MODMEP project description



Work Packages


Contributions to the Goals of the AFO 2000

Principal Investigator

Proposed Project Duration




Project flyer


Aerosol particles affect cloud microphysics (e.g. number and size of drops, precipitation formations, state of aggregation of the hydrometeors) with their size distribution, their number and chemical composition. In addition, clouds act as chemical reaction chambers, where gases and particles are scavenged and modified. The complexities of the cloud processes involved have discouraged investigators from simultaneously treating all aspects of multiphase chemistry and microphysics with equal rigor. The description of cloud processes in the currently available box models and Eulerian grid models (cloud or mesoscale models) focuses either on detailed microphysics or complex multiphase chemistry. The chemical conversions in the liquid phase are described only in a few aggregated drop classes (e.g., Chaumerliac et al., 2000), or strongly simplified chemical mechanisms are used (e.g., Wurzler et al, 2000).

The objective of the present project is the development of a cloud module which combines a complex multiphase chemistry with detailed microphysics. The description of both components is given for a fine-resolved drop spectrum. An efficient numerical solution of the entire complex model strongly requires the development of new numerical models (Wolke et al., 2000). The influence of simplifications within single components and the kind of their coupling on the simulation results will be investigated for different tropospheric situations. In the framework of the joint project, techniques will be provided and tested which allow the description of complex multiphase chemistry and of detailed microphysics in multidimensional chemistry-transport models. The development is performed in close cooperation with the joint cloud experiment project FEBUKO (Herrmann et al.).


Coupling of detailed microphysics with multiphase chemistry in several drop classes
The fine resolution of the drop spectrum used in microphysical cloud models will be extended to the treatment of detailed multiphase chemistry. It will be investigated how a reduction of the number of drop classes by aggregation affects the simulation results. In a first version, operator splitting will be used as coupling scheme. The aqueous phase concentrations are coupled both via the gas phase and via the liquid water transfer (coalescence, break up). The numerical integration of the multiphase system is done by an implicit integrator making use of the sparsity of the system. The microphysical parameters needed by the multiphase chemistry are supplied by the microphysical model. The sensitivity of the entire model will be analysed.

Adapted multiphase mechanisms and tools for their implementation into complex models
The existing aqueous phase mechanism CAPRAM 2.3 and 2.4 (Herrmann et al., 2000) already containing a detailed C1 and C2 chemistry will be developed towards a better process description for higher organic compounds which are identified in cloudwater and aerosols in field experiments. For higher organics not only uptake from the gas phase but additionally dissolution out of the cloud droplet precursor CCN has to be implemented as source processes. The extension of the chemical mechanism will focus on a better description of the oxidation of dissolved organic compounds within aqueous particles. For higher organics which may partition from the gas phase, phase transfer will be described by gas phase diffusion, mass accommodation and Henry solubility. Process parameters from the FEBUKO field experiments will directly be taken over into the complex chemistry model. Another objective consists in the development and the application of an automated method of analysis and reduction for multiphase reaction mechanisms. This leads to the derivation of reduced mechanisms for specifiable application purposes.

Methods for coupled time integration
Despite the strong coupling over different processes, the governing equations for dynamic, chemical and cloud parameters are commonly treated separately in chemistry-transport models. It is investigated whether and which processes can be integrated decoupled with which loss of exactness and which gain of efficiency. A major task is the development of implicit-explicit time integration methods which integrate all involved processes in a coupled manner. This includes a.o. the development and the test of solution methods for large, sparse, linear equation systems. An efficient solution of such systems is only possible utilizing its special structure (Wolke and Knoth, 2000).

Spatial description of clouds in 3D models
Clouds are dynamic objects with high spatial and temporal variability, whose formation depends on the existence of aerosols, their physical and chemical properties and the appearance of local supersaturation. At the boundaries of the clouds a phase transition process between dry air and air with cloud drops occurs. Eulerian grid models in general do not spatially resolve the cloud boundaries. Due to the gradual representation of the cloud in a grid box, an artificial, numerically caused smearing of the cloud properties over the grid box appears in the area of the cloud boundaries. This has an effect both on the computation of cloud-dynamical and cloud-microphysical parameters as well as on the simulation of cloud-chemical properties which are coupled with the gas phase concentrations of the related grid cells. The main goal is the development of a method for the additional prediction of the cloud boundary inside a model grid cell (Margolin et al., 1997). Hereby, by means of the Volume of Fluid method, the separate computation of microphysical and chemical processes in the clouded as in the cloud-free area of a grid cell is possible.


