Early Warning Signals for Regime Transition in the Stable Boundary Layer: A Model Study

Area of Science:

• Atmospheric Science
• Fluid Dynamics
• Geophysics

Background:

• The nocturnal boundary layer exhibits two distinct regimes: weakly stable (strong-wind/overcast) and very stable (clear-sky/weak-wind).
• Previous studies used Monin-Obukhov similarity, relying on turbulence closure assumptions, to model the transition dynamics.
• Understanding these transitions is crucial for predicting atmospheric stability and turbulence.

Purpose of the Study:

• To investigate the evening transition of the nocturnal boundary layer using direct numerical simulation (DNS).
• To verify previous predictions of transition dynamics without relying on turbulence parametrizations.
• To identify and quantify early warning signals for the regime shift to a very stable state.

Main Methods:

• Direct numerical simulation of an idealized nocturnal boundary layer model.
• Analysis of turbulent boundary layer dynamics to detect changes preceding regime shifts.
• Comparison with generic non-linear dynamical systems exhibiting critical transitions.

Main Results:

• DNS results confirm previous predictions regarding nocturnal boundary layer transition dynamics, independent of turbulence closure assumptions.
• Specific, predictable changes in turbulent boundary layer dynamics precede the shift to the very stable state.
• Quantitative estimates for the transition point can be inferred from these dynamic changes.

Conclusions:

• The study validates previous findings on nocturnal boundary layer transitions using a more robust simulation method.
• Early warning signals for critical transitions in stably stratified turbulent flows are identified.
• The nocturnal boundary layer system exhibits characteristics of generic non-linear systems undergoing critical transitions.