Technical note: Recursive rediscretisation of geo-scientific data in the Modular Earth Submodel System (MESSy)

This technical note describes a method for the rediscretisation of "gridded" geo-scientific data. A recursive algorithm (NREGRID) is derived to solve the rediscretisation problem for orthogonal grids (including curvilinear) of arbitrary dimension. The algorithm is used within the program NCREGRID to handle geo-scientific data. These data are typically 2-dimensional (latitude-longitude grid), or 3-dimensional (latitude-longitude grid with a vertical pressure, or hybrid pressure coordinate, as used in atmospheric modelling). NCREGRID can be used as stand-alone program for the transformation ("regridding") of data from and to data files in netCDF format. Moreover, NCREGRID constitutes the core of the Modular Earth Submodel System (MESSy) data import interface, providing a powerful tool for accessing data independently of the applied grid resolution ("automatic regridding")

Lightning and convection parameterisations ? uncertainties in global modelling

International audienceThe simulation of convection, lightning and consequent NOx emissions with global atmospheric chemistry models is associated with large uncertainties since these processes are heavily parameterised. Each parameterisation by itself has deficiencies and the combination of these substantially increases the uncertainties compared to the individual parameterisations. In this study several combinations of state-of-the-art convection and lightning parameterisations are used in simulations with the global atmospheric chemistry general circulation model ECHAM5/MESSy, and are evaluated against lightning observations. A wide range in the spatial and temporal variability of the simulated flash densities is found, attributed to both types of parameterisations. Some combinations perform well, whereas others are hardly applicable. In addition to resolution dependent rescaling parameters, each combination of lightning and convection schemes requires individual scaling to reproduce the observed flash frequencies. The resulting NOx profiles are inter-compared, however definite conclusions about the most realistic profiles can currently not be drawn

Technical Note: The Modular Earth Submodel System (MESSy) ? a new approach towards Earth System Modeling

International audienceGenerally, the typical approach towards Earth System Modeling has been to couple existing models of different domains (land, ocean, atmosphere, ...) offline, using output files of one model to provide input for the other. However, for a detailed study of the interactions and feedbacks between chemical, physical, and biological processes, it is necessary to perform the coupling online. One strategy is to link the existing domain-specific models with a universal coupler. In many cases, however, a much simpler approach is more feasible. To achieve the online coupling, we have developed the Modular Earth Submodel System (MESSy). Data are exchanged between a and several within one comprehensive model system. MESSy includes a generalized interface structure for the standardized control of the and their interconnections. The internal complexity of the is controllable in a transparent and user friendly way. This provides remarkable new possibilities to study feedback mechanisms (by two-way coupling), e.g., by applying MESSy to a general circulation model (GCM)

Quantifying atmospheric transport, chemistry, and mixing using a new trajectory-box model and a global atmospheric-chemistry GCM

We present a novel method for the quantification of transport, chemistry, and mixing along atmospheric trajectories based on a consistent model hierarchy. The hierarchy consists of the new atmospheric-chemistry trajectory-box model CAABA/MJT and the three-dimensional (3-D) global ECHAM/MESSy atmospheric-chemistry (EMAC) general circulation model. CAABA/MJT employs the atmospheric box model CAABA in a configuration using the atmospheric-chemistry submodel MECCA (M), the photochemistry submodel JVAL (J), and the new trajectory submodel TRAJECT (T), to simulate chemistry along atmospheric trajectories, which are provided offline. With the same chemistry submodels coupled to the 3-D EMAC model and consistent initial conditions and physical parameters, a unique consistency between the two models is achieved. Since only mixing processes within the 3-D model are excluded from the model consistency, comparisons of results from the two models allow to separate and quantify contributions of transport, chemistry, and mixing along the trajectory pathways. Consistency of transport between the trajectory-box model CAABA/MJT and the 3-D EMAC model is achieved via calculation of kinematic trajectories based on 3-D wind fields from EMAC using the trajectory model LAGRANTO. The combination of the trajectory-box model CAABA/MJT and the trajectory model LAGRANTO can be considered as a Lagrangian chemistry-transport model (CTM) moving isolated air parcels. The procedure for obtaining the necessary statistical basis for the quantification method is described as well as the comprehensive diagnostics with respect to chemistry. <br><br> The quantification method presented here allows to investigate the characteristics of transport, chemistry, and mixing in a grid-based 3-D model. The analysis of chemical processes within the trajectory-box model CAABA/MJT is easily extendable to include, for example, the impact of different transport pathways or of mixing processes onto chemistry. Under certain prerequisites described here, the results can be used to complement observations with detailed information about the history of observed air masses

