Modelling meteorology and air quality

Urban and global air quality modelling.

Heat mapping

Our group is using state-of-the-art modelling tools for addressing key issues in meteorology and air quality research. A special emphasis is being placed on the dynamical, physical and chemical processes that affect air quality on scales ranging from global down to turbulence scales.

See below to find out more about our recent and current work:

Global and regional scale interactions

Regional scale air quality is influenced by global scale dynamics and transformation processes. Global chemical transport and climate models are used to derive boundary conditions for regional models such as the Community Multi-scale Air Quality Modelling system (CMAQ).

Within CAIR, boundary conditions are employed from a number of global models such as:

STOCHEM

GEOS-Chem

EMAC/MATCH

GEMS

MACC/MACC II

HADGEM2

The methods to generate chemical boundary conditions for regional air quality varies from the use of a constant background concentrations of species, through use of idealised vertical profiles to dynamically varying boundary conditions from global atmospheric models.

References:

Appel, W., Chemel, C., Roselle, S.J., Francis, X.V., Hu, R.-M., Sokhi,  R.S., Rao, S.T., Galmarini, S., (2012) Examination of the Community Multiscale Air Quality (CMAQ) model performance over the North American and European domains, Atmospheric Environment (2011), doi: 10.1016/j.atmosenv.2011.11.016.

Kenneth Schere, Johannes Flemming, Robert Vautard, Charles Chemel, Augustin Colette, Christian Hogrefe, Bertrand Bessagnet, Frederik Meleux, Rohit Mathur, Shawn Roselle, Rong-Ming Hu, Ranjeet S. Sokhi, S. Trivikrama Rao, Stefano Galmarini (2012)  Trace gas/aerosol boundary concentrations and their impacts on continental-scale AQMEII modelling domains. Atmospheric Environment Volume 53, June 2012, Pages 38–50

Regional scales

Global models can provide the large-scale picture of atmospheric  dynamics and chemistry but their representation of features occurring on  smaller temporal and spatial scales is poor due to their coarse  horizontal resolutions. Indeed, spatial features smaller than a grid box  are averaged out. For that reason, limited-area or mesoscale models are  being used for investigations at resolutions of up to 1km.

Further information

Comparison of simple and advanced regional models

Impacts of urban areas on regional air quality

Cross-tropopause transport induced by deep tropical convection

Regional air quality and impacts of large point sources

Local/urban scales

The local scale refers to spatial scales of about hundred metres up to  about few kilometres. Urban scales usually refer to spatial scales of  about few kilometres to several tens of kilometres. At these scales  finer resolution models are employed and some features that are  traditionally considered 'subgrid' in mesoscale models, and thus needs  to be parametrized, become explicit. Convection is a good example. In  addition, the assessment of the airflow is more challenging as details  of the terrain such orographic or urban features becomes even more  important.

Further information

Urbanisation of the UK Met Office Unified Model

Effects of landuse heterogeneities on air quality over London

Modelling urban meteorology with MM5

Effects of orography on air quality in deep and narrow valleys

Integrated modelling system for air quality assessment

Turbulence scales - entrainment and mixing processes in stably stratified layers

The parametrization of turbulent mixing processes is crucial for the  transport, mixing and dry deposition of pollutants within the  atmospheric boundary layer. In all boundary-layer flow  parameterisations, the scale of the turbulent structures is assumed to  be much smaller than the grid spacing. This assumption is reasonable for  grid spacings of the order of a couple of kilometres or more but cannot  be used for grid resolution of one kilometre or less. Important  research issues are thus raised regarding this interface between  explicit representation and parametrization.

Recent research work has focused on entrainment and mixing processes  in stably stratified layers. The exchange processes coupling the  convectively boundary layer (CBL) with the free atmosphere above have  been detailed over homogeneous flat terrain. Entrainment and mixing  occur across the stably stratified density interface capping the mixed  layer. As a result of both encroachment and entrainment of air from the  free atmosphere above, the mixed layer deepens. The entrainment dynamics  was presented from both deterministic and statistical point of views  using results from a high-resolution large-eddy simulation (LES).

CBL simulation three comparison images

Results from a high-resolution large-eddy simulation of the CBL over homogeneous flat terrain. Snapshots of the vertical velocity field, w, in a vertical plane and in different planes located at various normalised heights z/ziabove the ground surface: (a) 0.1, (b) 0.5, and (c) 1.0, where z is the mean height of the mixed layer over the computational domain. The grey scale ranges from black (strong negative values) to white (strong positive values)

The  rate of growth of the mixed layer, i.e. the entrainment velocity, is  determined by the energy balance within the mixed and interfacial  layers. The entrainment law across this interfacial layer has been  revisited in light of the concept of mixing efficiency of the  entrainment process. A new parameterisation for the entrainment ratio R  in terms of mixing efficiency has been derived, yielding a new  formulation of convective entrainment. This formulation was found to be  in good agreement with results from the abovementioned LES.

