the university of reading helen dacre agu dec 2008 boundary layer ventilation by convection and...
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![Page 1: The University of Reading Helen Dacre AGU Dec 2008 Boundary Layer Ventilation by Convection and Coastal Processes Helen Dacre, Sue Gray, Stephen Belcher](https://reader035.vdocument.in/reader035/viewer/2022062511/5515f90f55034694308b4855/html5/thumbnails/1.jpg)
The University of Reading Helen Dacre AGU Dec 2008
Boundary Layer Ventilation by Convection and Coastal Processes
Helen Dacre, Sue Gray, Stephen Belcher
Didcot power station, Oxfordshire
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The University of Reading Helen Dacre AGU Dec 2008
Motivation
Boundary layer ~1km
Advection ?Convection ?
Mixing ?
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The University of Reading Helen Dacre AGU Dec 2008
Talk Outline
• Modelling Experiment
• Case study – May 9th 2005– Results from modelling experiments– Comparison with observations– Ventilation estimate
• Conclusions
• Future Work
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The University of Reading Helen Dacre AGU Dec 2008
Experimental Setup• Met Office Unified Model• Limited area domain over Europe, ∆x≈12km, ∆z≈100m in
boundary layer, 500m in mid-troposphere • Parameterized convection and boundary layer turbulent
mixing
• Pollution sources represented by constant emission of tracers over land emitted 20m above the surface
• Different ventilation mechanisms represented by advection, convection and turbulent mixing schemes
• Tracers transported by different combinations of transport schemes
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The University of Reading Helen Dacre AGU Dec 2008
May 2005 Case Study OverviewUK Met Office surface pressure analysis 00UTC
Nimrod radar composite 13:00 UTC
Modis Aqua visible 12:45 UTC
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The University of Reading Helen Dacre AGU Dec 2008
Advection
Tracer in free troposphere integrated over height
17 UTC
coastland sea
09 UTC
13 UTC
17 UTC
kg/m
2
wind direction
wind direction
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The University of Reading Helen Dacre AGU Dec 2008
Advection and Mixing
Tracer in free troposphere integrated over height
17 UTC
coastland sea
09 UTC
13 UTC
17 UTCkg
/m2
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The University of Reading Helen Dacre AGU Dec 2008
Advection, Mixing and Convection
09 UTC
13 UTC
17 UTC
land coast sea
kg/m
2
Tracer in free troposphere integrated over height
17 UTC
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The University of Reading Helen Dacre AGU Dec 2008
AMPEP Observations
Downwind air measurements(mass flux of pollution coming off UK)
Upwind air measurements (background concentrations)
• Direct measurements of the mass budgets of pollutants in the boundary layer over the UK
(Aircraft Measurements of chemical Processing and Export fluxes of Pollutants)
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The University of Reading Helen Dacre AGU Dec 2008
AMPEP flight
AMPEP flight path AMPEP flight height
IG
D
wind direction
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The University of Reading Helen Dacre AGU Dec 2008
Boundary layer profile in central England
AMPEP CO profileModel tracer profile
17:24 UTC
boundary layer top
background concentration
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The University of Reading Helen Dacre AGU Dec 2008
Polluted profile in English Channel
AMPEP CO profileModel tracer profile
15:34 UTC
elevated pollution
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The University of Reading Helen Dacre AGU Dec 2008
Quantifying Ventilation
? ?
?
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The University of Reading Helen Dacre AGU Dec 2008
Quantifying Ventilation
Percentage of tracer in free troposphere
Percentage of tracer above 2km
52%
26%
sunrise sunsetsunrisesunset
% o
f tr
acer
in f
ree
trop
osph
ere
% o
f tr
acer
abo
ve 2
km
Time of day (hours) Time of day (hours)
advadv + mixadv, mix + conv
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The University of Reading Helen Dacre AGU Dec 2008
Case Study Summary
Transport process
Ventilation
AdvectionLarge-scale ascent transported pollution up to 3km
Coastal venting enhanced by sea-breeze → layer of polluted air over clean air along coast
ConvectionWidespread convection → layer of polluted air in mid-troposphere over whole of the UK
Turbulent mixingTurbulence mixes pollution within boundary layer → increases ventilation by large-scale ascent
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The University of Reading Helen Dacre AGU Dec 2008
Conclusions• NWP model output is a useful tool and can complement
observational results in studying transport processes
• Assumption that ventilation of the boundary layer is negligible during non-frontal events is not always valid– Convection can transport polluted air out of bl and
form a distinct layer in the free troposphere– Coastal outflow can transport pollution above the
marine boundary layer– Sea-breeze can enhance coastal outflow and create
a layer of polluted air above a layer of clean air along the coast
• Turbulent mixing and convection can double the amount of pollution ventilated from the boundary layer
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The University of Reading Helen Dacre AGU Dec 2008
Further Work• How sensitive are model simulations of the distribution of
pollutants to the representation of convective transport?
• How well is the sea-breeze represented at 12km resolution?
• Are Meteorological models suitable for chemical dispersion studies?
• What are the relative importance of these ventilation processes on pollution distribution on longer timescales?
ReferenceH.F.Dacre, S.L.Gray and S.E.Belcher (2007), A case study of boundary layer
ventilation by convection and coastal processes, J. Geophys. Res., 12, D17106, doi:10.1029/2006JD007984
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The University of Reading Helen Dacre AGU Dec 2008
Schematic of boundary layer ventilation for 9th May 2005
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The University of Reading Helen Dacre AGU Dec 2008
15 UTC over sea
11 UTC over land
Advection• Cross-sections of
potential temperature• Bold contours indicate
inversions• Dashed line indicates
boundary layer top
09 UTC
13 UTC land coast seaHei
ght a
bove
sur
face
(m
)
coast
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The University of Reading Helen Dacre AGU Dec 2008
Introduction
• The dynamical processes that transport pollution within and above urban areas span a large range of spatial and temporal scales
100m10m
200m
Street scaleNeighbourhood scale
2km
City scale
20km
Regional scale
200km
1km10km
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The University of Reading Helen Dacre AGU Dec 2008
Introduction• There are many transport processes responsible for the
ventilation of pollution from the boundary layer into the free troposphere