Submitted as: model evaluation paper 10 Feb 2021

Submitted as: model evaluation paper | 10 Feb 2021

Review status: this preprint is currently under review for the journal GMD.

Model intercomparison of COSMO 5.0 and IFS 45r1 at kilometer-scale grid spacing

Christian Zeman1, Nils P. Wedi2, Peter D. Dueben2, Nikolina Ban3, and Christoph Schär1 Christian Zeman et al.
  • 1Institute for Atmospheric and Climate Science, ETH Zurich, Switzerland
  • 2European Centre For Medium-Range Weather Forecasts, Reading, UK
  • 3Department of Atmospheric and Cryospheric Sciences, University of Innsbruck, Austria

Abstract. The increase in computing power and recent model developments allow the use of global kilometer-scale weather and climate models for routine forecasts. At these scales, deep convective processes can be partially resolved explicitly by the model dynamics. Next to horizontal resolution, other aspects such as the applied numerical methods, the use of the hydrostatic approximation, and timestep size are factors that might influence a model's ability of resolving deep convective processes.

In order to improve our understanding of the role of these factors, a model intercomparison between the nonhydrostatic COSMO model and the hydrostatic Integrated Forecast System (IFS) from ECMWF has been conducted. Both models have been run with different spatial and temporal resolutions in order to simulate two summer days over Europe with strong convection. The results are analyzed with focus on vertical wind speed and precipitation.

Results show that even at around 3 km horizontal grid spacing the effect of the hydrostatic approximation seems to be negligible. However, timestep proves to be an important factor for deep convective processes, with a reduced timestep generally allowing for higher updraft velocities and thus more energy in vertical velocity spectra, in particular for smaller wavelengths. A shorter timestep is also causing an earlier onset and peak of the diurnal cycle. Furthermore, the amount of horizontal diffusion plays a crucial role for deep convection with more diffusion generally leading to larger convective cells and higher precipitation intensities. The study also shows that for both models the parameterization of deep convection leads to lower updraft and precipitation intensities and biases in the diurnal cycle with a precipitation peak which is too early.

Christian Zeman et al.

Status: open (extended)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on gmd-2021-31', Anonymous Referee #1, 06 May 2021 reply

Christian Zeman et al.

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Model intercomparison of COSMO and IFS at kilometer-scale grid spacing Zeman, Christian, Wedi, Nils P., Dueben, Peter D., Ban, Nikolina, and Schär, Christoph

Christian Zeman et al.


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Short summary
Kilometer-scale atmospheric models allow us to partially resolve thunderstorms and thus improve their representation. We present an intercomparison between two very distinct atmospheric models for two summer days with heavy thunderstorms over Europe. We show the dependence of precipitation and vertical wind speed on spatial and temporal resolution, and also discuss the possible influence of the used system of equations, numerical methods, and diffusion in the models.