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**Aerodynamic approach** - A flux-gradient approach that uses the Reynolds analogy to express the sensible heat flux density as a function of a measured temperature gradient and a wind gradient only.

**Aerodynamic conductance** (*g _{a}*) - relates the turbulent flux densities of mass, heat or momentum measured over a finite layer in the atmosphere to the gradient of the corresponding property over the same layer (in s

**Aerodynamic resistance** (*r _{a}*) - the inverse of the constant of proportionality that relates the turbulent flux densities of mass, heat or momentum measured over a finite layer in the atmosphere to the gradient of the corresponding property in the same layer (in s m

**Absorption** - the process in which incident radiant energy is retained by an object or a substance (e.g. gas).

**Absorptivity** (*ζ _{λ}*) - the relative fraction of the irradiance of a particular wavelength

**Active surface** - The height within a three dimensional land-atmosphere interface (e.g. forest), where most of the surface exchange / interaction takes place. The active surface might be different for different flux densities.

**Advection** - A term used to describe predominantly horizontal motion and processes in the atmosphere.

**Albedo** (*α*) - the ratio of the amount of short-wave radiation reflected by a surface to the short-wave irradiance (amount incident upon it).

**Amplitude** - half the magnitude from crest-to-trough of a wave. *See also period and phase shift*.

**Anemometer** - an instrument for measuring wind speed.

**Angular frequency** (*ω*) - Frequency in s^{-1} of a wave expressed as 2π/P, where P is the period of the wave in seconds.

**Apparent atmospheric emissivity** (*ε _{a}*) - The bulk emissivity in the long-wave part of the spectrum of the atmosphere.

**Apparent sky temperature** (*T _{k}*) - the calculated temperature of the upper (sky) hemisphere inferred from its measured long-wave atmospheric emittance (

**Apparent surface temperature** (*T _{r}*) - the calculated temperature of a surface inferred from its measured long-wave emittance (

**Atmospheric boundary layer ** - same as planetary boundary layer.

**Atmospheric 'window'** - part of the long-wave specturm where we observe a relative gap in the absorption for atmospheric gases (between 8 µm and 13 µm).

**Autotrophic respiration** - all respiration processes by living plant parts. This is manly root respiration but also respiration in above-ground plant parts.

**Beer's Law** - Physical law that describes attenuation of monochromatic radiation transmitted through a homogeneous medium.

**Black body** - an 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.

**Boundary layer** - A layer of fluid immediately above a material surface where the flow is significantly influenced by the surface's energy and mass exchange and surface friction.

**Bowen ratio** (*β*) - The ratio of sensible heat flux density to latent heat flux density at land-atmosphere interfaces.

**Buoyancy** (also: 'buoyant force') - The upward force exerted upon a parcel of fluid by virtue of the density difference between itself and the surrounding fluid.

**Concentration** - amount of a substance contained in a unit volume (in kg m^{-3} or mol m^{-3}).

**Conduction** - the transfer of energy in a substance by means of molecular motions without any macroscale motion.

**Continuum assumption** - A theoretical assumption that underlies the macroscopic descrition of motion and thermodynamics of a fluid without resolving the molecular motions. The continuum assumption is valid if the Knudsen-number of the system is significantly smaller than 1.

**Convection** - mass motions within a fluid resulting in transport and mixing of properties (e.g. energy and mass). We distinguish between mechanical convection and thermal convection.

**Convective boundary layer** - A state of the planetary boundary layer over land that is driven by strong surface heating and thermal convection. The CBL can be separated into a surface layer, a mixed layer and the entrainment zone above.

**Correlation coefficient** (*r _{ab}*, where

**Cosine law of illumination** - Describes how the irradiance caused by a direct beam decreases as the beam's angle varies from overhead (normal) to a more slanted path relative to the surface.

**Covariance** (*a'b'*, where *a* and *b* are any scalars or wind components) - The temporal average of the product of two turbulent fluctuations in a time series. Covariances with at least one wind component can be interpreted as turbulent flux densities. *See also variance*.

