Eberhard Renner

The parallel model system LM-MUSCAT for chemistry-transport simulations: Coupling scheme, parallelization and applications

Publisher Summary This chapter discusses the parallel model system LM-MultiScale chemistry aerosol transport (MUSCAT) for chemistry-transport simulations: coupling scheme, parallelization, and applications. The physical and chemical processes in the atmosphere are very complex. They occur simultaneously, coupled and in a wide range of scales. These facts have to be taken into account in the numerical methods for the solution of the model equations. The numerical techniques allow the use of different resolutions in space and also in time. Air quality models base on mass balances described by systems of time-dependent, three-dimensional advection-diffusion reaction equations. A parallel version of the multiscale chemistry-transport code MUSCAT is presented, which is based on multiblock grid techniques and implicit-explicit (IMEX) time integration schemes. The meteorological fields are generated simultaneously by the non-hydrostatic meteorological model LM. Both codes run in parallel mode on a predefined number of processors and exchange information by an implemented coupler interface. The ability and performance of the model system are discussed for a “Berlioz” ozone episode.

The Chemistry-Transport Modeling System lm-Muscat : Description and citydelta Applications

Air quality models base on mass balances described by systems of time-dependent, three-dimensional advection-diffusion-reaction equations. The solution of such systems is numerically expensive in terms of computing time. This requires the use of fast parallel computers. Multiblock grid techniques and implicit-explicit (IMEX) time integration schemes are suited to take benefit from the parallel architecture. A parallel version of the multiscale chemistry-transport code MUSCAT (MUiltiScale Chemistry Aerosol Transport) is presented which is based on these techniques (Wolke and Knoth, 2000).

Formation of Secondary Inorganic Aerosols by High Ammonia Emissions Simulated by LM/MUSCAT

Ammonia (NH3) is the most abundant gaseous base and responsible for neutralizing a large fraction of acidic gases promoting the formation of atmospheric particles. Therefore, the contribution of ammonia to the formation of secondary particles (PM2.5and PM10) in a regional scale is examined. The aerosols result from SO2 and NOx via sulfuric and nitric acid formation in the gas- and the liquid-phase and following subsequent reactions with ammonia.

Influence of Grid Resolution and Biomass Burning Emissions on Air Quality Simulations: A Sensitivity Study with the Modelling System COSMO-MUSCAT

Model evaluation studies are essential for determining model performance as well as assessing model deficiencies, and are the focus of the Air Quality Model Evaluation International Initiative (AQMEII). The chemistry-transport model system COSMO-MUSCAT participates in this initiative. In this paper the robustness and variability of the model results against changes in the model setup are analyzed. Special focus is given to the formation of secondary particulate matter and the ability to reproduce unusually high levels of PM10 in Central Europe caused by long-range transported smoke of widespread agricultural burning and forest fires in western Russia. Seven different model configurations are investigated in this study. The COSMO-MUSCAT results are evaluated in comparison with ground-base measurements in Central Europe. The analysis is performed for two selected periods in April/ May and October 2006 which are characterized by elevated concentrations of PM. The model sensitivity is studied against changes in the used grid resolution, the meteorological forcing and the applied aerosol module. Possible reasons for differences in model results will be discussed.

The Impact of Meteorological Uncertainties on the Prediction of PM in Urban Areas

The transport and transformation of PM is mainly forced by meteorological processes. Therefore, an appropriate description of these processes is of essential interest in chemistry transport modelling. In the paper, the influence of two different meteorological drivers on the simulated particle concentrations is analyzed. For this purpose, the chemistry transport code MUSCAT was online-coupled with WRF as well as with the COSMO model of the German Weather Service. Furthermore, WRF-Chem simulations are also considered in the model comparison. The combination of two meteorological and two chemistry-transport models has a great potential for a detailed analysis of the meteorological dependencies and its impact on the aerosol processes. The simulation results were compared with a comprehensive set of ground-based and profile measurements. The influence of several meteorological parameters (e.g., PBL height, humidity, precipitation) on the simulated concentration fields was analyzed. Main differences are caused by deviations in PBL and cloudiness.

The Urban Impact on the Regional Climate of Dresden

The building effect parameterization (BEP) module [3] was implemented in a high resolution version of the COSMO model (DWD) in order to take into account the urban impact on the airflow and the radiation budget. Based on urban structure data of Dresden, relevant input parameters of the BEP module were developed. By means of this model setup it is possible to investigate the interactions between the city structure and the meteorological variables with different types of artificial cities, ranging from densely built-up areas to suburban areas in order to illuminate the impact of the city type on the dynamical and thermal properties of the atmosphere.

Influence of Turbulence Parameterization on the Mixing Layer Height Prediction with a Mesoscale Model

In the case study presented here, a maximal MLH hit rate of 88 % in the CBL over the northern part of the model domain is achieved. However, despite of its tolerant definition,the maximal MLH hit rate over the mountainous terrain in the South does not exceed 39 %. Possible explanations for the low hit rate may be shortcomings of the applied parameterizations schemes over complex, inhomogeneous terrain, grid resolution and model forcing. As the prognostic TKE closure does not provide basically better results than the countergradient schemes, we conclude that the characteristic velocity scale is properly considered in the countergradient schemes. Therefore, the low hit rate may be also adressed to the parameterization of the integral length scale. This is suggested by sensitivity studies with the TKE closure, showing a strong influence of the integral turbulence length scale on the MLH prediction. Nevertheless, the verification results presented here are similar to findings from Sorensen and Rasmussen (1997), who exemplary quote a correlation coefficient of r≈0.4 for their 24-h MLH forecast using the DMI-HIRLAM NWP model (Danish Meteorological Institute, High Resolution Limited Area Model) verified against ETEX radiosounding data (European Tracer Experiment 1994). Our findings are also in the range of evaluation results from Schaller and Wenzel (1999) for five regional atmospheric chemistry transport models showing correlation coefficients between 0.4 and 0.6 and overall hit rates between 49 and 69 % for a 24-h MLH forecast period of TRACT, 16.09.92.

An Implicit-Explicit Algorithm for Chemistry-Transport Models

The systems of ordinary differential equations (ODEs) resulting from regional atmospheric chemistry transport models are nonlinear, highly coupled and extremely stiff. Because explicit ODE solvers require numerous short time steps in order to maintain stability, most current techniques solve stiff ODEs implicitly or semi implicitly. In this paper we investigate a new approach based on backward differential formula (BDF) which solves the sparse linear equations during the Newton iteration by linear Gauss-Seidel iterations (Knoth, Wolke, 1994). The Jacobian is computed explicitly and not by finite differences.