Topic 3: Boundary Layer Physics Flashcards
What are four characteristics of the Boundary layer?
- lowermost part of the troposphere 2. layer most affected by the surface 3. varies from m to km thick 4. strong diurnal cycle
What surface properties affect the boundary layer? 1. 2.
- surface stress (friction) - changes the wind properties 2. surfaces fluxes (radiative fluxes, latent heat and moisture fluxes) - distribute heat in the vertical
What are three properties of the boundary layer?
- varies diurnally, controlled by surface radiative balance 2. ~100m at night and a few km during the day 3. characterised by unstable thermals and eddies mixing heat throughout its depth
Describe the Laminar boundary layer 1. 2. 3. 4.
- direct contact with surface 2. only a few mm thick 3. dominated by molecular diffusion 4. smooth flow
Describe the roughness layer 1. 2. 3. 4. 5.
- normally three times the depth of individual elements 2. has roughness elements 3. highly irregular flow and strongly affected by individual elements 4. flow starts to be turbulent 5. normally there is a height in which wind speed is zero due to friction
Describe the surface layer 1. 2. 3.
- has intense small scale eddies generated by surface roughness (friction) and thermals 2. mixture of thermals and mechanical turbulence 3. strong vertical gradient (not well mixed)
What is turbulence?
fully three dimensional, irregular seemingly random motion. is diffusive (mixes) and dissipative. produced by thermal or mechanic energy.
What are the two types of radiation?
short wave (solar) and long wave (earth emitted)
What is the Planck Function?
empirical relationship found in the lab for a black body. intensity of radiation depends on wavelength and temperature
What is a black body?
a surface that completely absorbs all incident radiation. A black body emits or re-emits that received radiation at a different wavelength and energy based on its temperature.
what is the formula for total emitted radiative flux?
What is the solar constant?
1368 Wm^2
What is the albedo of the earth and black body temperature?
0.3 and 255K
What is the equation that describes the emition of radiation from the earth?
4piR^2
What is emissivity?
the ratio of emitted radiation to the amount that would be emitted if it were a “black-body”. equal or less than 1. Depends on the molecular property of each material
What is Albedo?
the ratio of reflected energy flux to the incident energy flux. 1 is a perfect reflector 0 absorbs all the radiation received. Earth has a planetary albedo of about 0.3
The Surface radiation budget formulas:
- incoming shortwave
- outgoing shortwave
- incoming longwave
- outgoing longwave
- direct + scattered
- albedo x incoming
- from atmosphere
- from surface
What is the resultant transport of heat?
The result is an upwad transport of heat
Define sensible heat flux
vertical transport of heat by turbulant eddies. Laminar layer at surface is heated and distributed vertically.
What does the following formula represent? what does each component mean?
U = the east west velocity at any one instant
Ubar = large scale flow over a sustained period (the overbar indicates a time average)
u’ = tubulant flow or eddies
formula defines the mean and turbulant part of flow
What do the following formulas define? what are they all part of?
- N-S velocity
- Vertical velocity
- potential temperature
- specific humidity or mixing ratio
All general pertubation theories
What is the sensible heat flux formula? units?
Qh = rho Cp (W’theta’ overbar) given in Wm^-2
If Qh is negative which direction is the transport of heat? if Qh is positive?
negative = downward transport of heat
Positive = upward transport of heat
Define latent heat flux
the distribution of water vapour by turbulant eddies. Latent heat flux at surface is the result of evoporation/condensation to/from the surface
What is the formula for the latent heat flux? units?
Qe = rho Lv (w’q’) given in Wm^-2
If there is evoporation the latent heat flux is positive or negative? condensation?
positive = evaporation
negative = condensation
Define convective heat fluxes. example
convective fluxes are fluxes in which energy is transferred by air turbulance. eg. latent heat flux or sensible heat flux. transported away from surface during the day.
Define conduction fluxes. example
heat that is transported through soil. e.g. soil or ground heat flux
What is the formula for soil or ground heat flux
Qg = -Ks (dT/dZb)
Where Zb = distance below ground
T = soil temperature
Ks = soil thermal conductivity Wm^-1K-1
If Qg is positive flux is…?