**Damping depth** (*D*) - The depth in m in a soil where the amplitude of an ideal temperature wave has decayed to e^{-1} (37%) of its value at the surface. Defined as the square root of twice the thermal diffusivity divided by the angular frequency of the wave.

**Density** - Entity expressed per unit volume. If no further specifications are given we typically mean the mass density (*ρ*) in kg m^{-3}.

**Diabatic** - A non-neutral situation in the atmosphere, i.e. either unstable or stable.

**Diffuse reflection** - is an introtropically diffused reflection of irradiance on an object or surface. The reflected radiation is evenly dirstributed on all solid angles of the hemisphere (Lambertian diffusion). *See also specular reflection*.

**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 (and/or reflected from nearby objects).

**Diffusometer** - An instrument to measure diffuse short-wave irradiance made of a pyranometer and a shade devive. *See also pyrheliometer*.

**Direct-beam short-wave radiation** (*S*) - that portion of short-wave radiation received in a parallel beam 'directly' from the Sun.

**Dissipation** - The conversion of macroscopic (turbulent) motion to heat. *See also energy cascade*.

**Dynamic viscosity ** (*μ*) - A molecular propery of a fluid. Internal resistance (force) of the fluid to deformation by shear stress in N m^{-2} s = kg m^{-1} s^{-1} . *See also kinematic viscosity*.

**Ecosystem respiration** (*R _{e}*) - the sum of all respiration processes in an ecosystem. It includes above-ground autotrophic respiration and soil respiration.

**Eddies** - Coherent parts in a turbulent flow that have roughly same temperature, vorticity, momentum, and/or mass concentrations.

**Eddy covariance method** - An approach to directly measure flux densities of momentum, mass and heat 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.

**Eddy diffusivity** (*K*) - The constant of proportionality (in m^{2} s^{-1}) in the K-Theory that relates a gradient of concentration of anything to the corresponding flux density.

**Ejection** - an event in a turbulent flow that transports a momentum deficit from a low velocity region to a high velocity region (upwards in the planetary boundary layer). Opposite is a sweep.

**Emission** - the process in which an object or surface actively releases radiant energy.

**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.

**Energy cascade** - The breakdown of large eddies to smaller ones in a turbulent flow finally leading to dissipation.

**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 zone (EZ)** - An strong inversion that separates the well mixed convective boundary layer from the free atmopshere above.

**Equation of time** - difference between local apparent time and local mean solar time due to the differing rates of the Earth's rotation through a year because of its elliptical orbit.

**Evaporation** - the process by which liquid water is transformed into water vapour without physiological control. *See also transpiration*.

**Evapotranspiration** - the sum of evaporation and transpiration from a given vegetated surface.

**Extinction** - for radiation same as attenuation.

**Extinction coefficient** (*k*) - Parameter in Beer's Law that describes the magnitude of the decay of a radiant flux density when transmitted through a homogeneous medium.

**Extra-terrestrial irradiance ** (*K _{Ex}*) - short-wave radiant flux density received at the 'top' of the Earth's atmosphere from the sun.

**Far infrared radiation** - the same as long-wave radiation.

**Fetch** - distance, measured in the upwind direction.

**Flux** - rate of flow of some quantity (examples: energy flux, mass flux).

**Flux density** (*Q*)- the flux of any entity through a unit surface area (examples: energy flux density, mass flux density and momentum flux density).

**Flux density convergence** - the state where the sum of the incoming flux densities of any quantity is larger than the sum of the outgoing ones. Typically refers to an atmospheric layer, a volume or three-dimensional system. Opposite is flux density divergence.

**Flux density divergence** - the state where the sum of the incoming flux densities of any quantity is lower than the sum of the outgoing ones. . Typically refers to an atmospheric layer, a volume or three-dimensional system. Opposite is flux density convergence.

**Flux-gradient relationships** - All practical approaches that use vertical gradients of mean entities in a turbulent flow to calculate turbulent flux densities such as the K-Theory or the aerodynamic approach.

