Further comments on the equilibrium height of neutral and stable planetary boundary layers

The height of the neutrally and stably stratified turbulent planetary boundary layers (PBLs) in the atmosphere and hydrosphere is controlled by numerous factors of different natures. The PBL deepening is facilitated by turbulent mixing caused. first of all, by the velocity shear at the surface, and prevented by the Earth's rotation and the negative buoyancy forces caused by the flow-surface interaction and the stable static stability in the free atmosphere above the PBL. The balance between these factors is often complicated by the baroclinic shears and large-scale vertical motions (usually disregarded in field experiments), not to mention temporal and horizontal variability of natural PBL flow. It is not surprising that until now many alternative (sometimes contradictory) formulations for the stable PBL height have been proposed and no consensus achieved. To the authors' opinion, the most natural way to investigate this complex problem is to consider step by step the PBL height in comparatively simple, basic equilibrium regimes (controlled by limited factors), to develop and to validate theoretical models for these regimes, and only afterwards to switch to general diagnostic (equilibrium) and prognostic (non-steady) equations. In the present paper, we briefly summarize recent investigations following the above strategy, and present new data supporting our earlier theoretical conclusions and recommendations. We would not recommend using PBL height equations inconsistent with firmly established basic-regime formulations. Copyright (c) 2007 Royal 'Meteorological Society.