On the limitations of data-driven weather forecasting models

As in many other areas of engineering and applied science, Machine Learning (ML) is having a profound impact in the domain of Weather and Climate Prediction. A very recent development in this area has been the emergence of fully data-driven ML prediction models which routinely claim superior performance to that of traditional physics-based models. In this work, we examine some aspects of the forecasts produced by an exemplar of the current generation of ML models, Pangu-Weather, with a focus on the fidelity and physical consistency of those forecasts and how these characteristics relate to perceived forecast performance. The main conclusion is that Pangu-Weather forecasts, and by extension those of similar ML models, do not have the fidelity and physical consistency of physics-based models and their advantage in accuracy on traditional deterministic metrics of forecast skill can be attributed, to a large extent, to these peculiarities. Similarly to other current post-processing technologies, ML models appear to be able to add value to standard NWP outputs for specific forecast applications and combined with their extremely low computational cost during deployment, will likely provide an additional, useful source of forecast information.