Constant flux layer
From AMS Glossary
constant flux layer
A layer of air tens of meters thick at the bottom of the atmosphere where the variation of vertical turbulent flux with altitude is less than 10% of its magnitude.
This layer is also called the surface layer, which is roughly the bottom 10%: of the atmospheric boundary layer. While the flux is not perfectly uniform with height within this layer, the idealization of a constant flux layer permits certain theoretical approaches, such as Monin–Obukhov similarity theory to describe the logarithmic wind profile.