**Flux Richardson number** (*Rf*) - A dimensionless number that expresses the ratio of buoyancy production of TKE to shear production of TKE in terms of the directly measured flux densities of momentum and sensible heat. *See also Gradient Richardson number*.

**Fog drip** (I) - Precipitation formed by objects (typically plants) that extract / collect moisture from drifting fog.

**Forced convection** - same as mechanical convection.

**Form drag** - Drag observed when wind is hitting solid obstacles (e.g. tres, buildings) as a consequence of flow separation and pressure differences. Creates forced convection.

**Fourier's Law** - A physical law that relates a temperature gradient in a body to the heat flux density by conduction. The constant of proportionality between temperature gradient and heat flux density is the thermal coductivity.

**Fourier transform** - the mathematical transformation of a stationary time series (time vs. energy) to a spectrum (frequency vs. energy).

**Free convection** - same as thermal convection.

**Frequency** (*ν*) - number of events (e.g. crests in a sine signal) per second (in s^{-1}).

**Friction velocity** (*u _{*}*) - A charateristic velocity in m s

**Gradient** - The change of anything over distance. Examples in this course are the wind gradient and the temperature gradient.

**Gradient Richardson number** (*Ri*) - A dimensionless number that expresses the ratio of buoyancy production of TKE to shear production of TKE in terms of the gradients of mean wind and temperature.*See also Flux Richardson number*.

**Grey body** - As opposed to a black body a body with an emissivity < 1. A grey body absorbs only a fraction of the radiation striking it, i.e. shows reflection and/or transmission.

**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 also: specific heat*.

**Heat conduction** - see conduction

**Heat flux** - rate of flow of sensible or latent heat in Joules per second (J s^{-1}).

**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}).

**Heterotrophic respiration** (*R _{sh}*) - respiration processes by microbes in soil and litter that oxidize organic carbon to carbon dioxide.

**Histogram** - a binned (discrete) form of a probability density function.

**Horizontal homogeneity** - An often applied simplification in micrometeorology and microclimatology that assumes that changes in averages and flux densities are much stronger along the vertical axis compared to along the horizontal axis.

**Hour angle** (*h*) - The angle Earth must turn to bring the solar altitude at the observer's location to it's highest diurnal value ('noon') in degrees or radians.

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

**Interception** (I) - The amount of precipitation in a forest or crop canopy that remains on the surface of the plant and does not immediately reach the ground.

**Inviscid fluid ** - An ideal fluid that has no viscosity.

**Irradiance** - total radiant flux density received by a surface in W m^{-2} in a given wavelength band.

**Irradiation** - same as irradiance.

**IRT** - see thermal infrared thermometer

**Joint probability density function** - a two- (or higher) dimensional probability density fuction that describes the co-occurence of any two (or more) variables.

**Joule** (J) - SI-unit for energy (1 J = 1 N m = 1 kg m^{2} s^{-2}).

**K-Theory** A flux-gradient approach that relates any flux density to a measured concentration gradient by an eddy diffusivity.

**Katabatic flow** - any flow / wind blowing down an incline in topographic terrain due colder temperatures of the moving air mass (density) compared to the sourrounding air.

**Kelvin** (K) - SI base unit for thermodynamic temperature (*T* (K) = *T* (°C) + 273.15).

**Kinematic viscosity ** (*ν*) - Dynamic viscosity divided by the mass density of the fluid in m^{2} s^{-1}.

**Kirchhoff's Law** - says that the absorptivity of a body is equal its emissivity in a wavelength. It assumes no transmission. Note, the law refers to the ability of absporption (absorptivity) and the ability of emission (emissivity), not the actual absorptance and emittance.

**Knudsen Number ** (*Kn*) - In a gas, the ratio of the free molecular path length to the characteristic length scale of turbulent motions.

**Laminar boundary layer (LBL)** - the layer immediately next to a fixed boundary in a fluid in which laminar flow prevails.