Project coordination (IfT)

Development of size-resolved cloud models with detailed microphysics and complex multiphase chemistry

  1. Coupling between detailed microphysics and complex multiphase chemistry (IfT-Num, IfT-Mod, MPI)
  2. Modelling of microphysical processes (MPI, IfT-Mod)
  3. Model test for selected scenarios (IfT-Mod, IfT-Num, MPI)
  4. Interpretation of data from the FEBUKO field experiment (IfT-Mod, IfT-Chem)

Development of adapted multiphase mechanisms and of tools for their implementation into complex models

  1. Chemical mechanisms for the description of tropospheric cloud processes (IfT-Chem, BTU)
  2. Reduction of multiphase reaction mechanisms (BTU, IfT-Chem)
  3. Preprocessor for the input of chemical reaction systems (IfT-Num, BTU)
  4. Preparation of test scenarios (IfT-Chem, IfT-Mod, BTU)

Coupling and time integration (IfT-Num, MPI)

Spatial description of clouds in 3D models (IfT-Num, MPI)

The connection and interaction of the working packages are schematically shown in Fig.1.


The goal of the joint project is the development of suitable tools and methods for the treatment of complex cloud-chemical processes under consideration of several drop classes for parcel and multidimensional chemistry transport models (cloud resolving or mesoscale models). At the end of the project multiphase reaction mechanisms and moduls of different complexity are available for the coupled simulation of multiphase-chemical and microphysical processes.


Multiphase processes are appreciated to be of increasing importance in the comprehension of atmospheric processes. On the one hand, they directly influence the life cycles of trace constituents and facilitate conversions of these trace constituents, which are not or very inefficiently possible in the pure gas phase. On the other hand, they strongly influence cloud formation and the radiation budget of the atmosphere. Multiphase processes are in close interaction with each other as well as with other atmospheric processes. They must be understood as part of the entire system "troposphere". Their essential interchange effects must be taken into consideration, in order to understand their inner mechanisms and the effects to this complex system. Atmospheric models of varying complexity can make an important contribution to a better understanding of the complex processes in the troposphere.


Ralf Wolke, Hartmut Herrmann

Institute for Tropospheric Research, Leipzig,


3 years


This research is supported by the BMBF (AFO 2000 atmospheric research program, project 07ATF40).


Cooperation with AFO 2000 groups:

National and international cooperation with:


Herrmann H, Ervens B, Jacobi H-W, Wolke R, Nowacki P, Zellner R (2000): CAPRAM2.3: a chemical aqueous phase radical mechanism for tropospheric chemistry. J Atm Chem 36:231-284

Hindmarsh A.C. (1983): ODEPACK: a systematized collection of ODE solvers. In: Steplman et al. (ed) Scientific Computing, North-Holland, Amsterdam, pp 55-74

Margolin, L., Reisner, J.M. and Smolarkiewicz, P. (1997): A systematized collection of ODE solvers. In: Steplman et al. (ed.): Scientific Computing, North Holland, Amsterdam, pp 55-74

Schwartz S.E. (1986): Mass transport considerations pertinent to aqueous phase reactions of gases in liquid water clouds. In: Jaeschke W (ed) In Chemistry of Multiphase Atmospheric Systems, Springer Verlag, Berlin, pp 415-471

Wolke R., Knoth O. (2000): Time-integration of multiphase chemistry in size-resolved cloud models. Appl Num Math, submitted

Wolke R., Knoth O., Herrmann H. (2000): Numerical treatment of aqueous phase chemistry in atmospheric chemistry transport modelling. In: Gryning SE and Schiermeier FA (ed.) Air Pollution Modeling and Its Application XIV, Kluwer Academic / Plenum Publishers, New York, in press


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Last change: 2004/04/13