The Residual Layer and Nocturnal Boundary Layer

The Residual Layer.

• Approximately 1/2 hour before sunset, the thermals in the convectively mixed boundary layer have shut off as the surface is cooling.
• Hence, above the stable boundary layer, the residual layer is found, and can be thought of as a left-over convective mixed layer.
• The residual layer, therefore, has all the properties of the recently decayed convective mixed layer.
• The static stability of this layer of air is then: Answer   
• The residual layer does not come in direct contact with the ground, and therefore, is strictly speaking, not a boundary layer.

The Stable (Nocturnal) Boundary Layer.

• As evening progresses and the surface cools via radiational cooling, a shallow stable layer of air forms that is in direct contact with the ground.  This stable layer is often called the radiation inversion.
• The nocturnal boundary layer can be anywhere from 0-200m or so deep and is characterized by:
  • strong static stability
  • weak/sporadic turbulence - often occurs in short bursts
  • weak/calm winds at the surface, but increasing to supergeostrophic speeds aloft -> 
  • This wind speed profile is often referred to as a low-level, or nocturnal jet.  The low-level flow is often decoupled from the flow aloft within the low-level jet.  It is possible for the surface winds to be calm, while, a few 10's of meters aloft, the winds are 30-40 m/s.
• Q:  How is turbulence within the nocturnal boundary layer generated???? Answer
• Q:  Is the flow direction at point A in Fig. 1.11 directed:
  • parallel to the isobars
  • crosses the isobars from H to L
  • crosses the isobars from L to H
  • Answer
• Q:  Is the flow direction at point B in Fig. 1.11 directed:
  • parallel to the isobars
  • crosses the isobars from H to L
  • crosses the isobars from L to H
  • Answer
• Q:  Is the flow direction at point C in Fig. 1.11 directed:
  • parallel to the isobars
  • crosses the isobars from H to L
  • crosses the isobars from L to H
  • Answer
• Waves are often observed within the nocturnal boundary layer, more specifically, gravity (buoyancy) waves.  Gravity waves are generated in statically stable layers of air.  The nocturnal boundary layer, therefore, is a prime location for gravity wave generation.

Q:  What do the nocturnal boundary layer and residual layers look like in a sounding?  Take a look...

EXAMPLE HERE.