1. 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

2. The University of Reading Helen Dacre AGU Dec 2008 Motivation Boundary layer ~1km Advection ? Convection ? Mixing ?

3. The University of Reading Helen Dacre AGU Dec 2008 Talk Outline Modelling Experiment Case study – May 9 th 2005 –Results from modelling experiments –Comparison with observations –Ventilation estimate Conclusions Future Work

4. The University of Reading Helen Dacre AGU Dec 2008 Experimental Setup Met Office Unified Model Limited area domain over Europe, x12km, z100m 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

5. The University of Reading Helen Dacre AGU Dec 2008 May 2005 Case Study Overview UK Met Office surface pressure analysis 00UTC Nimrod radar composite 13:00 UTC Modis Aqua visible 12:45 UTC

6. The University of Reading Helen Dacre AGU Dec 2008 Advection Tracer in free troposphere integrated over height 17 UTC coastlandsea 09 UTC 13 UTC 17 UTC kg/m 2 wind direction

7. The University of Reading Helen Dacre AGU Dec 2008 Advection and Mixing Tracer in free troposphere integrated over height 17 UTC coastlandsea 09 UTC 13 UTC 17 UTC kg/m 2

8. The University of Reading Helen Dacre AGU Dec 2008 Advection, Mixing and Convection 09 UTC 13 UTC 17 UTC land coastsea kg/m 2 Tracer in free troposphere integrated over height 17 UTC

9. 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)

10. The University of Reading Helen Dacre AGU Dec 2008 AMPEP flight AMPEP flight pathAMPEP flight height I G D wind direction

11. 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

12. The University of Reading Helen Dacre AGU Dec 2008 Polluted profile in English Channel AMPEP CO profileModel tracer profile 15:34 UTC elevated pollution

13. The University of Reading Helen Dacre AGU Dec 2008 Quantifying Ventilation ? ? ?

14. The University of Reading Helen Dacre AGU Dec 2008 Quantifying Ventilation Percentage of tracer in free troposphere Percentage of tracer above 2km 52% 26% sunrisesunsetsunrisesunset % of tracer in free troposphere % of tracer above 2km Time of day (hours) adv adv + mix adv, mix + conv adv adv + mix adv, mix + conv

15. The University of Reading Helen Dacre AGU Dec 2008 Case Study Summary Transport process Ventilation Advection Large-scale ascent transported pollution up to 3km Coastal venting enhanced by sea-breeze layer of polluted air over clean air along coast Convection Widespread convection layer of polluted air in mid-troposphere over whole of the UK Turbulent mixing Turbulence mixes pollution within boundary layer increases ventilation by large-scale ascent

16. 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

17. 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? Reference H.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

18. The University of Reading Helen Dacre AGU Dec 2008 Schematic of boundary layer ventilation for 9 th May 2005

19. 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 coastsea Height above surface (m) coast

20. 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 100m 10m 200m Street scaleNeighbourhood scale 2km City scale 20km Regional scale 200km 1km 10km

21. 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