Beyond Closure Models: Learning Chaotic-Systems via Physics-Informed Neural Operators

Accurately predicting the long-term behavior of chaotic systems is crucial for various applications such as climate modeling. However, achieving such predictions typically requires iterative computations over a dense spatiotemporal grid to account for the unstable nature of chaotic systems, which is expensive and impractical in many real-world situations.An alternative approach to such a fully-resolved simulation (FRS) is using a coarse grid and then correcting its errors through a \textit{closure model}, which approximates the overall information from fine scales not captured in the coarse-grid simulation. Recently, ML approaches have been used for closure modeling, but they typically require a large number of expensive FRS training samples. In this work, we prove an even more fundamental limitation, i.e., the standard approach to learning closure models suffers from a large approximation error no matter how expressive the model is, and it stems from the non-uniqueness of the mapping. We also prove that other existing methods leveraging history information and randomness can neither resolve this limitation.We propose an alternative end-to-end learning approach using a physics-informed neural operator (PINO) that overcomes this limitation by not using a closure model or a coarse-grid solver. We first train the PINO model on data from a coarse-grid solver and then fine-tune it with (a small amount of) FRS and physics-based losses on a fine grid. The discretization-free nature of neural operators means that they do not suffer from the restriction of a coarse grid that closure models face, and they can provably approximate the long-term statistics of chaotic systems. In our experiments, our PINO model achieves a 120x speedup compared to FRS with a relative error ~5%. In contrast, the closure model coupled with a coarse-grid solver is $58$x slower than PINO while having a higher error ~205% when trained on the same FRS dataset.