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JDonda
- 1. Collapse of turbulencein the atmospheric nocturnal boundary layer
- 2. Two nocturnal regimes Weaklystable boundary layer Very stable boundary layer Cloudy sky, Strong wind Warm surface Clear sky, weak wind Cold surface
- 3. Relevance: why isit important ? Fog, Slippery road, Smog, frozen blossom… Current forecast models have difficulties to predict cold extremes
- 4. Challenge: why itis difficult ? ?
- 5. Atmospheric boundary layer duringdaytime Heating Strong mixing ~ 1 km
- 6. Cooling Weak mixing Atmospheric boundary layer duringthe night ~ 100 m
- 7. Weakly stable boundarylayer Warm Radiative cooling Turbulent mixing Moderately cold Strong wind
- 8. Very stable boundarylayer Warm Radiative cooling Turbulent mixing ‘Very cold’ Weak wind
- 9. Turbulentheatflux Temperature gradient Cooling rate Strongwind Maximum Heat Flux Warm Radiative cooling Turbulent mixing Moderately cold
- 10. Warm Radiative cooling Turbulent mixing ‘Verycold’ Maximum Heat Flux
- 11. Vary wind Vary cooling
- 12. Collapse of theturbulence in DNS Goal: predict collapse Turbulentkineticenergy time ‘laminar’ turbulent
- 13. Prediction of collapse Degreeofturbulence weakcooling WarmCold Simulation theory strong cooling Critical cooling
- 14. What after collapse? initial state1 ‘laminar’ state2 ‘laminar’ state2 Turbulentheatflux Temperature gradient Cooling rate initial state1 Acceleration revival? 3 revival state 3
- 15. What after collapse? Velocity Height Comparison with prediction Final state ‘laminar’ state initial state1 2 3
- 16. Basic competition betweencooling and turbulent mixing explains ‘observed’ collapse in simulations of surface cooled flow Conclusion
Editor's Notes
- #3 I would like to start by presenting the two regimes that can occur in the nocturnal boundary layer. First, the weakly stable boundary layer which corresponds to a night with cloudy sky and strong wind. And then, the very stable boundary layer which corresponds to a night with clear sky and weak wind. In my thesis I am studying the collapse of turbulence which means the transition from weakly stable to very stable. This transition is also named laminarization of the flow and generates a strong stratification of the flow and cold extremes at the surface.
- #4 These cold extremes are nowadays very difficult to predict by the forecast models. However, they can have a great impact on our lives, with for example, development of fog or slippery road. These may be problematic for the safety of the road of airplane traffic. It can also be very expensive if flights have to be canceled. They can also generate the development of smog in the cities and be problematic for the health of the population. And I would like to give one last example in the agriculture area, with the frozen blossom which can be devastating for the fruits harvest.
- #5 The transition from laminar to turbulent is studied since a while in the literature, with for instance the introduction of perturbations in the laminar flow and then, we look if the perturbations will grow into turbulence. However the transition from turbulent to laminar is still a unknown issue. So, this is the aim of my project to understand this transition and to predict it for the numerical simulations of a surface cooled flow.
- #6 During the day, the sun is heating up the surface which generates warm air at the surface and relatively cold air at the top of the boundary layer. Therefore, large convective cells develops.
- #7 However , during the night
- #13 So, the aim of my Ph.D is to predict this collapse of turbulence in my numerical simulations for a surface cooled flow. Therefore we translate this conceptual graph to a mathematical, technical framework.