Turbulent Layers Associated with a Critical Level in the Planetary Boundary Layer

Abstract

Formation of thin turbulent layers in a stably stratified fluid is discussed in combination with a critical level and internal gravity waves.
A series of numerical simulations by using nonlinear model of an incompressible fluid reveal that very thin unstable layers (local Richardson number <0.25) are formed in the vicinity of the critical level. Mini-K-H billows can be expected to grow up and break into turbulence in the thin unstable layers. Long-time numerical calculations show that such thin unstable layers which contain the mini-K-H billows and turbulence roll up at their edges and form Cat's eye patterns associated with large amplitude K-H billows deformed by the critical level. This result also seems to open a new perspective to the critical level theory itself.
Some observations in the planetary boundary layer provide good evidences to the results of the present numerical simulations.