**Laminar flow** - a flow in which the fluid moves smoothly in parallel streamlines; in contrast to turbulence.

**Latent heat** - the energy released or taken up per unit mass by a system in changing phase.

**Latent heat flux density** (*Q _{E}*) - The heat flux density of latent heat. A term of the surface energy balance.

**Lateral wind component** (*v*) - see wind components.

**Logarithmic wind law** A law that desribes the vertical wind gradient.

**Local apparent time (LAT)** - A non-uniform time that is varying through the year according to the equation of time. It ensures that highest solar altitude is always observed at noon.

**Local mean solar time** - Measure of time based on the avrage rotation of the Earth. Mean solar time is fixed and ensures that on average, highest solar altitude is observed at noon (but not at each day throughout the year, see local apparent time).

**Longitudinal wind component** (*u*) - see wind components.

**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.).

**Lysimeter** an instrument for measuring evapotranspiration by monitoring the weight changes of a representative soil plus vegetation monolith.

**Mass flux** - rate of flow of mass in kilogram per second (kg s^{-1}).

**Mass flux density** - rate of flow of mass per unit surface area in kilogram per second and square meter (kg s^{-1} m^{-2}).

**Mechanical convection** - Turbulent motion induced by a gradient in wind speed due to mechanical forces such as deflection or friction.

**Mechanical production** - The process of creating mechanical convection - and therefore TKE - by extracting kinetic energy from the mean wind. Opposed to thermal production.

**Metabolic heat** (*Q _{M}*) - Energy flux released by a living organism due to chemical reactions that occur in its cells.

**Microclimatology** - sub-discipline of Climatology that deals with long-term integral effects (from hours to decades) of micrometeorological processes, namely the average and variability of fluxes of energy, mass and momentum and the average state and variability of the planetary boundary layer with respect to different geographic settings.

**Micrometeorology** - sub-discipline of Meteorology that focuses on short-term fluctuations (typically up to one hour) and small scale processes (from a few mm up to 1 km). Micrometeorology investigates the detailed physical processes (e.g turbulence) that drive exchange of energy, mass and momentum and the short-term dynamics of the planetary boundary layer.

**Mie scattering** - scattering on objects that have approximately the same wavelength as the radiation hitting it. Mie scattering in the Atmosphere is important on droplets and aerosols and is strongly directed. *See also Rayleigh scattering
*

*
Mixed layer - Major part of the daytime convective boundary layer over land where properties are well mixed and thermal convection dominates. Located above the surface layer*.

**Mixing length** - the characteristic vertical distance mixing occurs in a turbulent flow. It is realted to the size of eddies.

**Monin-Obukhov Similarity** - A theoretical relationship / framework describing mean flow and turbulence statistics in the atmospheric surface layer as a function of the height above ground and the Obukhov length.

**Monochromatic** - radiation of a single wavelength.

**Momentum** - A property of a moving particle or air parcel which is given by the product of its mass with its velocity in kg m s^{-1}.

**Momentum flux density** (*τ*, same as 'Shear stress') - The flux density of horizontal momentum in (kg m s^{-1}) (m^{-2} s^{-1}) = N m^{-2} (Drag per unit area). In a fully turbulent flow the momentum flux density is mainly described by Reynolds stress.

**Near infrared radiation (NIR)** - Radiation in the range 0.7 µm to 3 µm. Part of the short-wave spectrum that is beyond the part visible to the human eye.

**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.

**Net ecosystem exchange (NEE)** - The difference between ecosystem respiration and ecosystem photosynthesis expressed a a mass flux denisty of carbon between surface and atmosphere. NEE is positive if the net effect is resulting in carbon dioxide to be emitted by the surface into the atmosphere (surface is a net carbon source). NEE is negative if carbon dioxide is taken up by the surface from the atmopshere (surface is a net carbon sink).

**Net long-wave radiative flux density** (*L**, also 'net long-wave radiation') - The net long-wave radiant flux density in W m^{-2}, i.e. long-wave irradiance minus long-wave surface emittance / reflectance.

