Mechanical internal boundary layer
From AMS Glossary
mechanical internal boundary layer
(Abbreviated MIBL.) An internal boundary layer caused by advection of air across a discontinuity in surface roughness.
When the new surface is rougher than the old one, the MIBL depth grows roughly as the 0.8 power of the ratio of the two roughness lengths. In this example, the MIBL grows to include the whole surface layer.