16.

Radiant energy - the energy of any type of electromagnetic radiation. See prerequisite courses GEOB 204 and 300.

Radiant flux - the energy flux of radiant energy (in J s-1). See prerequisite courses GEOB 204 and 300.

Radiant flux density - the energy flux density of radiant energy (in J s-1 m-2 = W m-2). See prerequisite courses GEOB 204 and 300.

Radiant flux density divergence - the flux density divergence of radiant energy. See for example Lecture 13, slide 16.

Radiation - the process by which radiant energy is propagated through free space by virtue of joint undulatory variations in the electric and magnetic fields in space. See prerequisite courses GEOB 204 and 300.

Reflectance - the radiant flux density in W m-2 a surface returns back by reflection. See prerequisite courses GEOB 204 and 300.

Reflection - the process in which incident radiant energy is returned back where it originated (mirrored or diffused). We distingish between specular and diffuse reflection. See prerequisite courses GEOB 204 and 300.

Reflectivity (αλ, also 'spectral reflectivity') - the relative fraction of the irradiance of a particular wavelength λ reaching a surface of an object that is reflected back. Values for reflectivity range between 0 (no reflection) to 1 (e.g. a perfect mirror). See Lecture 2, slide 5 and prerequisite courses GEOB 204 and 300.

Roughness length (z0) - a length (in m) that describes the surface roughness impact on the logarithmic wind law and formally descibes the height where the logarithmic wind law becomes zero. See Lecture 18, slide 3 and prerequisite course 300.

Roughness sublayer (RSL) - part of the surface layer extending from ground up to about 2 to 5 times the height of the roughness elements (buildings, trees) that includes the urban canopy layer. Within the RSL flow is three-dimensional and is governed by the size and shape of the elements. See Lecture 3, slide 9.


S

Sensible heat - that heat (energy) able to be sensed (e.g. with a thermometer). Used in contrast to latent heat. See prerequisite courses GEOB 204 and 300.

Sensible heat flux density (QH) - The heat flux density of sensible heat. A term of the surface energy balance. See prerequisite courses GEOB 204 and 300.

Short-wave radiation (=solar radiation) - radiation with a wavelength between 0.15 µm and 3 µm. Short-wave radiation in the climate system originates from the sun. See prerequisite courses GEOB 204 and 300.

Skimming flow - Flow pattern over roughness elements (e.g. buidlings) that are closely spaced, so that flow skimms over elements. Air in cavities in between the elements is not well coupled with the flow above. See Lecture 17, slide 8.

Sky view factor (Ψsky) - the fraction of the radiant flux leaving a certain surface (or object) that is intercepted by the sky (as opposed to by other ground objects). See Lecture 7, slide 4.

Smog - urban air pollution with some visible manifestation. See Lecture 5, slide 12.

Soil Vegetation Atmosphere Transfer scheme (SVAT) - a sub-model in weather forecasting and climate models that handles the surface-atmosphere exchange, typically in 1-D solving different layers. See Lecture 30, slide 7.

Sonic anemometer - same as ultrasonic anemometer

Source area - The area that influences the measurements of a particular sensor See Lecture 27, slide 22.

Specific heat (c) - the amount of energy absorbed (or released) by unit mass of a system for a corresponding temperature rise (or fall) of 1 Kelvin in J kg-1 K-1. See also Heat capacity, Lecture 2, slide 5 and prerequisite courses GEOB 204 and 300.

Stagnation point - is a point on the object's (e.g. building's) surface where the velocity of the flow is zero and the dynamic pressure is highest. See Lecture 16, slide 7.

Stefan Boltzmann Law - A basic radiation law that describes the integral emittance of a black body as a fuction of its absolute temperature in Kelvin. See prerequisite courses GEOB 204 and 300.

Storage heat flux densityQS) - All sensible heat storage in soil, buildings and other urban ecosystem components by heating up the material. See prerequisite courses.

Street canyon - see urban canyon.

Subsidence - Sinking or descending of air. See Lecture 21, slide 13.

Surface cover - see urban surface cover.

Surface layer - the lowest part of the planetary boundary layer adjacent to the surface of Earth where mechanical (forced) generation of turbulence by shear exceeds buoyant (free) production by thermal instability. Over a city it can be further subdivided into the roughness sublayer and the intertial sublayer. See Lecture 3, slide 4.

Surface energy balance - The energy conservation equation applied to a land-atmosphere interface. For urban surfaces we can write that the net all-wave radiative flux density is equal the sensible heat flux density plus the latent heat flux density plus the storage heat flux density plus the anthropogenic heat flux density, i.e. Q* = QH + QE + ΔQS + QF. See prerequisite courses GEOB 204 and 300.

Surface temperature (To) - Thermodynamic temperature of an object's surface as opposed to the apparent surface temperature. See prerequisite courses GEOB 204 and 300.