The influence of the vertical distribution of emissions on tropospheric chemistry

The atmospheric chemistry general circulation model EMAC (ECHAM5/MESSy atmospheric chemistry) is used to investigate the effect of height dependent emissions on tropospheric chemistry. In a sensitivity simulation, anthropogenic and biomass burning emissions are released in the lowest model layer. The resulting tracer distributions are compared to those of a former simulation applying height dependent emissions. Although the differences between the two simulations in the free troposphere are small (less than 5%), large differences are present in polluted regions at the surface, in particular for NO<sub>x</sub> (more than 100%), CO (up to 30%) and non-methane hydrocarbons (up to 30%), whereas for OH the differences at the same locations are somewhat lower (15%). Global ozone formation is virtually unaffected by the choice of the vertical distribution of emissions. Nevertheless, local ozone changes can be up to 30%. Model results of both simulations are further compared to observations from field campaigns and to data from measurement stations

Technical note: The new comprehensive atmospheric chemistry module MECCA

In this technical note we present the multi-purpose atmospheric chemistry model MECCA. Owing to its versatility and modular structure, it can be used for tropospheric as well as stratospheric chemistry calculations. Extending the code to other domains (e.g. mesospheric or oceanic chemistry) is easily possible. MECCA contains a comprehensive atmospheric reaction mechanism that currently includes: 1) the basic O₃, CH₄, HO_x, and NO_x chemistry, 2) non-methane hydrocarbon (NMHC) chemistry, 3) halogen (Cl, Br, I) chemistry, and 4) sulfur chemistry. Not only gas-phase chemistry but also aqueous-phase and heterogeneous reactions are considered. Arbitrary subsets of the comprehensive mechanism can be selected according to the research objectives. The program code resulting from the chemical mechanism can easily be used in any model, from a simple box model to a comprehensive global general circulation model

What does the global mean OH concentration tell us?

International audienceThe global mean OH concentration ([OH]GM ) has been used as an indicator of the atmospheric oxidizing efficiency or its changes over time. It is also used for evaluating the performance of atmospheric chemistry models by comparing with other models or with observationally-based reference [OH]GM levels. We contend that the treatment of this quantity in the recent literature renders it problematic for either of these pur-poses. Several different methods have historically been used to compute [OH]GM: weighting by atmospheric mass or volume, or by the reaction with CH4 or CH3CCl3. In addition, these have been applied over different domains to represent the troposphere. While it is clear that this can lead to inconsistent [OH]GM values, to date there has been no careful assessment of the differences expected when [OH]GM is computed using various weightings and domains. Here these differences are considered using four different 3D OH distributions, along with the weightings mentioned above applied over various atmospheric domains. We find that the [OH]GM values computed based on a given distribution but using different domains for the troposphere can result in differences of 10% or more, while different weightings can lead to differences of up to 30%, comparable to the uncertainty which is commonly stated for [OH]GM or its trend. Thus, at present comparing [OH]GM values or trends from different studies does not provide clearly interpretable information about whether the OH amounts are actually similar or not, except in the few cases where the same weighting and domain have been used in both studies. Furthermore, we find that the only direct indicator of the global atmospheric oxidizing efficiency of OH with respect to a particular gas (e.g. CH4 or CH3CCl3 ) is the [OH]GM value weighted by the reaction with that gas; the mass-weighted and volume-weighted [OH]GM values, in contrast, are generally poor indicators of the atmospheric oxidizing efficiency on a global basis (regionally they are better). We recommend that in future studies the [OH]GM value weighted by the reaction with CH4 , along with the CH4 turnover time, be given as the primary indicators of the atmospheric oxidizing efficiency, and that serious evaluations of modeled OH concentrations be done with air mass weighted [OH]GM broken down into atmospheric sub-compartments, especially focusing on the tropics, where the atmospheric oxidizing efficiency is greatest