**Net pyrradiometer** - An instrument to measure the net all-wave radiation *Q**. It integrates over both solid angles 2π - from above and below.

**Net short-wave radiative flux density** (*K**, also 'net short-wave radiation') - The net short-wave radiant flux density in W m^{-2}, i.e. short-wave irradiance minus short-wave reflectance.

**Newton** (N) - The SI unit for a force (1 N = 1 kg m s^{-2}).

**Obukhov length** (*L*) - A length (in m) that describes the height above surface where theoretically the magnitude of shear production and buoyancy production of TKE are equal.

**Period** (*P*) - The distance (or duration) from crest to crest of a wave. *See also amplitude and phase shift*.

**Phase lag** (also: phase shift) - The absolute location (or timing) of a wave relative to another wave. *See also amplitude and period*.

**Photosynthesis** - The biological process of carbohydrate formation from carbon dioxide and water in the chlorophyll-containing tissures of living plants ith the use of photosynthetic active ratiation.

**Photosynthetically active radiation (PAR)** - Short-wave radiation in the range 0.4 µm to 0.7 µm (approx. same as visible radiation) that can be used by plants in the process of photsynthesis.

**Planck's Law** - A basic radiation law that describes the emittance from a black body as a fuction of wavelength and absolute temperature in Kelvin (Planck curves). Integrating a Plack curve over all wavelengths results in the Stafan Boltzmann Law. The location of the maximum of a plack curve is descibed by Wien's Displacement Law.

**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.

**Porometer** - an instrument to measure the degree of stomatal opening by means of resistance to the diffusion of water vapour.

**Porosity** - The volume fraction in a soil that is occupied by water and/or air in m^{3} m^{-3}.

**Potential photosynthesis** - The maximal rate of photosynthesis for a plant if it is well watered under optimal conditions.

**Probability density function (PDF)** - a gradual function that desribes the probability of occurence of any variable (probability distribution). *See also histogram*.

**Pyranometer** - An instrument to measure the short-wave irradiance *K↓* (or short-wave reflectance *K↑* if installed upside down) of a surface. It integrates over the solid angle 2π.

**Pyrgeometer** - An instrument to measure the long-wave irradiance *L↓* (or long-wave emittance plus reflectance *L↑* if installed upside down) of a surface. It integrates over the solid angle 2π.

**Pyrheliometer** - An instrument to measure direct-beam short-wave irradiance *S*.

**Pyrradiometer** - An instrument to measure the all-wave (i.e. short-wave plus long-wave) irradiance *Q↓* (or all-wave emittance plus reflectance *Q↑* if installed upside down) of a surface. It integrates over the solid angle 2π.

**Radian** (rad) - the SI unit to express a planar angle. One radian is the angle which is subtended at the centre of a circle of radius *r*, by an arc of length *r*.

**Radiant energy** - the energy of any type of electromagnetic radiation.

**Radiant flux** - the energy flux of radiant energy (in J s^{-1}).

**Radiant flux density** - the energy flux density of radiant energy (in J s^{-1} m^{-2} = W m^{-2}).

**Radiant intensity** - is the radiant flux per unit solid angle from a point source in W sr^{-1}.

**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.

**Radiometer** - An instrument that measures radiation. We distinguish between Pyranometer, Pyrheliometer, Pyrgeometer, and Pyrradiometers.

**Rayleigh scattering** - scattering on molecules that are much smaller than the wavelength of the radiation hitting them. Rayleigh is more effective for shorter wavelengths and responsible for blue skies. *See also Mie scattering*.

**Reflectance** - the radiant flux density in W m^{-2} a surface returns back by reflection.

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

**Reflection coefficient** - average reflectivity in a wavelength range weighted by the distribution (spectrum) of the irradiance in the same range. The reflection coefficient of the short-wave part of the spectrum is called albedo.

**Reflectivity** (*α _{λ}*, also 'spectral reflectivity') - the relative fraction of the irradiance of a particular wavelength

**Residual layer ** (also: weak mixed layer) - During night, the remainder of the daytime mixed layer in the planetary boundary layer over land that is neutral and inactive and sits above the shallow stable boundary layer.