Surface urban heat island (SUHI) - the phenomena of (on average) increased surface temperatures in a city compared to the rural background. Typically approximated by remote sensing approaches through apparent surface temperature. See Lecture 11, slide 4.

Sweep - an event in a turbulent flow that transports excess momentum from a high velocity region to a low velocity region (downwards in the planetary boundary layer). Opposite is an ejection. See Lecture 19, slide 11.


T

Tethered balloon - a helium-filled balloon attached to a line so it can be winched up and down. It carries meteorological sensors and data is transmitted to a base station via radio. See Lecture 27, slide 15.

Tetroon - a helium-filled constant-volume balloonof tetrahedron shape with neutral lift. It's path reflects the trajectory of a wind parcel. A tetroon is typically tracked via RADAR and may carry meteorological sensors. See Lecture 27, slide 15.

Thermal admittance (µ) - a surface thermal property that governs the ease with which it will take up or release heat. It is the square root of the product of the thermal conductivity and heat capacity. See Lecture 2, slide 5 and prerequisite courses GEOB 204 and 300.

Thermal anisotropy - the anisotropy of surface temperatures of an urban surface. See Lecture 12, slide 23.

Thermal conductivity (k) - a physical property of a substance describing its ability to conduct heat by molecular motion across a given temperature gradient. Its units are W m-1 K-1. See Lecture 2, slide 5 and prerequisite courses GEOB 204 and 300.

Thermal diffusivity (κ) - the ratio of the thermal conductivity to the heat capacity of a substance. It determines the rate of heating due to a given temperature distribution in a given substance. It tells us how fast temperature waves will propagate in a substance. Its units are m2 s-1. See Lecture 2, slide 5 and prerequisite courses GEOB 204 and 300.

Thermal infrared (TIR) - Radiation in the long-wave part of the spectrum. See prerequisite courses GEOB 204 and 300.

Thermal infrared thermometer (IRT) - a sensor that uses the Stefan-Boltzmann law to convert measured long-wave (typically in the atmospheric window) radiant flux densities in a given field of view to apparent surface temperatures. See prerequisite courses GEOB 204 and 300.

Thermal mass scheme (TMS) - A method to estimate / calculate the urban storage heat flux density by carefully tracking the temperature change over time of the different subsystems (walls, floors, roofs). See Lecture 9, slide 7.

Town energy balance (TEB) model - A specific urban canopy scheme developped by Meteo France to represent cities in numerical weather forecasting models. See Lecture 28, slide 9.

Tracer - an artificially released chemical compound not found in a natural system that can for example be used to track flow and dispersion in the atmosphere. See Lecture 20, slide 7.

Transmission - the process in which incident radiant energy is passing through an object. See prerequisite courses GEOB 204 and 300.

Transmissivity (Ψλ) - the relative fraction of the irradiance of a particular wavelength λ on the surface of an object that is transmitted through the object. Values for transmissivity range between 0 (no transmission, opaque body) to 1 (perfect transmitter). See Lecture 2, slide 5 and prerequisite courses GEOB 204 and 300.

Transpiration - the process by which liquid water is transformed into water vapour within and on plant tissues which is physiologically controlled. See also evaporation and prerequisite courses GEOB 204 and 300.

Trajectory - The path an air parcel will follow over time. See Lecture 21, slide 12.

Turbulence (also: turbulent flow or turbulent motion) - a state of a flow in which the instantaneous velocities and streamlines exhibit irregular and apparently random fluctuations so that in practice only statistical properties can be recognized. These fluctuations are capable of transporting atmospheric properties (e.g. heat, water vapour, momentum, etc.) at rates far in excess of molecular processes. Used in contrast to laminar flow. See prerequisite courses GEOB 204 and 300.

Turbulent fluctuations - The deviation of any value from its temporal (or spatial) average. See prerequisite courses GEOB 204 and 300.

Turbulent footprint - same as turbulent source area.

Turbulent kinetic energy (TKE) - The kinetic energy of the three-dimensional, random turbulent fluctuations of the wind components in a flow. See Lecture 19, slide 7.

Turbulent source area - The area on the surface that influences the current measurement of a particular turbulent flux. See Lecture 27, slide 20.


U

Ultrasonic anemometer - An instrument that outputs all wind components at high frequency by measuring of the speed of sound. See prerequisite courses.

Ultraviolet radiation (UV) - Radiation in the range 0.15 µm to 0.38 µm of the short-wave spectrum. It is not visible to the human eye. See Lecture 5, slide 13.

Urban boundary layer (UBL) - internal boundary layer formed when the planetary boundary layer flows over a city. It is a mesoscale phenomenon the properties of which are affected by the nature of the urban surface. See Lecture 3, slide 5.