This work is currently pursued focusing on the applicability of this  new formulation for an explicit treatment of the entrainment process in  classical boundary-layer parameterization schemes implemented in  mesoscale models. Indeed, the vertical mixing and the growth of the  (thermally- or mechanically driven) CBL are generally modelled  separately in subgrid boundary-layer flow parameterization schemes. This  is inconsistent since both the vertical mixing and the growth of the  convective mixed layer are driven by the vertical heat transfer.

Whilst vertical fluxes in the convective boundary layer are carried  by large convective motions spanning most of the mixed layer, vertical  motions are strongly inhibited in the stable boundary layer (SBL),  especially in nocturnal conditions. The essential issue here is the  understanding of turbulence and mixing in stable layers and its  representation in numerical models, i.e. the parameterisation of  momentum and heat fluxes. This issue is currently being investigated in  the cases of the SBL over homogeneous flat terrain as well as complex  heterogeneous terrain, such urban areas and mountainous terrain.

Acknowledgements & collaborations:

C. Staquet & J.-P. Chollet (Laboratory of Geophysical and Industrial Fluid Flows, Joseph Fourier University, Grenoble, France) have provided invaluable help via idea generation and exchange, transfer of know-how and mutual interactions

Selected references

Chemel, C. and C. Staquet, 2008. Generation of internal gravity waves by a katabatic wind in an idealized valley. Submitted to Met. Atm. Phys. Chemel, C., C. Staquet, and J.-P. Chollet, 2008. Eulerian- and  Lagrangian-based estimates of convective entrainment rate from  large-eddy simulation. To be submitted pretty soon. Chemel, C. and C. Staquet, 2007. A convective entrainment formulation in terms of mixing efficiency. J. Fluid Mech. 580, 169–178.

Bridging the scales: downscaling and up-scaling

Progress on scale and process interactions has been limited because of  the tendency to focus mainly on issues arising at specific scales.  However the inter-relating factors between the sources of air pollution  and their impacts on the environment rely on atmospheric processes and  interactions operating on a whole range of scales. Therefore, a  consistent, integrated framework bringing together the treatment of  meteorology, emissions and chemistry is required.

Numerical weather and air pollution prediction models are now able to  approach a few kilometre horizontal resolution, as detailed input data  are becoming more often available. As a result the conventional concepts  of down- (and up-) scaling for air pollution prediction need revision  along the lines of integration of multi-scale meteorological and  chemical transport models.

MEGAPOLI (a new project funded by the European Commission under FP7) aims at developing a comprehensive integrated modelling  framework usable by the research community which will be tested and  implemented for a range of megacities within Europe and across the world  to increase our understanding of how large urban areas and other  hotspots affect air quality and climate on multiple scales.

Within MEGAPOLI, we are looking at a strategy to integrate the range  of scales. The integration is not focused on any particular  meteorological and/or air pollution modelling system. The approach  considers an open integrated framework with flexible architecture  (module/interface structure) and with a possibility of incorporating  different meteorological and chemical transport models. The structure of  the framework will enable the coupling across the whole range of scales  by minimizing the scale-dependence of the interfaces. This multi-scale  approach is especially crucial for an efficient integration strategy.

Indeed, atmospheric flows include frequently locally forced features,  which interact with regional- and global-scale processes such as fronts  and convection. Scale interaction is a challenge for both weather and  air pollution predictions, especially at regional scales. Depending on  both time and space scales, certain atmospheric processes can no longer  be explicitly resolved or treated as sub-grid scale features and thus  needs to be parameterised. The framework will contain methods for the  aggregation of episodic and long-term results, model downscaling as well  as nesting.

With the application to megacities in mind, the 'integration' needs  to be fully achieved down to urban scales. MEGAPOLI is thus expected to  address the difficulties arising from the treatment of the multi-scale  and multi-process nature of the integration procedure down to the city  scale, including feedback processes.

image

Interlinking atmospheric modelling on different scales

With the same sort of idea as MEGAPOLI in mind, an intercomparison   exercise is being designed for tropical regions. Here, the   parameterisation of deep convection is playing a key role. The idea is   to provide a strategy to upscale the information obtained from campaign   measurements, such as those obtained during ACTIVE, and mesoscale  models  to the global scale. This will be done by evaluating convection  and  convective transport in global models against mesoscale model  results  and campaign data in a statistical framework, therefore  improving our  assessment and our confidence in the global model  treatment of such  processes. This activity is coordinated by the Centre  for Atmospheric  Science at the University of Cambridge under the  SCOUT-O3 EU project.

Acknowledgements & collaborations:

The MEGAPOLI project is funded by the European Commission (FP7 project). The group benefits from interactions with the Centre for Atmospheric Science at the University of Cambridge, UK.