**Respiration** - Generally the oxidation of carbohydrates with the release of energy by living organisms. The process of respiration is resulting in a release of carbon dioxide. We can distinguish between autotrophic respiration and heterotrophic respiration.

**Reynolds analogy** - An assumption that all eddy diffusivities for all heat flux densities, mass flux densities, and momentum flux density are equal.

**Reynolds decomposition** (also: Reynolds averaging) - The mathematical concept of splitting a stationary time series *a(t)* into its average *a* and the deviation thereof i.e. the turbulent fluctuation a'(t).

**Reynolds number** (*Re*) - A number that can be used to describe the state of a flow (turbulent or laminar). The Reynolds number is the ratio of inertial forces (*ρ u*) to viscous forces (*μ/L*) where *u* is the flow's characteristic velocity (m s^{-1}) and *L* is its characteristic length scale (m).

**Reynolds stress** - the tangential force per unit area (in N m^{-2}) that is deforming a fluid parcel by means of macroscale mass exchange (turbulent eddies). Reynolds stress can be interpreted as a momentum flux density. *See also Viscous shear stress*.

**Roughness length** (*z _{0}*) - 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.

**Scalar** (any italic symbol) - a parameter without any directional information. Used in contrast to vector. Examples of scalars are temperature, pressure, humidity, etc.

**Scattering** - the process of diffusing a portion of the incident radiation in all directions by molecules, air droples or aerosols. We distinguish between Rayleigh scattering and Mie scattering.

**Schlieren photography** - an experimental photographic technique for detecting the presence of slight density (and hence temperature /pressure) variations in fluids by virtue of refraction effects.

**Sensible heat** - that heat (energy) able to be sensed (e.g. with a thermometer). Used in contrast to latent heat.

**Sensible heat flux density** (*Q _{H}*) - The heat flux density of sensible heat. A term of the surface energy balance.

**Shear stress** (*τ*) - same as momentum flux density in a moving fluid.

**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. Short-wave radiation is further separated into ultraviolet radiation, visible radiation, and near infrared radiation.

**Skin drag** - Drag retarding a flow close to a surface as a consequence of surface friction. Creates forced convection.

**Sky view factor** (*Ψ _{sky}*) - 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).

**Soil heat flux plate** - A sensor to directly measure the soil heat flux density A thermopile measures the temperature gradient across a thin plate of known thermal conductivity.

**Soil heat flux density** (*Q _{G}*) - the sensible heat transported away from the surface by diffusion and stored by heating up the soil material.

**Soil respiration** (*R _{s}*) - the sum of autotrophic respiration (root respiration part) and heterotrophic respiration (microbial respiration) that occurs below ground.

**Solar altitude** (*β*) - vertical direction of the Sun above the observer's horizon (assuming a flat Earth's surface) expressed in degrees or radians. The reciprocal of zenith angle, i.e. *Z* = 90° - β

**Solar azimuth** (*Ω*) - horizontal direction of the Sun relative to a reference direction (usually true North) expressed in degrees or radians.

**Solar constant** (*S0*) - solar (= short-wave) radiant flux density at the top of the atmosphere normal to the solar beam at Earth's mean distance from Sun. It's value has been determined by satellite measurements to 1366 W m^{-2}.

**Solar declination** (*δ*) - Planar angle between Sun's rays and Earth's Equatorial plane in degrees or radians. Solar declination is a fucntion of time of year and ranges between -23.4° and +23.4°.

**Solar radiation** same as short-wave radiation.

**Sonic anemometer** - same as ultrasonic anemometer

**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*.

**Spectral energy** - The turbulent kinetic energy of all eddies of a selected frequency (or size) only. A graph of frequency (size) vs. spectral energy is a turbulence spectrum.

**'Spectral gap'** - A region of reduced spectral energy between 10 min and 10 hours. The 'spectral gap' separates the energy of large-scale synoptic motions (i.e. weather systems, > 10 hours) from the small-scale turbulent motions (< 10 min) and is the theoretical prerequisite for Reynolds averaging.

