Twelve times faster yet accurate: a new state‐of‐the‐art in radiation schemes via performance and spectral optimization

Ukkonen, P. ORCID: https://orcid.org/0000-0001-8565-8079 and Hogan, R. J. ORCID: https://orcid.org/0000-0002-3180-5157 (2024) Twelve times faster yet accurate: a new state‐of‐the‐art in radiation schemes via performance and spectral optimization. Journal of Advances in Modeling Earth Systems, 16 (1). e2023MS003932. ISSN 1942-2466 doi: 10.1029/2023ms003932

Abstract/Summary

Radiation schemes are critical components of Earth system models that need to be both efficient and accurate. Despite the use of approximations such as 1D radiative transfer, radiation can account for a large share of the runtime of expensive climate simulations. Here we seek a new state‐of‐the‐art in speed and accuracy by combining code optimization with improved algorithms. To fully benefit from new spectrally reduced gas optics schemes, we restructure code to avoid short vectorized loops where possible by collapsing the spectral and vertical dimensions. Our main focus is the ecRad radiation scheme, where this requires batching of adjacent cloudy layers, trading some simplicity for improved vectorization and instruction‐level parallelism. When combined with common optimization techniques for serial code and porting widely used two‐stream kernels fully to single precision, we find that ecRad with the TripleClouds solver becomes 12 times faster than the operational radiation scheme in ECMWF's Integrated Forecast System (IFS) cycle 47r3, which uses a less accurate gas optics model (RRMTG) and a more noisy solver (McICA). After applying the spectral reduction and extensive optimizations to the more sophisticated SPARTACUS solver, we find that it’s 2.5 times faster than IFS cy47r3 radiation, making cloud 3D radiative effects affordable to compute in large‐scale models. The code optimization itself gave a threefold speedup for both solvers. While SPARTACUS is still under development, preliminary experiments show slightly improved medium‐range forecasts of 2‐m temperature in the tropics, and in year‐long coupled atmosphere‐ocean simulations the 3D effects warm the surface substantially.

Item Type	Article
URI	https://reading-clone.eprints-hosting.org/id/eprint/114859
Item Type	Article
Refereed	Yes
Divisions	Science > School of Mathematical, Physical and Computational Sciences > Department of Meteorology
Uncontrolled Keywords	General Earth and Planetary Sciences, Environmental Chemistry, Global and Planetary Change
Publisher	American Geophysical Union (AGU)
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