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Atmospheric Sciences > Topic 3: Boundary Layer Physics > Flashcards

Topic 3: Boundary Layer Physics Flashcards

1
Q

What are four characteristics of the Boundary layer?

A
  1. lowermost part of the troposphere 2. layer most affected by the surface 3. varies from m to km thick 4. strong diurnal cycle
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2
Q

What surface properties affect the boundary layer? 1. 2.

A
  1. surface stress (friction) - changes the wind properties 2. surfaces fluxes (radiative fluxes, latent heat and moisture fluxes) - distribute heat in the vertical
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3
Q

What are three properties of the boundary layer?

A
  1. 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
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4
Q

Describe the Laminar boundary layer 1. 2. 3. 4.

A
  1. direct contact with surface 2. only a few mm thick 3. dominated by molecular diffusion 4. smooth flow
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5
Q

Describe the roughness layer 1. 2. 3. 4. 5.

A
  1. 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
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6
Q

Describe the surface layer 1. 2. 3.

A
  1. 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)
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7
Q

What is turbulence?

A

fully three dimensional, irregular seemingly random motion. is diffusive (mixes) and dissipative. produced by thermal or mechanic energy.

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8
Q

What are the two types of radiation?

A

short wave (solar) and long wave (earth emitted)

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9
Q

What is the Planck Function?

A

empirical relationship found in the lab for a black body. intensity of radiation depends on wavelength and temperature

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10
Q

What is a black body?

A

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.

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11
Q

what is the formula for total emitted radiative flux?

A
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12
Q

What is the solar constant?

A

1368 Wm^2

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13
Q

What is the albedo of the earth and black body temperature?

A

0.3 and 255K

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14
Q

What is the equation that describes the emition of radiation from the earth?

A

4piR^2

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15
Q

What is emissivity?

A

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

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16
Q

What is Albedo?

A

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

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17
Q

The Surface radiation budget formulas:

  1. incoming shortwave
  2. outgoing shortwave
  3. incoming longwave
  4. outgoing longwave
A
  1. direct + scattered
  2. albedo x incoming
  3. from atmosphere
  4. from surface
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18
Q

What is the resultant transport of heat?

A

The result is an upwad transport of heat

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19
Q

Define sensible heat flux

A

vertical transport of heat by turbulant eddies. Laminar layer at surface is heated and distributed vertically.

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20
Q

What does the following formula represent? what does each component mean?

A

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

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21
Q

What do the following formulas define? what are they all part of?

A
  1. N-S velocity
  2. Vertical velocity
  3. potential temperature
  4. specific humidity or mixing ratio

All general pertubation theories

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22
Q

What is the sensible heat flux formula? units?

A

Qh = rho Cp (W’theta’ overbar) given in Wm^-2

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23
Q

If Qh is negative which direction is the transport of heat? if Qh is positive?

A

negative = downward transport of heat

Positive = upward transport of heat

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24
Q

Define latent heat flux

A

the distribution of water vapour by turbulant eddies. Latent heat flux at surface is the result of evoporation/condensation to/from the surface

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25
Q

What is the formula for the latent heat flux? units?

A

Qe = rho Lv (w’q’) given in Wm^-2

26
Q

If there is evoporation the latent heat flux is positive or negative? condensation?

A

positive = evaporation

negative = condensation

27
Q

Define convective heat fluxes. example

A

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.

28
Q

Define conduction fluxes. example

A

heat that is transported through soil. e.g. soil or ground heat flux

29
Q

What is the formula for soil or ground heat flux

A

Qg = -Ks (dT/dZb)

Where Zb = distance below ground

T = soil temperature

Ks = soil thermal conductivity Wm^-1K-1

30
Q

If Qg is positive flux is…?

