Chapter 6: Thermally stratified BL

Question 1

unstably stratified ABL

Answer 1

or convective boundary layer (CBL)

• occurs when strong surface heating (due to the sun) produces
  
  • thermal instability or convection in the form of thermals and plumes, and
  • when upside-down convection is generated by cloud-top radiative cooling.

Question 2

mixed layer

Answer 2

In strongly unstable conditions driven by surface heating, the outer layer in particular is dominated by convective motions

Question 3

Entrainment happens at a variety of scales:

Answer 3

• lateral entrainment by small eddies into the sides of thermals, and
• vertical entrainment on the thermal scale into the whole mixed layer.

Question 4

Three layers can be identified within the convective boundary layer:

Answer 4

• the surface layer in the bottom 5 to 10%
• the mixed layer composing the middle 35 to 80%, and
• the entrainment zone in the top 10 to 60%.

Question 5

The Surface Layer (SL)

Answer 5

• there are small-scale structures such as
  
  • buoyant vertical plumes,
  • convergence lines,
  • sheets of rising air, and
  • dust devils.
• characterized by:
  • T: a superadiabatic lapse rate,
  • moisture decrease with height,
    
    • become relatively small at the top of the SL
    • Evaporation/transpiration from the surface occur if the ground is moist or vegetated
  • strong wind shear.
  • virtual potential temperature decreases rapidly across the microlayer
    
    • from the hot surface skin temperature to warm temperature at bottom of SL
    • the strong gradient in the microlayer suppot molecular conduction
    • in the remainder of the SL, the virual pot temp gradient decrease smoothly with ht. becoming nearly zero at the top of the SL
      
      • the gradient in the reminder of the SL support down-gradient (small eddy) turbulent diffusion.

Question 6

The Mixed Layer (ML)

Answer 6

named because intense vertical mixing tends to leave conserved variables such as potential temperature and humidity nearly constant with height

• wind speed and direction are nearly constant over the bulk of the ML
• The top of the whole convective mixed layer, zi, is often defined as the level of most negative heat flux
• This level is near the middle of the entrainment zone, often at the height where the capping inversion is strongest
  • CI acts like an interface between the ML and the FA

Another measure of the average ML depth is the height at which an undiluted air parcel rising from the surface becomes neutrally buoyant.

Question 7

Another measure of the average ML depth is the height at which an undiluted air parcel rising from the surface becomes neutrally buoyant.

Answer 7

Mixing can be generated mechanically by

• shears or
  
  • (ML) favours horizontal motions
  • shear near the ground are more impotant for generating mixing than shears across the top of the ML
    
    • shear at ML top cause _seperate laye_r to form
• convectively by buoyancy
  
  • tend to be more uniformly mixed than ones driven mechanically
    
    • (ML) anistropy in convection favours vertical motion
  • mixed layer dominated by buoyant turbulence generation: convective boudary layer (CBL)

Question 8

The Entrainment Zone (EZ)

Answer 8

is the region of statically stable air at the top of the ML, where there is entrainment of FA air downward and overshooting thermals upward. The EZ can be quite thick - averaging about 40% of the depth of the ML.

alternate definition: the region where the buoyancy flux w’O-‘ is negative

Question 9

The Entrainment Zone (EZ) profile

Answer 9

• The top of the entrainment zone
  
  • defined as the altitude, h₂ of the top of the highest thermal within a region.
  • altitude of local ML top varies between the EZ top and middle of the ML
    
    • its shape looks like a series if inverted U’s marking the tops of penetrating thermals
• The bottom
  
  • h_o is more difficult to define because there is no sharp demarcation.
  • taken as that altitude where about 5 to 10 % of the air on a horizontal plane has FA characteristics.

The average ML depth, which we traditionally call z_i, is the altitude where 50% of the air has FA characteristics on a horizontal average. Thus, z_i is just below the midpoint between h₂ and h₀.

Question 10

The Stable BL (SBL)

Answer 10

• The boundary layer can become stably stratified whenever the surface is cooler than the air.
• it can form:
  
  • SBL forms at night over land (Nocturnal boundary layer)
  • by advection of warmer air over cooler surface

Question 11

The stable BL typical profile of mean variables in SBL for the case of weak turbulent mixing

Answer 11

• The greatest static stability is near the ground, with stability decreasing smoothly toward neutral with height.
• classified as temperature inversion if
  
  • stabilities are great enough near the surface to cause
    
    • temperature to increase with hight

sometimes the whole SBL is loosely called a nocturnal inversion.

Question 12

SBL winds

Answer 12

very complex charactaristics

in the lowest 2 to 10m:

• cold air will drain down hill
  
  • wind direction determined by local topography
  • wind speed is governed by
    
    • buouyancy
    • friction
    • entrainment
• wind can become calm
  
  • in flat areas or at
  • the bottom of valleys or
  • topographic depression
• wind speed increase with height reaching maximum near the top of the stable layer
• wind direction often veer (clockwise) with height

Question 13

SBL forcings become important

Answer 13

higherin the SBL synoptic and mesoscale forcings

Question 14

nocturnal jet

Answer 14

• the layer of peak wind speed is sometimes greater than the geostophic speed
• above the jet
  
  • wind speed and direction smoothly change to geostrophic
  • wind profile is often not in steady state and evolve with time during the night

Question 15

SBL humidity

Answer 15

difficult to classify because

• sometimes evaporation from the surface continues at night
• while at other locations or times condensation results in dew or frost formation.

Question 16

SBL in case of reduced turbulences

Answer 16

• strong moisture gradients can occur near the surface.
• The delicate balance between turbulence and stability is apparent in the formation of fog vs. dew, and in the fog processes of dissipation, thickening, or lifting.

Question 17

The Maritime BL (MBL)

Answer 17

Stably stratified marine boundary layers, with vertical temperature moisture and wind profiles analogous to the nighttime land profiles, are observed in regions of warm advection, where warm air is flowing over colder water

For example, as air that has resided over the Gulf Stream flows northward over the cold Labrador Current to the southeast of Nova Scotia it becomes stably stratified.

Question 18

For example, as air that has resided over the Gulf Stream flows northward over the cold Labrador Current to the southeast of Nova Scotia it becomes stably stratified.

under these conditions

Answer 18

the air just above the surface is often chilled to its dew point, resulting in widespread fog.

Question 19

Advection fog is also common when

Answer 19

air approaching the coast of California passes over the narrow zone of coastal upwelling just offshore, and when warm, humid air masses off the Gulf of Mexico flow northward over snow-covered ground in the midwestern United States during winter

Question 20

Over the Arctic and Antarctic, the boundary layer becomes

Answer 20

• highly stratified in response to the uninterrupted radiative cooling of the surface during the extended polar night.
• Under conditions of light winds,
  • boundary-layer turbulence virtually disappears and
  • the capping inversion settles to the ground
• the surface air temperature may be more than 20 °C lower than the air temperature at the top of the inversion just tens or hundreds of meters above the ground.
  
  • temperature may rise sharply whenever the wind speed picks up mixing warmer air downward from above the inversion or when a cloud layer moves overhead, increasing the downward flux of longwave radiation incident on the Earth’s surface.

Question 21

During cold, calm intervals, Arctic cities and towns such as Fairbanks, Alaska, experience

Answer 21

episodes of ice fog when water vapor emitted by automobiles and wood stoves is trapped below the inversion.

• The frost point of the air is so low that even the emissions from a small urban complex are sufficient to supersaturate the air,
  
  • resulting in the formation of tiny, sparsely distributed ice crystals referred to by local residents as
    
    • “diamond dust” because of the way they sparkle in the sunlight.