Assessment of Climate Biases in OpenIFS Version 43R3 across Model Horizontal Resolutions and Time Steps

We examine the impact of horizontal resolution and model time step on the climate of the OpenIFS version 43r3 atmospheric general circulation model. A series of simulations for the period 1979–2019 are conducted with various horizontal resolutions (i.e. ∼100, ∼50, and ∼25 km) while maintaining the s...

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Bibliographic Details
Published in:Geoscientific Model Development
Main Authors: Savita, Abhishek, Kjellsson, Joakim, Kedzierski, Robin P., Latif, Mojib, Rahm, Tabea, Wahl, Sebastian, Park, Wonsun
Format: Article in Journal/Newspaper
Language:English
Published: Copernicus Publications (EGU) 2024
Subjects:
Online Access:https://oceanrep.geomar.de/id/eprint/59011/
https://oceanrep.geomar.de/id/eprint/59011/7/gmd-17-1813-2024.pdf
https://gmd.copernicus.org/articles/17/1813/2024/gmd-17-1813-2024.html
https://doi.org/10.5194/gmd-17-1813-2024
Description
Summary:We examine the impact of horizontal resolution and model time step on the climate of the OpenIFS version 43r3 atmospheric general circulation model. A series of simulations for the period 1979–2019 are conducted with various horizontal resolutions (i.e. ∼100, ∼50, and ∼25 km) while maintaining the same time step (i.e. 15 min) and using different time steps (i.e. 60, 30, and 15 min) at 100 km horizontal resolution. We find that the surface zonal wind bias is significantly reduced over certain regions such as the Southern Ocean and the Northern Hemisphere mid-latitudes and in tropical and subtropical regions at a high horizontal resolution (i.e. ∼25 km). Similar improvement is evident too when using a coarse-resolution model (∼100 km) with a smaller time step (i.e. 30 and 15 min). We also find improvements in Rossby wave amplitude and phase speed, as well as in weather regime patterns, when a smaller time step or higher horizontal resolution is used. The improvement in the wind bias when using the shorter time step is mostly due to an increase in shallow and mid-level convection that enhances vertical mixing in the lower troposphere. The enhanced mixing allows frictional effects to influence a deeper layer and reduces wind and wind speed throughout the troposphere. However, precipitation biases generally increase with higher horizontal resolutions or smaller time steps, whereas the surface air temperature bias exhibits a small improvement over North America and the eastern Eurasian continent. We argue that the bias improvement in the highest-horizontal-resolution (i.e. ∼25 km) configuration benefits from a combination of both the enhanced horizontal resolution and the shorter time step. In summary, we demonstrate that, by reducing the time step in the coarse-resolution (∼100 km) OpenIFS model, one can alleviate some climate biases at a lower cost than by increasing the horizontal resolution.