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Keller, Christoph

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Keller

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Christoph

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Keller, Christoph
Author's Publications: search.filters.isAuthorOfPublication.4be6f95e-6f44-42cf-9d1b-f3e76210ef87

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    Development of a grid-independent GEOS-chem chemical transport model as an atmospheric chemistry module for Earth System Models
    (Copernicus GmbH, 2014) Long, Michael; Yantosca, Robert; Nielsen, J. E.; Keller, Christoph; da Silva, A.; Sulprizio, Melissa; Pawson, S.; Jacob, Daniel
    The GEOS-Chem global chemical transport model (CTM), used by a large atmospheric chemistry research community, has been re-engineered to also serve as an atmospheric chemistry module for Earth System Models (ESMs). This was done using an Earth System Modelling Framework (ESMF) interface that operates independently of the GEOS-Chem scientific code, permitting the exact same GEOS-Chem code to be used as an ESM module or as a stand-alone CTM. In this manner, the continual stream of updates contributed by the CTM user community is automatically passed on to the ESM module, which remains state-of-science and referenced to the latest version of the standard GEOS-Chem CTM. A major step in this re-engineering was to make GEOS-Chem grid-independent, i.e., capable of using any geophysical grid specified at run time. GEOS-Chem data "sockets" were also created for communication between modules and with external ESM code via the ESMF. The grid-independent, ESMF-compatible GEOS-Chem is now the standard version of the GEOS-Chem CTM. It has been implemented as an atmospheric chemistry module into the NASA GEOS-5 ESM. The coupled GEOS-5/GEOS-Chem system was tested for scalability and performance with a tropospheric oxidant-aerosol simulation (120 coupled species, 66 transported tracers) using 48–240 cores and MPI parallelization. Numerical experiments demonstrate that the GEOS-Chem chemistry module scales efficiently for the number of processors tested. Although inclusion of atmospheric chemistry in ESMs is computationally expensive, the excellent scalability of the chemistry module means that the relative cost goes down with increasing number of MPI processes.
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    HEMCO v1.0: a versatile, ESMF-compliant component for calculating emissions in atmospheric models
    (Copernicus GmbH, 2014) Keller, Christoph; Long, Michael; Yantosca, Robert; Da Silva, A. M.; Pawson, S.; Jacob, Daniel
    We describe the Harvard–NASA Emission Component version 1.0 (HEMCO), a stand-alone software component for computing emissions in global atmospheric models. HEMCO determines emissions from different sources, regions, and species on a user-defined grid and can combine, overlay, and update a set of data inventories and scale factors, as specified by the user through the HEMCO configuration file. New emission inventories at any spatial and temporal resolution are readily added to HEMCO and can be accessed by the user without any preprocessing of the data files or modification of the source code. Emissions that depend on dynamic source types and local environmental variables such as wind speed or surface temperature are calculated in separate HEMCO extensions. HEMCO is fully compliant with the Earth System Modeling Framework (ESMF) environment. It is highly portable and can be deployed in a new model environment with only few adjustments at the top-level interface. So far, we have implemented HEMCO in the NASA Goddard Earth Observing System (GEOS-5) Earth system model (ESM) and in the GEOS-Chem chemical transport model (CTM). By providing a widely applicable framework for specifying constituent emissions, HEMCO is designed to ease sensitivity studies and model comparisons, as well as inverse modeling in which emissions are adjusted iteratively. The HEMCO code, extensions, and the full set of emissions data files used in GEOS-Chem are available at http://wiki.geos-chem.org/HEMCO.
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    Ozone and organic nitrates over the eastern United States: Sensitivity to isoprene chemistry
    (Wiley-Blackwell, 2013) Mao, Jingqiu; Paulot, Fabien; Jacob, Daniel; Cohen, Ronald C.; Crounse, John D.; Wennberg, Paul O.; Keller, Christoph; Hudman, Rynda C.; Barkley, Michael P.; Horowitz, Larry W.
    We implement a new isoprene oxidation mechanism in a global 3-D chemical transport model (GEOS-Chem). Model results are evaluated with observations for ozone, isoprene oxidation products, and related species from the International Consortium for Atmospheric Research on Transport and Transformation aircraft campaign over the eastern United States in summer 2004. The model achieves an unbiased simulation of ozone in the boundary layer and the free troposphere, reflecting canceling effects from recent model updates for isoprene chemistry, bromine chemistry, and HO2 loss to aerosols. Simulation of the ozone-CO correlation is improved relative to previous versions of the model, and this is attributed to a lower and reversible yield of isoprene nitrates, increasing the ozone production efficiency per unit of nitrogen oxides (NOx ≡ NO + NO2). The model successfully reproduces the observed concentrations of organic nitrates (∑ANs) and their correlations with HCHO and ozone. ∑ANs in the model is principally composed of secondary isoprene nitrates, including a major contribution from nighttime isoprene oxidation. The correlations of ∑ANs with HCHO and ozone then provide sensitive tests of isoprene chemistry and argue in particular against a fast isomerization channel for isoprene peroxy radicals. ∑ANs can provide an important reservoir for exporting NOx from the U.S. boundary layer. We find that the dependence of surface ozone on isoprene emission is positive throughout the U.S., even if NOx emissions are reduced by a factor of 4. Previous models showed negative dependences that we attribute to erroneous titration of OH by isoprene.