The ARP-GEM1 Global Atmosphere Model: Description, Speedup Analysis, and Multiscale Evaluation up to 6 km
Geoffroy, Olivier ; Saint-Martin, David
This article provides a complete description of the global, efficient, and multiscale version of ARPEGE (ARP-GEM) global atmosphere model version 1 and its evaluation across simulations from 55- to 6-km resolution. ARP-GEM1 is approximately 15 times faster than its base model, ARPEGE-Climat v6.3, while maintaining accurate simulations. We first present the model's new physics and acceleration factors aimed at improving computational efficiency. The significant speedup results from a combination of factors, rather than a single one, with each factor's contribution quantified. Additionally, a detailed decomposition of the model's speedup per component is presented. While some developments are specific to ARP-GEM, the results presented provide insights into various possibilities for efficiency-oriented improvements of other climate models. Simplifying and optimizing the model reduce computational costs, enhance transparency and portability, and maximize the benefits of future advanced computing technologies. ARP-GEM1 is then evaluated through a set of 30-yr prescribed sea surface temperature simulations at 55-, 25-, 12.6-, and 6.3-km resolutions, with minimal tuning between resolutions. The model demonstrates reliability in representing global climate metrics, comparable to the top-performing CMIP6 models, while maintaining high computational efficiency at all resolutions. These simulations further demonstrate the feasibility of O(10)-km climate simulations and highlight their added value, particularly for capturing phenomena such as tropical cyclones. Ultimately, these exploratory simulations should be considered an intermediate step toward the development and tuning of even higher-resolution, convection-permitting kilometer-scale configurations. The results presented here suggest that kilometer-scale global climate simulations should become feasible in the near future.</p>
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