chapter 6: thermally stratified BL (notes)

Question 1

the thermally stratified boundary layer

Answer 1

• unstable boundary layer (convective BL)
• Stable boundary layer (nocturnal BL)
• marine BL (marine BL)

Question 2

convective boundary layer (unstable boundary layer)

Answer 2

BCL occurs when:

• thermal instability due to strong surface heating (convection in the form of thermals)
• upside down convection generated by cloud top radiational cooling

Question 3

Convective boundary layer (unstable boundary layer) components

Answer 3

• surface layer
• mixed layer (ML)
• entrainment zone

Question 4

Convective boundary layer (unstable boundary layer) surface layer

Answer 4

• Decrease with ht
  
  • T
    
    • rate of decrease of temperature is rapid
    • the SL is characterised by SUPERADIABATIC lapse rate (due to intense surface heating) >9.8 c/km
    • transfer of fluxes due to conduction
  • O-
  • q
• increase with ht
  
  • wind

Question 5

Convective boundary layer (unstable boundary layer) mixed layer (ML)

Answer 5

• characterised by intense vertical mixing
  
  • because of the O- and q remain constant with ht
• wind speed and direction remain constant with ht
• T decrease at a slower rate than T at the surface

Question 6

Convective boundary layer (unstable boundary layer) entrainment zone

Answer 6

• top of mixing layer (zi) is near the middle of entrainment zone
  
  • the level where the capping inversionis very strong
  • winds becomes constant when it reaches geostrophic wind
• zi can also be defined as the level of most negative heat flux
  
  • gradient from low to high = flux from high to low

Question 7

what is the entrainment zone

Answer 7

• a region of stable air at the top of mixing layer
• thermals in mixing layer extend into the environment zone

h₀: when air has 5 to 10% of the FA charactaristics (bottom of EZ)

h_z: top of the EZ (top of heighest thermals in the region)

Question 8

Stable boundary layer

Answer 8

the BL becomes stable whenever the surface becomes colder than air

• Stable BL forms at night due to radiational cooling and is called nocturnal BL
• Stable BL forms by advection of warm air over colder surface

Question 9

charactaristics of stable BL winds:

Answer 9

• wind direction is
  
  • determined by local topography
  • veers with ht (turns clockwise)
• wind speed is determined by
  
  • buoyancy
  • friction
  • entrainment
• higher in the Stable BL synoptic and mesoscale forces are important

Question 10

marine boundary layer (MBL)

Answer 10

a stable layer with profile similar to night time land profiles