Evaluating the prediction of subgrid eddy kinetic energy from a machine learning model

Abstract

Global climate model resolution is still too coarse to fully resolve mesoscale processes, therefore subgrid closures are required to approximate the feedback of eddies onto the resolved flow. With high-resolution datasets now readily available, there has been a rapid growth in the use of machine learning to develop new eddy closures that could help reduce model biases and uncertainty. In this work we employ an offline learning approach with a convolutional neural network to predict subgrid eddy kinetic energy ($KE_e$), laying the initial groundwork for a machine-learned scale-aware GEOMETRIC eddy closure for the UK's Earth System Model. We address the question of whether global parameterizations are needed or if regional closures are sufficient by evaluating a number of neural network models through region and feature ablation experiments. ML parameterization models generalize to unseen regions when their training data share similar feature and target distributions with those regions. Models that fail to generalize incur distributional shift, or employ different filtering scales for the true eddy kinetic energy field. When holding out features, there is improvement among the generalizable ML models, with some feature spaces leading to improvements in some regions but degrading performance in others. Generally, power spectral density of $KE_e$ tend to diverge at small scales, though some feature spaces are able to minimize this divergence. Overall, the best performing model trained on all regions achieves time-mean and anomaly R2 scores of R2$\approx 0.95$ and R2$\approx 0.8 - 0.9$, respectively.