CHRISTOPHE SHÄR – ETH Zürich

Towards Convection-Resolution Climate Modeling

Currently comprehensive efforts are underway to refine the horizontal resolution of global and regional climate models to O(1 km), with the intent to represent convective clouds explicitly rather than using semi-empirical parameterizations. This refinement would move the governing equations closer to first principles and is expected to reduce the uncertainties of climate models. High resolution is particularly attractive in order to better represent critical cloud feedback processes (e.g. related to global climate sensitivity and extratropical summer convection) and extreme events (such as heavy precipitation events, floods, and hurricanes).
The presentation will be illustrated using two sets of decade-long simulations at 2 km horizontal resolution, one covering the European continent on a mesh with 1536x1536x60 grid points, the other covering the Alpine region. To accomplish such simulations, use is made of emerging heterogeneous supercomputing architectures. We use a version of the COSMO limited-area weather and climate model that is able to run entirely on GPUs (rather than CPUs). It is shown that kilometer-scale resolution dramatically improves the simulation of precipitation in terms of the diurnal cycle and short-term extremes. Using the high-resolution methodology, we investigate the temperature-dependent scaling of daily and hourly precipitation extremes, and discuss prospects for the representation of climate feedback processes.
It is argued that already today, modern supercomputers would in principle enable global atmospheric convection-resolving climate simulations, provided appropriately refactored codes were available, and provided solutions were found to cope with the rapidly growing output volume.
Ban N., J. Schmidli and C. Schär, 2015: Heavy precipitation in a changing climate: Does short-term summer precipitation increase faster? Geophys. Res. Lett., 42 (4), 1165–1172 http://dx.doi.org/10.1002/2014GL062588 <http://dx.doi.org/10.1002/2014GL062588>
Leutwyler, D., O. Fuhrer, X. Lapillonne, D. Lüthi, C. Schär, 2016: Towards European-scale convection-resolving climate simulations with GPUs: a study with COSMO 4.19. Geosci. Model Dev., 9, 3393–3412, http://dx.doi.org/10.5194/gmd-9-3393-2016 <http://dx.doi.org/10.5194/gmd-9-3393-2016>
Leutwyler, D., D. Lüthi, N. Ban, O. Fuhrer and C. Schär, 2017: Evaluation of the Convection-Resolving Climate Modeling Approach on Continental Scales. J. Geophys. Res. Atmos., revised