SuperdropNet: a Stable and Accurate Machine Learning Proxy for Droplet-based Cloud Microphysics

TL;DR

SuperdropNet is a machine learning emulator that accurately and stably replicates droplet-based cloud microphysics, outperforming previous ML models and matching traditional schemes in many cases, thus enabling more precise climate and weather predictions.

Contribution

We introduce SuperdropNet, a novel ML-based emulator that improves accuracy and stability in simulating droplet-based cloud microphysics using multi-step autoregressive training and physical constraints.

Findings

SuperdropNet predicts hydrometeor states more accurately than previous ML models.

It matches or exceeds the performance of bulk moment schemes in many scenarios.

Multistep autoregressive training enhances model stability and accuracy.

Abstract

Cloud microphysics has important consequences for climate and weather phenomena, and inaccurate representations can limit forecast accuracy. While atmospheric models increasingly resolve storms and clouds, the accuracy of the underlying microphysics remains limited by computationally expedient bulk moment schemes based on simplifying assumptions. Droplet-based Lagrangian schemes are more accurate but are underutilized due to their large computational overhead. Machine learning (ML) based schemes can bridge this gap by learning from vast droplet-based simulation datasets, but have so far struggled to match the accuracy and stability of bulk moment schemes. To address this challenge, we developed SuperdropNet, an ML-based emulator of the Lagrangian superdroplet simulations. To improve accuracy and stability, we employ multi-step autoregressive prediction during training, impose physical constraints, and carefully control stochasticity in the training data. Superdropnet predicted hydrometeor states and cloud-to-rain transition times more accurately than previous ML emulators, and matched or outperformed bulk moment schemes in many cases. We further carried out detailed analyses to reveal how multistep autoregressive training improves performance, and how the performance of SuperdropNet and other microphysical schemes hydrometeors' mass, number and size distribution. Together our results suggest that ML models can effectively emulate cloud microphysics, in a manner consistent with droplet-based simulations.