Planetary boundary layer

Remarks

The layer is usually divided into a surface layer, roughly its lowest tenth, in which the turbulent fluxes are nearly constant with height, and an outer layer above. In the outer layer of a neutral, rotating boundary layer the wind turns with height towards the geostrophic wind, as in the Ekman layer. Its structure depends strongly on stratification: a convective boundary layer during the day contains a well-mixed layer capped by an inversion, and a stable boundary layer forms at night.

Example: over land on a sunny afternoon the boundary layer is typically $1$ to $2$ km deep, and the potential temperature is nearly constant with height in its mixed layer.

Non-example: on a calm night with a stable boundary layer a few hundred metres deep, the air at $5$ km height is part of the free atmosphere. Its wind is close to geostrophic balance and does not respond to the surface within hours.

The layer is a turbulent boundary layer with very large Reynolds number. Molecular viscosity is negligible except within millimetres of the surface, and turbulent fluxes must be parametrised in models.