1. Processes in the Planetary Boundary Layer
Niels Woetmann Nielsen
Danish Meteorological Institute

2. Overview General remarks about the PBL
The turbulent kinetic energy equation
Buoyancy generation of turbulence
Shear generation of turbulence
Surface fluxes
Temperature and wind profiles
Heterogeneity
Air pollution and turbulence
Soundings/observations
Exercise

3. ------------------------ --------- S B
Turbulent kinetic energy equation in
horizontally homogeneous turbulence
S B
Mean turbulent kinetic energy
Shear production of turbulent kinetic energy
Buoyancy production of turbulent kinetic energy

4. Buoyancy generation of turbulence

6. Shear generation of turbulence

8. Turbulent surface fluxes
Surface momentum flux:
Surface sensible heat flux:
Surface latent heat flux:

9. Temperature and wind profiles

13. Friction in the
Atmospheric/Planetary Boundary Layer

14. Due to friction the wind veers with height in the Northern Hemisphere Atmospheric Boundary Layer (ABL).
Cold and warm advection in the ABL contribute to backing and veering with height, respectively.

15. Synoptic scale effect of friction in the Atmospheric Boundary Layer

16. Heterogeneity

17. Left: Diurnal variation of temperature
and wind speed at selected heights
from 2 m to 123 m. Location: Risoe
Right: Monthly variation of wind speed
at seven heights from 7 m to 123 m.
Location: Risoe

18. Effect of surface roughness change on mean wind profile
in neutral stratification

20. Ventilation of the PBL by deep convection embedded in shallow convection with cloud tops near the top of the PBL

22. Air pollution and turbulence

23. After sunrise the convective PBL grows, reaches the plume with highly polluted air and turbulent eddies begin to mix polluted air down to the surface (fumigation)

24. “Small-scale” storm on 18 November 2009

25. During the passage of the cyclone shear generation of turbulence increases substantially, contributing to a closer to neutral stratification and a deeper PBL.

26. Narssarsuaq (South Greenland) Karup (Jutland)
18 November UTC November UTC

27. Surface pressure tendency 15 UTC Surface pressure tendency 18 UTC
and wind velocity at 10m and wind velocity at 10m

28. Ship ldwr (Norwegian Sea) Ekofisk (North Sea)
18 November UTC November UTC

29. Exercise Discuss qualitatively how the PBL evolves downstream
from central Jutland on 25 January 2010 (see next slide S1)
– consider
• PBL height,
• static stability,
• relative role of shear and buoyancy generated turbulence
• sense (upward, downward) of turbulent fluxes of
momentum, sensible and latent heat

30. s1 s1 s1
Temperature (2m), wind (10m) and weather

31. Ekofisk (North Sea) 12 UTC 25 January
2910
Karup (Jutland) 11 UTC 25 January 2010