Processes in the Planetary Boundary Layer

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Presentation transcript:

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

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

------------------------ --------- 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

Buoyancy generation of turbulence

Shear generation of turbulence

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

Temperature and wind profiles

Friction in the Atmospheric/Planetary Boundary Layer

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.

Synoptic scale effect of friction in the Atmospheric Boundary Layer

Heterogeneity

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

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

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

Air pollution and turbulence

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)

“Small-scale” storm on 18 November 2009

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

Narssarsuaq (South Greenland) Karup (Jutland) 18 November 2009 11 UTC 18 November 2009 10 UTC

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

Ship ldwr (Norwegian Sea) Ekofisk (North Sea) 18 November 2009 12 UTC 18 November 2009 12 UTC

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

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

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