**Spectral reflectivity** - see reflectivity.

**Specular reflection** - is a mirror-like reflection of direct-beam irradiance on an object or surface. The reflected beam is at same angle to the plane as the incident beam. * See also diffuse reflection*.

**Stable boundary layer (SBL)** - A state of the planetary boundary layer over land that is driven by strong surface cooling and thermal supression. The SBL is only a few 10 to 100 m thick and topped typically by a residual layer.

**Standard deviation** (*σ*) - a measure for the random dispersion around a mean value, defined as the square root of the variance.

**Stationarity** - An often applied simplification in micrometeorology and microclimatology that assumes that for carefully selected runs, the averages and other statistics (e.g. flux densities, variances) do not change over time.

**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.

**Stemflow** (Sf) - The amount of precipitation in a forest canopy that is drained along leafs, branches and finally flows down along the tree stems.

**Steradian** (sr) - is the SI unit to express a solid angle. One steradian is the solid angle subtended at the centre of a sphere of radius *r*, by an area on the surface of the sphere of *r*^{2}.

**Stomatal resistance** - The resistance of a leaf to the exhange of water and carbon dioxide between the leaf's interior and its laminar boundary layer physiologically controlled by the aperture of the stomata (guard cells).

**Storage heat flux densities** - a more general term for all energy storage within surface-atmosphere interface. It can include storage of sensible heat or latent heat in the interface airspace, or sensible heat in plant parts, buildings and other ecosystem components. A special case of storage heat flux density is the net biochemical energy storage due to photosynthesis and respiration .

**Streamline** - a line whose tangent at any point in a fluid is parallel to the instantaneous velocity of the fluid. A map of streamlines gives an instantaneous 'snap-shot' of the flow.

**Stress** - a force per unit surface area that is able to deform a body (in N m^{-2}). We distingush between pressure, viscous shear stress and Reynolds stress. The latter two are tangential forces that represent a momentum flux density.

**Surface energy balance** - The energy conservation equation applied to a land-atmosphere interface. For flat and hroizontally homogeneous 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 soil heat flux density, i.e. *Q** = *Q _{H}* +

**Surface layer** - The lowerst 10% of the planetary boundary layer where mechanical convection typically dominates and flux densities are roughly constant with height.

**Surface temperature** (*T _{o}*) - Thermodynamic temperature of an object's surface as opposed to the apparent surface temperature.

**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.

**Système International d'Unités (SI)** - international scientific system of units which is mandatory in climatology. The SI utilises seven base units from which other derived units can be obtained. *See your prerequisites for details.*

**Taylor's hypothesis** - relates duration and size of eddies in a turbulent flow. Assumes that for special cases turbulence might be considered 'frozen'.

**Temperature gradient** - The change of temperature with distance in K m^{-1}.

**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.

**Thermal belt** (*k*) - The regions half-way up the slopes in a valley that have typically the higest air temperatures during night.

**Thermal conductivity** (*k*) - a physical property of a substance describing its ability to conduct heat by molecular motion across a given temperature gradient according Fourier's Law. Its units are W m^{-1} K^{-1}.

**Thermal convection** - motion caused only by density differences in a fluid (e.g. due to different surface heating).

**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 m^{2} s^{-1}.

**Thermal infrared radiation (TIR)** - the same as long-wave radiation.

**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.

**Thermal production** - The process of creating thermal convection - and therefore TKE - due to buoyancy in a fluid. Opposed to mechanical production.

**Throughfall** (Tf) - The amount of precipitation in a forest or crop canopy that is directly falling through the canopy or deflected by plant parts to the ground.

**Transmission** - the process in which incident radiant energy is passing through an object.

**Transmissivity** (*Ψ _{λ}*) - the relative fraction of the irradiance of a particular wavelength

**Transpiration** - the process by which liquid water is transformed into water vapour within and on plant tissues which is physiologically controlled. *See also evaporation*.

