Boundary Layer Depth and Structure

Over the Oceans:

• boundary layer depth varies relatively slowly is space and time...., why? ANSWER
• most changes in boundary layer depth over the oceans are caused by synoptic or mesoscale processes, creating vertical motion and/or advection of different air masses over the sea surface

Over the Tropics:

• this structure also varies slowly in space and time
• near the ground is located a surface layer
• a shallow mixed layer (less than 1 km deep) is found above the surface layer
• the mixed layer is capped by a transition layer at just about 1 km above the surface.  This also corresponds to cloud base
• often you will find a trade wind cumulus layer on order of 1 km deep between roughly 1-2 km AGL. 
• The cumulus layer is capped by the trade wind inversion layer
• Of course, in areas of stronger forcing (e.g., easterly waves, ITCZ, etc.) or where the trade wind inversion is weaker, the cumulus will grow in Cb.

Effect of Highs and Lows:

Near a region of high pressure:

• Over both land and oceans, the boundary layer tends to be shallower near the center of high pressure regions. 
  • This is due to the associated subsidence and divergence.
  • few clouds form in this region, why? 
• boundary layer depth increases on the periphery of the high where the subsidence is weaker.
  • clouds are more apt to form in this region

Near a region of low pressure:

• the rising motion associated with the low transports boundary layer air up into the free troposphere. 
• Hence, it is often difficult to find the top of the boundary layer in this region.
• In a situation such as this where the boundary layer is not evident in a sounding, cloud base is often used at the top of the boundary layer.
• this may be somewhat arbitrary, but it is objective and makes some physical sense.