The Daytime Convectively-Mixed Boundary Layer

During the daytime hours, you will find;

1. a surface layer.  The surface layer is in direct contact with the earth's surface.  The lowest few centimeters of air in the surface layer is often called the microlayer, or interfacial layer.
   • The interfacial layer is at most, a few centimeters deep.
   • Within this layer, molecular transport of heat, moisture, and momentum is much more effective than turbulent transport. 
   • This makes some sense since this layer is in direct contact with the ground
   • above the interfacial layer, in the heart of the surface layer, turbulent transport dominates.
   • gradients of temperature, moisture and winds can be very large in the surface layer, especially in the interfacial layer.
   • The lapse rate within the surface layer tends to be super-adiabatic.
2. the mixed layer.  This layer is located above the surface layer and below the entrainment zone.
   • heat, moisture and momentum are uniformly mixed within the mixed layer.  This is accomplished by turbulence within the mixed layer.
   • Turbulence within the mixed layer is largely convectively driven from two main sources:
     • heat transfer from the warm ground to the interfacial layer via conduction and then convective transport of this heat by thermals up into the mixed layer.
     • radiative cooling from the top of the cloud layer creating "upside down" thermals of cool, sinking air.
   • wind shear can also generate mechanical turbulence within the mixed layer
   • Notice from the above figure that the mixed layer begins to grow vertically approximately 1/2 hour after sunrise.  It grows rapidly during the morning hours and reaches a maximum depth in the afternoon

Q:  How does the mixed layer grow?

• The turbulence (largely the convectively driven thermals) mixes (entrains) down potentially warmer, usually drier, less turbulent air down into the mixed layer
• These variable are then well-mixed within the mixed layer.
• Hence, heat, moisture and momentum are well mixed, or distributed uniformly within the mixed layer. See Figure 1.9.

3.  The Entrainment Layer.    

• As shown in the above figures, the entrainment layer (zone) is a stable layer above the mixed layer.
• It acts as a lid to rising thermals
• It is often an inversion layer, but not always.
• Waves can often be seen propagating on top of the mixed layer within and above the entrainment zone.

OK, that said, what do the surface layer, mixed layer, and entrainment zone look like in a sounding??????