the university of reading helen dacre agu dec 2008 boundary layer ventilation by convection and...
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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
The University of Reading Helen Dacre AGU Dec 2008
Motivation
Boundary layer ~1km
Advection ?Convection ?
Mixing ?
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
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
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
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
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
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
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)
The University of Reading Helen Dacre AGU Dec 2008
AMPEP flight
AMPEP flight path AMPEP flight height
IG
D
wind direction
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
The University of Reading Helen Dacre AGU Dec 2008
Polluted profile in English Channel
AMPEP CO profileModel tracer profile
15:34 UTC
elevated pollution
The University of Reading Helen Dacre AGU Dec 2008
Quantifying Ventilation
? ?
?
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
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
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
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
The University of Reading Helen Dacre AGU Dec 2008
Schematic of boundary layer ventilation for 9th May 2005
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
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
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