Technical Note: The new comprehensive atmospheric chemistry module MECCA

International audienceIn this technical note we present the multi-purpose atmospheric chemistry model MECCA. Owing to its versatility and modular structure, it can be used for tropospheric as well as stratospheric chemistry calculations. Extending the code to other domains (e.g. mesospheric or oceanic chemistry) is easily possible. MECCA contains a comprehensive atmospheric reaction mechanism that currently includes: 1) the basic O3, CH4, HOx, and NOx, chemistry, 2) non-methane hydrocarbon (NMHC) chemistry, 3) halogen (Cl, Br, I) chemistry, and 4) sulfur chemistry. Not only gas-phase chemistry but also aqueous-phase and heterogeneous reactions are considered. Arbitrary subsets of the comprehensive mechanism can be selected according to the research objectives. The program code resulting from the chemical mechanism can easily be used in any kind of model, from a simple box model to a sophisticated global general circulation model

The 1-way on-line coupled atmospheric chemistry model system MECO(n) – Part 1: Description of the limited-area atmospheric chemistry model COSMO/MESSy

The numerical weather prediction model of the Consortium for Small Scale Modelling (COSMO), maintained by the German weather service (DWD), is connected with the Modular Earth Submodel System (MESSy). This effort is undertaken in preparation of a new, limited-area atmospheric chemistry model. Limited-area models require lateral boundary conditions for all prognostic variables. Therefore the quality of a regional chemistry model is expected to improve, if boundary conditions for the chemical constituents are provided by the driving model in consistence with the meteorological boundary conditions. The new developed model is as consistent as possible, with respect to atmospheric chemistry and related processes, with a previously developed global atmospheric chemistry general circulation model: the ECHAM/MESSy Atmospheric Chemistry (EMAC) model. The combined system constitutes a new research tool, bridging the global to the meso-γ scale for atmospheric chemistry research. MESSy provides the infrastructure and includes, among others, the process and diagnostic submodels for atmospheric chemistry simulations. Furthermore, MESSy is highly flexible allowing model setups with tailor made complexity, depending on the scientific question. Here, the connection of the MESSy infrastructure to the COSMO model is documented and also the code changes required for the generalisation of regular MESSy submodels. Moreover, previously published prototype submodels for simplified tracer studies are generalised to be plugged-in and used in the global and the limited-area model. They are used to evaluate the TRACER interface implementation in the new COSMO/MESSy model system and the tracer transport characteristics, an important prerequisite for future atmospheric chemistry applications. A supplementary document with further details on the technical implementation of the MESSy interface into COSMO with a complete list of modifications to the COSMO code is provided

Will climate change increase ozone depletion from low-energy-electron precipitation?

We investigate the effects of a strengthened stratospheric/mesospheric residual circulation on the transport of nitric oxide (NO) produced by energetic particle precipitation. During periods of high geomagnetic activity, energetic electron precipitation (EEP) is responsible for winter time ozone loss in the polar middle atmosphere between 1 and 6 hPa. However, as climate change is expected to increase the strength of the Brewer-Dobson circulation including extratropical downwelling, the enhancements of EEP NO<sub>x</sub> concentrations are expected to be transported to lower altitudes in extratropical regions, becoming more significant in the ozone budget. Changes in the mesospheric residual circulation are also considered. We use simulations with the chemistry climate model system EMAC to compare present day effects of EEP NO<sub>x</sub> with expected effects in a climate change scenario for the year 2100. In years of strong geomagnetic activity, similar to that observed in 2003, an additional polar ozone loss of up to 0.4 μmol/mol at 5 hPa is found in the Southern Hemisphere. However, this would be approximately compensated by an ozone enhancement originating from a stronger poleward transport of ozone from lower latitudes caused by a strengthened Brewer-Dobson circulation, as well as by slower photochemical ozone loss reactions in a stratosphere cooled by risen greenhouse gas concentrations. In the Northern Hemisphere the EEP NO<sub>x</sub> effect appears to lose importance due to the different nature of the climate-change induced circulation changes