Boundary layer ventilation by baroclinic systems

1
Boundary layer ventilation by baroclinic systems

• Victoria Sinclair
• Suzanne Gray Stephen Belcher

2
Questions

• Conveyor belts are often assumed to be the
  dominant process in pollution transport is this
  correct?
• How do pollutants get from surface based sources
  to heights where conveyor belts originate?
• How and where are pollutants transported out of
  the boundary layer into the free troposphere ?

3
Methodology

• Idealised dry baroclinic lifecycles in the
  Reading IGCM
• LC1 and LC2
• differ by the addition of barotropic cyclonic
  shear to background state
• Realistic boundary layer scheme
• T42 L39 well resolved BL
• Tracer initialised
• uniform concentration at lowest model level
• acted upon by resolved winds and turbulent
  motions
• no sources or sinks

4
Turbulent mixing. Day 3
Tracer concentration (kg kg-1) Red line
indicates boundary layer depth
Depth integrated tracer mass (kg) and surface
pressure Thick black line marks cross-section
5
Ekman motions. Day 4
Tracer concentration and wind vectors. Note
different colour scales

• Divergence out of anticyclones - low
  concentrations
• Convergence into cyclonic regions - higher
  concentrations

6
Ventilation regions. Day 7
Tracer flux (x 10 5 kg s-1) out of the
boundary layer 0.5 cm s-1 vertical velocity
contours on the boundary layer top Surface
pressure contours. 4mb contour interval
7
Total mass and tracer fluxes
LC1
no sources
Total mass flux (solid line) and tracer flux
(dashed line) out of the boundary layer
8
Transport by conveyor belts
LC1
LC2
Two equal, distinct branches
Cyclonic branch dominates
9
Boundary layer ventilation rate

• LC1 ventilates 26 of tracer and LC2 28
• Similar considering the different frontal
  evolution

10
Conclusions

• Transport occurs by three distinct processes
• Turbulent mixing
• Ekman divergence and convergence
• Advection by warm conveyor belts
• Boundary layer processes control the amount of
  pollutants in the ventilation regions
• Only the warm conveyor belt footprint region of
  the boundary layer is ventilated

11
Questions?

• v.a.sinclair_at_reading.ac.uk
• www.met.reading.ac.uk/swr05vas

12
Boundary layer structure
depth
h/L

• Across synoptic scales there are significant
  variations in the depth and stability of the
  boundary layer
• The boundary layer structure is strongly coupled
  to the large scale dynamics.

13
Questions

• Conveyor belts are often assumed to be the
  dominant process in pollution transport is this
  correct?
• How do pollutants get from surface based sources
  to heights where conveyor belts originate?
• How and where are pollutants transported out of
  the boundary layer into the free troposphere?

14
Transport Processes