Assessment of Climate Biases in OpenIFS Version 43R3 across Model Horizontal Resolutions and Time Steps
Abstract. We examine the impact of horizontal resolution and model time step on climate of the OpenIFS version 43R3 atmosphere 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 minutes) and using different time steps (i.e., 60, 30 and 15 minutes) at 100 km horizontal resolution. We find that the surface zonal wind bias reduces significantly over certain regions such as the Southern Ocean, the Northern Hemisphere mid-latitudes, and in tropical and subtropical regions at 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 minutes). We also find improvements in Rossby wave amplitude and phase speed as well as 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 resolution or smaller time step, 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 OpenIFS model, one can alleviate some climate biases at a lower cost than by increasing the horizontal resolution.
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