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Machine Learning the Warm Rain Process
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  • Andrew Gettelman,
  • David John Gagne,
  • Chih-Chieh Chen,
  • Matthew Christensen,
  • Zachary Lebo,
  • Hugh Morrison,
  • Gabrielle Gantos
Andrew Gettelman
National Center for Atmospheric Research (UCAR)

Corresponding Author:andrew@ucar.edu

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David John Gagne
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Chih-Chieh Chen
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Matthew Christensen
University of Oxford
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Zachary Lebo
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Hugh Morrison
National Center for Atmospheric Research (UCAR)
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Gabrielle Gantos
National Center for Atmospheric Research
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Clouds are one of the most critical yet uncertain aspects of weather and climate prediction. The complex nature of sub-grid scale cloud processes makes traceable simulation of clouds across scales difficult (or impossible). Often models and measurements are used to develop empirical relationships for large-scale models to be computationally efficient. Machine learning provides another potential tool to improve our empirical parameterizations of clouds. To explore these opportunities, we replace the warm rain formation process in a General Circulation Model (GCM) with a detailed treatment from a bin microphysical model that causes a 400\% slowdown in the GCM. We analyze the changes in climate that result from the use of the bin microphysical calculation and find improvements in the rain onset and frequency of light rain compared to detailed models and observations. We also find a resulting change in the cloud feedback response of the model to warming, which will significantly impact the climate sensitivity. We then emulate this process with an emulator consisting of multiple neural networks that predict whether specific tendencies will be nonzero and the magnitude of the nonzero tendencies. We describe the risks of over-fitting, extrapolation, and linearization of a non-linear problem by using perfect model experiments with and without the emulator and show we can recover the solutions with the emulators in almost all respects, and recover nearly all the speed to get simulations that perform as the detailed model, but with the computational cost of the control simulation.
Feb 2021Published in Journal of Advances in Modeling Earth Systems volume 13 issue 2. 10.1029/2020MS002268