Exploring Pathways to More Accurate Machine Learning Emulation of Atmospheric Radiative Transfer

Machine learning (ML) parameterizations of subgrid physics is a growing research area. A key question is whether traditional ML methods such as feed‐forward neural networks (FNNs) are better suited for representing only specific processes. Radiation schemes are an interesting example, because they compute radiative flows through the atmosphere using well‐established physical equations. The sequential aspect of the problem implies that FNNs may not be well‐suited for it. This study explores whether emulating the entire radiation scheme is more difficult than its components without vertical dependencies. FNNs were trained to replace a shortwave radiation scheme, its gas optics component, and its reflectance‐transmittance computations. In addition, a novel recurrent NN (RNN) method was developed to structurally incorporate the vertical dependence and sequential nature of radiation computations. It is found that a bidirectional RNN with an order of magnitude fewer model parameters than FNN is considerably more accurate, while offering a smaller but still significant 4‐fold speedup over the original code on CPUs, and a larger speedup on GPUs. The RNN predicts fluxes with less than 1% error, and heating rates computed from fluxes have a root‐mean‐square‐error of 0.16 K day −1 in offline tests using a year of global data. Finally, FNNs emulating gas optics are very accurate while being several times faster. As with RNNs emulating radiative transfer, the smaller dimensionality may be crucial for developing models that are general enough to be used as parameterizations.