Urban canopy - the assemblage of buildings, trees and other objects comprising a city. Extends from the ground up to the mean height of the buildings and trees. Analagous to a vegetation canopy formed by a crop or forest. See Lecture 3, slide 16.

Urban canopy layer (UCL) - the layer of air within the urban canopy. It is dominated by microscale properties of the many surface types present in the urban canopy. It consists of an exterior layer of air between the buildings and trees and an interior portion of air inside the buildings. See Lecture 3, slide 17.

Urban canopy scheme - a sub-model in weather forecasting models that handles the surface-atmosphere exchange in a city by typically modelling all factets of a simplified 2-D street canyon. See Lecture 28, slide 4.

Urban canyon - (also: 'street canyon') the characteristic geometric form of a street and its flanking buildings. See Lecture 1, slide 15.

Urban canyon vortex - A specific flow pattern in an urban street canyon where air parcels are rotating in the street canyon. See Lecture 16, slide 11.

Urban climate - the climate of an area that is mainly affected by the presence of a town or city.

Urban climatology - The study of the statistically-preferred states of urban weather, which includes quantitative description of the urban climate (climatography), the use of urban meteorological knowledge to explain climatic differences (physical climatology) and the use of climatic data to solve practical problems (applied climatology). See Lecture 1, slide 12.

Urban dome - the volume of urban-influenced air in the planetary boundary layer under low-wind conditions. See Lecture 3, slide 6.

Urban CO2 'dome' - The phenomena of enhanced carbon dioxide concentrations in and above an urban surface during low-wind situatuions See also urban dome and Lecture 26, slide 6.

Urban fabric - the materials comprising urban surfaces, e.g. asphalt, concrete, stone, wood, soil, etc. See Lecture 2, slide 4.

Urban heat island (UHI) - characteristic warmth of a city, often approximated by urban-rural temperature differences. We distinguish between the subsurface urban heat island, the surface urban heat island (SUHI), the canopy layer urban heat island (CLUHI), and the boundary layer urban heat island (BLUHI). See Lecture 12, slide 3.

Urban heat island intensity - A measure of the magnitude of an urban heat island expressed as the difference between the highest sustained urban temperature and the the background rural temperature. See Lecture 13, slide 11.

Urban metabolism - the processes accompanying the functioning of a city involving the intake of resources such as energy, water and raw materials and the release (emissions) of heat, vapour, particles, gases and products. Analagous to the metabolism of an organism. See Lecture 2, slide 16.

Urban meteorology - The study of the physical, chemical and biological processes operating to produce or change the state of the urban atmosphere. See Lecture 1, slide 12.

Urban plume - downwind of a city as a new rural boundary layer forms it isolates the residual urban boundary layer aloft. The resulting layer of urban-modified air therefore resembles a giant plume of emissions from a chimney (the city). See Lecture 3, slide 7.

Urban structure - the geometrical arrangement of the buildings, trees and other objects of a city involving measures of the dimensions of the elements and the spaces between them (street widths, building spacing). See Lecture 2, slide 10.

Urban surface cover - type of materials covering a given area of the surface of a city as viewed from above, e.g. vegetation, buildings, roads, water, etc. Often expressed as a plan area ratios of each type. See Lecture 2, slide 6.


V

Veering - clockwise change of wind direction; the opposite of backing. In international usage this definition applies in both Hemispheres. See Lecture 21, slide 10.

Visibility - the distance, under daylight conditions, at which the contrast between a target and its background becomes visible.See Lecture 5, slide 8.

View factor - fraction of one object's view (usually hemispherical) that is occupied by another object (or the sky, see sky view factor). See Lecture 7, slide 4.

von Kármán's constant (k) - A dimensionless constant in the logarithmic wind law (0.4). See Lecture 18, slide 3.


W

Wake interference flow - Flow pattern over roughness elements (e.g. buidlings) that are moderately spaced, so that flow disturbed by an element can not readjust before hitting the next element. See Lecture 17, slide 8.

Wake zone - A zone in the lee of an obstacle (e.g. building) extending 10 to 25 times the height of the obstacle where the wind is still reduced and turbulence is increased. See Lecture 16, slide 4.

World Meteorological Organization (WMO) - The United Nations agency for meteorology and climatology with the aim to facilitate worldwide cooperation and communication of weather and climate data and the standardization of meteorological measurements (i.e. siting and exposure of instruments). See Lecture 27, slide 5.


Z

Zero-plane displacement (zd) - a length (in m) that describes how the logarithmic wind law is shifted upward (displaced) over tall roughness. See Lecture 18, slide 6 and prerequisite course GEOB 300.

 

Last additions: March 21, 2017. Selected definitions are from T. R. Oke (1987): 'Boundary Layer Climates', p. 400 to 405, and from T. R. Oke et al. (2017): 'Urban climates', with permission from the authors.