Processes in the Planetary Processes in the Planetary Boundary LayerBoundary Layer

Niels Woetmann NielsenNiels Woetmann Nielsen

Danish Meteorological InstituteDanish Meteorological Institute

OverviewOverview General remarks about the PBLGeneral remarks about the PBL The turbulent kinetic energy equationThe turbulent kinetic energy equation Buoyancy generation of turbulenceBuoyancy generation of turbulence Shear generation of turbulenceShear generation of turbulence Surface fluxes Surface fluxes Temperature and wind profilesTemperature and wind profiles Heterogeneity Heterogeneity Air pollution and turbulenceAir pollution and turbulence Soundings/observationsSoundings/observations ExerciseExercise

DWPTwg

zv

wvzu

wute

vv

''''''

Turbulent kinetic energy equation in horizontally homogeneous turbulence

------------------------ --------- S B

e

S

S

Mean turbulent kinetic energy

Shear production of turbulent kinetic energy

B Buoyancy production of turbulent kinetic energy

Buoyancy generation of turbulenceBuoyancy generation of turbulence

Shear generation of turbulenceShear generation of turbulence

Turbulent surface fluxesTurbulent surface fluxes

Surface momentum flux:2

)('' rms zuCwu 2

)('' rms zvCwv

Surface sensible heat flux:

)()))(('' rsrhpsp zVzCcwc

Surface latent heat flux:

)())(('' rsrqese zVqzqCLwqL

Temperature and wind profilesTemperature 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

HeterogeneityHeterogeneity

Left: Diurnal variation of temperatureand wind speed at selected heightsfrom 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 Ventilation of the PBL by deep convection embedded in shallow convection with cloud tops near the top of the PBLconvection with cloud tops near the top of the PBL

Air pollution and turbulenceAir 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” stormSmall-scale” stormonon

18 November 200918 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)Narssarsuaq (South Greenland) Karup (Jutland)18 November 2009 11 UTC 18 November 2009 10 UTC18 November 2009 11 UTC 18 November 2009 10 UTC

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

Ship ldwr (Norwegian Sea) Ekofisk (North Sea)Ship ldwr (Norwegian Sea) Ekofisk (North Sea)

18 November 2009 12 UTC 18 November 2009 12 UTC18 November 2009 12 UTC 18 November 2009 12 UTC

Exercise

Discuss qualitatively how the PBL evolves downstreamfrom 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

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

s1 s1

Ekofisk (North Sea) 12 UTC 25 January2910

Karup (Jutland) 11 UTC 25 January 2010