A

Downward into the ground. temperature decreases with depth

31
Q

During the daytime which direction are Q*, Qh, Qe and Qg/

A

Q* is positive and directed TOWARDS the surface

Qe, Qh and Qg are also positive but directed AWAY from the surface

32
Q

Define the radiation budget in terms of the fluxes

A

Q* = Qh + Qe + Qg + deltaQs

33
Q

Compare and contrast surface energy budgets for arid vs vegetated surfaces

A

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

34
Q

What does the surface energy budget look like at night

A

small neg upward Q*, small neg Qh ~small neg Qe, neg Qg

35
Q

What is the Bowen ratio? formula?

A

ratio of latent heat and sensible heat

beta = Qh/Qe

36
Q

Which environments have higher bowen ratios:

Desert

rainforest

Ocean

A

Desert>rainforest>Ocean

37
Q

What is the relationship between Qh and Qe if there is no vegetation?

A

Qh > Qe

38
Q

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?

A

if Qh > 0 unstable conditions

if Qe > 0 evaporation at surface

39
Q

What is the role of surface drag (friction) in the boundary layer?

A

reduces wind speed to zero near or at the surface. the height at which wind speed is 0 is the roughness length.

40
Q

what does Zg stand for? what influences its height?

A

Zg is the height at which the wind reachs its geostrophic value. the more frictiont the terrain provides the higher Zg is

41
Q

What are the two components of geostrophic flwo?

A

Coriolis and the pressure gradient

42
Q

In which way does geostrophic wind flow? on which side are the low pressure systems in the SH?

A

it flows parallel to the idobars and leaves low pressure systems to the right

43
Q

what is the Ekman spiral?

A

When the geostrophic balance is brokenand wind rotates with height, bending towards the low pressure

44
Q

How does wind behave in the surface layer with:

  1. neutral conditions
  2. unstable conditions
  3. stable conditions
A
  1. neutral = wind increases logarithmically with height
  2. unstable = eddies are streched out and weaken the vertical wind shear. in well mixed conditions wind is constant with height
  3. stable = eddies are compressed and wind shear is strengthened. almost linear
45
Q

What is the formula for the logarithmic wind profile? what does each component stand for?

A

u* = friction velocity

K = von Karmans constant (~0.4)

Z0 = roughness length

46
Q

What influences the value of U*?

A

wind speed and stability conditions. determined empiracally

47
Q

What influences Z0 (roughness length)

A

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

48
Q

What is Charnocks relation?

A

the formula used to approximate roughness length over oceans. alpha c is a constant 0.018 is the typical value

49
Q

What does this formula define? what do each of its components represent?

A

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

50
Q

What does this formula find?

A

horizontal momentum flux. the way wind shear is redistributed by horizontal momentum

51
Q

If there is no vertical windshear what does this imply?

A

production of mechanical turbulance is zero (friction velocity vanishes)

52
Q

If a strong wing region sits above a weak wind region which direction is the transport of momentum?

A

downward

53
Q

Why is the study of turbulance so imortant? (4)

A
  1. key process in the distribution of momentum, heat ad moisture.
  2. determines pollution dispersion
  3. has aviation applications
  4. no turbulance - no rain!
54
Q

Define isotropic turbulance

A

turbulance is the same when measured in different directions (veritcally horizontally etc.)

55
Q

What are the two ways in which turbulance can be generated in the boundary layer?

A

(1) Stress (drag) - mechanical tubulance or shear turbulance
(2) convective turbulacnce - buoyancy driven or buoyancy turbulance

56
Q

in the formula d(TKE)/dt = S + B - D + Tr what do S, B D and Tr represent?

A

S= shear

B= Buoyancy ]

D = dissipation or viscosity

Tr = Transport

57
Q

Define Richardson number

A

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

58
Q

What is Kelvin-Helmholtz instability?

A

warm layer above cold layer. statically stable and strongly sheared. visually - wispy billows or waves in the clouds

59
Q

When discussing turbulent length scales in which direction is the cascade of energy?

A

cascade of energy from large to small eddies untill the eddies are dissipated by molecular diffusion.

60
Q

what happens during the night in terms of heat fluxs and the surface layer?

A

a shallow, stable, inversion layer forms

Atmospheric Sciences (8 decks)

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