chapter 7: the cloud topped BL

Question 1

The cloud-topped boundary layer (CTBL) can be broadly identified as

Answer 1

a turbulent region in which ensembles of stratus, stratocumulus and cumulus clouds reside beneath a capping inversion

• dominant feature of
  
  • the lower atmosphere of many areas of the globe, particularly over the sea

important component of the climate system

Question 2

why is The cloud-topped boundary layer (CTBL) studied?

Answer 2

• clouds have a large impact on
  
  • surface weather
  • boundary-layer structure
• fog may exist in the lower regions of the ABL in the form of
  
  • radiation fog,
  • frontal fog,
  • advection fog and
  • ice snow fog.

Question 3

Clouds that are limited in their vertical extent by the capping inversion are

Answer 3

an intrinsic feature of the cloud-topped boundary layer (CTBL),

and consist mainly of three cloud types:

• shallow fair weather cumulus (Cu)
• stratocumulus (Sc) and
• stratus (St).

Question 4

CTBL over Land

Answer 4

• cloud base determined by
  
  • local lifting condensation level for rising unsaturated air parcel
• if LCL beneath capping inversion = CTBL exist
  
  • cloud top conciding with inversion level or lower

Question 5

On days when sufficient moisture is present in the boundary layer

Answer 5

• the LCL can be below zi.
• the tops of the thermals that extend above the LCL are filled with cumulus clouds.

Question 6

During fair weather regimes over land (early morning)

Answer 6

• BL rising slowly
• stable layer being burned off
• LCL rise rapidly due to warming surface air
• LCL above BL
• RH decrease
• no BL clouds till mid morning

Question 7

During fair weather regimes over land (late morning)

Answer 7

• nocturnal inversion disapear
• BL rise rapidly
• zi rise to ht of capping inversion from previous day (1 to 2 km)
• zi > LCL
• fair wx cumulus clouds form

Question 8

The degree of vertical development of the clouds depends on the

Answer 8

height difference between the LCL (which marks the cloud base) and zi (which marks the cloud top).

Question 9

CTBL over Oceans

Answer 9

different from land

• RH of surface air is higher (>75%)
  
  • cloud cover (mostly stratus and stratocumulus) is more extensive
  • some oceanic regions are cloud covered most of the time
  • Cloud layer strengthen the Capping inversion
• radiative transfere plays a more imporant and comlex role in the BL heat balance
• in some regions drizzle has a significant role in the BL heat and water balance

Question 10

Cloud layer strengthen the Capping inversion through:

Answer 10

• reduce the entrainment of dry air into the cloud layer
  
  • maintain the clouds
• its impact on the radiation balance of the underlying layers (i.e., the ocean plus the atmospheric boundary layer).
  
  • when BL is shallow (zi<= 1km)
    
    • radiative cooling of the mixed layer
      
      • reduction in insolation due to cloud (cloud top is not much cooler than sea surface bellow) >reduction in outgoing LW radiation from underlaying water
      • help maintain clouds by
        
        • keeping the relative humidity high
• for BL depth ~ 2km the two effects nearly cancel
  
  • the cooling is less efficient