Downward into the ground. temperature decreases with depth
During the daytime which direction are Q*, Qh, Qe and Qg/
Q* is positive and directed TOWARDS the surface
Qe, Qh and Qg are also positive but directed AWAY from the surface
Define the radiation budget in terms of the fluxes
Q* = Qh + Qe + Qg + deltaQs
Compare and contrast surface energy budgets for arid vs vegetated surfaces
Arid surface: Q* pos downward, large pos Qh, small pos Qe, small pos Qg
Vegetated surface: Q* pos downward, pos Qh~Qe, larger pos Qg
What does the surface energy budget look like at night
small neg upward Q*, small neg Qh ~small neg Qe, neg Qg
What is the Bowen ratio? formula?
ratio of latent heat and sensible heat
beta = Qh/Qe
Which environments have higher bowen ratios:
Desert
rainforest
Ocean
Desert>rainforest>Ocean
What is the relationship between Qh and Qe if there is no vegetation?
Qh > Qe
The following shows Qe and Qh with the eddy diffusivitie of heat and moisture included. What conditions does this imply is associated with pos values of Qe and Qh?
if Qh > 0 unstable conditions
if Qe > 0 evaporation at surface
What is the role of surface drag (friction) in the boundary layer?
reduces wind speed to zero near or at the surface. the height at which wind speed is 0 is the roughness length.
what does Zg stand for? what influences its height?
Zg is the height at which the wind reachs its geostrophic value. the more frictiont the terrain provides the higher Zg is
What are the two components of geostrophic flwo?
Coriolis and the pressure gradient
In which way does geostrophic wind flow? on which side are the low pressure systems in the SH?
it flows parallel to the idobars and leaves low pressure systems to the right
what is the Ekman spiral?
When the geostrophic balance is brokenand wind rotates with height, bending towards the low pressure
How does wind behave in the surface layer with:
- neutral conditions
- unstable conditions
- stable conditions
- neutral = wind increases logarithmically with height
- unstable = eddies are streched out and weaken the vertical wind shear. in well mixed conditions wind is constant with height
- stable = eddies are compressed and wind shear is strengthened. almost linear
What is the formula for the logarithmic wind profile? what does each component stand for?
u* = friction velocity
K = von Karmans constant (~0.4)
Z0 = roughness length
What influences the value of U*?
wind speed and stability conditions. determined empiracally
What influences Z0 (roughness length)
characteristics of the surface:
solid ground = height of roughness elements (vegetation = plant hegiht /7)
over water = wind speed as this is what dictates height of roughness elements
What is Charnocks relation?
the formula used to approximate roughness length over oceans. alpha c is a constant 0.018 is the typical value
What does this formula define? what do each of its components represent?
rho = density of air in kgm^-3
Km is eddy viscosity m^2s^-1
magnitude of the horizontal wind can be du or dv
fomula for shear stress
What does this formula find?
horizontal momentum flux. the way wind shear is redistributed by horizontal momentum
If there is no vertical windshear what does this imply?
production of mechanical turbulance is zero (friction velocity vanishes)
If a strong wing region sits above a weak wind region which direction is the transport of momentum?
downward
Why is the study of turbulance so imortant? (4)
- key process in the distribution of momentum, heat ad moisture.
- determines pollution dispersion
- has aviation applications
- no turbulance - no rain!
Define isotropic turbulance
turbulance is the same when measured in different directions (veritcally horizontally etc.)
What are the two ways in which turbulance can be generated in the boundary layer?
(1) Stress (drag) - mechanical tubulance or shear turbulance
(2) convective turbulacnce - buoyancy driven or buoyancy turbulance
in the formula d(TKE)/dt = S + B - D + Tr what do S, B D and Tr represent?
S= shear
B= Buoyancy ]
D = dissipation or viscosity
Tr = Transport
Define Richardson number
Rio = N^2/ (du/dz)^2 ( N is brunt vaisala frequency
Ri is the ratio of termal stability to wind shear. Ri <0.25 - > turbulant
What is Kelvin-Helmholtz instability?
warm layer above cold layer. statically stable and strongly sheared. visually - wispy billows or waves in the clouds
When discussing turbulent length scales in which direction is the cascade of energy?
cascade of energy from large to small eddies untill the eddies are dissipated by molecular diffusion.
what happens during the night in terms of heat fluxs and the surface layer?
a shallow, stable, inversion layer forms