**Troposphere** - the lowest 10-20 km of the Atmosphere, characterized by decreasing temperature with height, appreciable water vapour and vertical motion, and weather.

**Transmittance** - the radiant flux density in W m^{-2} that is passing through an object by transmission.

**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.

**Turbulence intensities** (*I _{u}*,

**Turbulence spectrum** - A way to analyze the size distribution of eddies in a turbulent flow. We draw frequency on the x-axis (which is related to the size of the eddies, see Taylor's Hypothesis) vs. spectral energy on the y-axis. Turbulence spectra can be numerically calculated by applying the Fourier transform to a time series.

**Turbulent fluctuations** (*a'*, where *a* is any scalar or wind component) - The deviation of any value from its temporal (or spatial) average when applying Reynolds averaging.

**Turbulent kinetic energy (TKE)** (*e*) - The kinetic energy of the three-dimensional, random turbulent fluctuations of the wind components in a flow.

**Ultrasonic anemometer** - An instrument that measures all wind components at high frequency by measuring of the speed of sound.

**Ultraviolet radiation (UV)** - Radiation in the range 0.15 µm to 0.4 µm of the short-wave spectrum. It is not visible to the human eye.

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

**Variance** - (*a' ^{2}*, where

**Vertical wind component** (*w*) - see wind components.

**Vector** (any bold lowercase symbol, **a**=(*a _{1}, a_{2}, a_{3}*)) - A variable with a directional information i.e. magnitude and direction. The mostly used vector in this course is the three dimensional wind vector with its three wind components.

**Virtual temperature ** (T_{v}) - A theoretical temperature that is the temperature a dry air parcel would have to reach the same density a moist air parcel acutually has.

**Viscosity ** - Internal resistance of a fluid to deformation. We can quantify viscosity by either the dynamic viscosity or the kinematic viscosity.

**Viscous shear stress** - the tangential force per unit area (in N m^{-2}) that is deforming a fluid parcel on a molecular level opposing viscosity and can be viewed as a momentum flux denisty. Viscous shear stress acts without any macroscale mass exchange. *See also Reynolds stress*.

**Visible radiation ** - Short-wave radiation in the range 0.4 µm to 0.7 µm which is visible to the human eye. In this course this is considered equivalent to photosynthetically active radiation.

**Volume fraction** (*θ*) - The volume that a soil compound (organic material, mineral material, water, air) occupies per total soil volume in m^{3} m^{-3}.

**Volumetric water content** (*θ _{w}*) - The volume fraction of a soil that is currently occupied by water in in m

**von Kármán's constant** (*k*) - A dimensionless constant in the logarithmic wind law. The fraction of the vertical distance the `average' eddy takes up from ground to a certain height under neutral conditions (0.41).

**Watt ** (W)- The SI unit for power (1 W = 1 J s^{-1} = 1 kg m^{2} s^{-3}).

**Wien's Displacement Law** - A basic radiation law that describes the wavelength of maximum emittance of a black body as a fuction of its absolute temperature in Kelvin.

**Wind components** - The components of the three dimensional wind vector **u**=(*u, v, w*) in m s^{-1}. Typically, the coordinate system is aligned in a way that the longitudinal wind component *u* points into mean wind (*x*-axis), the lateral wind component *v* is perpendicular to the mean wind in the horizonatal plane (*y*-axis) and *w* is the vertical wind component (*z*-axis).

**Wind gradient** - The change of mean wind with distance in m s^{-2} m^{-1}. Typically refers to the vertical change of horizontal wind ∂u/∂z.

**Zenith** - that point in the sphere surrounding an observer that lies directly above her or him.

**Zenith angle** (*Z*) - vertical direction of the Sun relative to the observer's zenith expressed in degrees or radians. The reciprocal of solar altitude, i.e. *Z* = 90° - β.

**Zero-plane displacement** (*z _{d}*) - a length (in m) that describes how the logarithmic wind law is shifted upward (displaced) over tall roughness.