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Planetary Boundary Layer > Chapter 1 > Flashcards

Chapter 1 Flashcards

1
Q

Layers in the atmosphere

A
  • Surface layer 50m
  • Atmospheric boundary layer 1 km
  • Troposphere 10 km
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2
Q

Planetary boundary layer definition

A

the lowest part of the atmosphere that is in contact with the surface of the earth

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3
Q

PBL is a region where earth’s surface strongly influence:

A
  • temperature
  • moisture
  • wind
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4
Q

Importance of PBL

A

PBL plays an important role in many fields:

  • air pollution
  • agro. met
  • hydrolody
  • aviation
  • ….
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5
Q

Free atmosphere:

A

the remaining air through the atmosphere above the PBL

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6
Q

energy in FA is ……………………. through ………………..

A

dissipated through the PBL

friction and turbulence

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7
Q

properties:

A
  • friction
  • turbulence
  • thickness
  • mixing
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8
Q

friction in PBL and FA

A

PBL:

  • significant drag
  • energy dissipation (due to friction)

FA:

  • Insignificant drag
  • little to no energy dissipation
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9
Q

Turbulence in PBL and FA

A

PBL:

  • continuous turbulences throughout the layer

FA:

  • turbulence only in convective clouds and in thunderstorms
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10
Q

thickness in PBL and FA

A

PBL:

  • between 100 and 3000 m
  • diurinal variation

FA:

  • between 3 and 18 km
  • little variation
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11
Q

Mixing in PBL and FA

A

PBL:

  • Rapid mixing in vertical and horizontal

FA:

  • rapid horizontal mixing
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12
Q

PBL can be subdevided into:

A
  • surface
  • mixed layer (convective mixing layer)
  • stable layer
  • residual layer
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13
Q

PBL depends on:

A
  • temperature
  • structure
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14
Q

PBL structure varies with:

A
  • season
    • higher in summer
  • time of day
    • higher in daytime
  • weather conditions
    • higher in instability (1000’s of meters)
    • lower in stability (10’s of meters)
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15
Q

Surface layer:

A
  • closest to the earth’s surface
  • turbulent stresses are relatively constant
  • height of surface layer is 10% of PBL height
    • varies from 20 to 200m
  • exchange of
    • heat
    • momentum
    • water
    • airpollutants
      by turbulent fluxes between surface and atmosphere
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16
Q

Mixing layer (convective mixed layer)

A
  • above the surface layer
    • mixed layer (during day)
    • stable boundary layer (during night)
      both depend on temperature structure of PBL
  • after sunrise the convective mixed layer grows rapidly due to intense turbulences
  • this layer is capped by a stable environment zone
  • during daytime the mixing layer reaches heights greater than 1 km and make upthe entire layer of the PBL above the surface layer
17
Q

Stable boundary layer

A
  • Near sunset the mixed layer collapses and in its place a stable (nocturnal) BL is formed
  • the bottom part of this layer is stabalized by night time radiational cooling of the surface
  • the height of PBL decreases during night time
18
Q

Residual layer

A
  • above the stabe BL is the residual layer
  • while
    • nocturnal BL has a stable profile
    • RL has a neutral profile
  • RL doesnt hace contact with the earth’s surface, therefore it is not concidered as part of BL
  • residual layer is bounded by a capping inversion (at a height of daytime BL height), which prevents environment aloft
19
Q

PBL charactaristics:

A
  • Physical state
  • energy exchange
  • Turbulence
20
Q

PBL physical state is described with:

A

(state variability)

  • temperature
  • humidity
  • pressure
  • wind
  • trace gasses

related to dynamical aspects (of PBL)

  • turbulence
  • diffusion
  • stability
21
Q

changes in state variability are controled by

A

processes that transfer (rate of transfer are quantified in the form of fluxes)

  • mass
  • energy
  • momentum

within the BL and between the BL & FA

22
Q

energy exchange:

A

radiation exhange regulate the physical and chemical state of the BL

radiation (solar energy):

  • absorbed during nighttime (warms PBL) or emiited during nighttime (cools PBL)
  • supply energy for evapotranspiration
23
Q

Turbulence:

A

in convective BL turb. eddies are generated by:

  • wind shear (DAY or NIGHT)
  • bouyancy associated with surface heating (DAY)

some of the large eddies penetrate into the FA generating convective clouds

24
Q

PBL modeling:

A

(turbulent and fluxes)

PBL model is an attempt to describe the statistical behaviours of the PBL turbulence through simplified equations

25
Q

PBL modeling is also called

A

PBL parametarisation

26
Q

A PBL model provide

A
  • PBL depth
  • surface stresses
  • surface fluxes (couple the atm and land surface processes)
    • heat
    • position
    • pollutants
      and their profiles within the PBL.
      ​time rate of change of
      • wind speed
      • wind direction
      • temperature
      • RH
  • cloud amount (if low clouds present)
    • determine the amount of radiation in and out of the surface
27
Q

PBL model produces

A
  • shear turbulence
  • buoyancy turbulence

(all meteorological foreecast models need a PBL model

Planetary Boundary Layer (10 decks)

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