Processes in the Planetary Boundary Layer
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Transcript of Processes in the Planetary Boundary Layer
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
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
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.
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
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
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)
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