Urban and global air quality modelling.
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:
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:
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.
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
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.
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.
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).
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
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.
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.
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.