Huge Ensembles Part II: Properties of a Huge Ensemble of Hindcasts Generated with Spherical Fourier Neural Operators

TL;DR

This paper introduces a massive ensemble of 7,424 hindcasts generated with Spherical Fourier Neural Operators, demonstrating improved sampling of extreme events and better uncertainty quantification for weather forecasting.

Contribution

It presents the creation and analysis of a huge ensemble (HENS) that efficiently samples forecast variability and extremes, with detailed technical requirements and applications for climate and weather prediction.

Findings

HENS samples the tails of the forecast distribution effectively.

HENS improves the skill and coverage of ensemble forecasts.

HENS reduces outlier events and enhances uncertainty quantification.

Abstract

In Part I, we created an ensemble based on Spherical Fourier Neural Operators. As initial condition perturbations, we used bred vectors, and as model perturbations, we used multiple checkpoints trained independently from scratch. Based on diagnostics that assess the ensemble's physical fidelity, our ensemble has comparable performance to operational weather forecasting systems. However, it requires orders of magnitude fewer computational resources. Here in Part II, we generate a huge ensemble (HENS), with 7,424 members initialized each day of summer 2023. We enumerate the technical requirements for running huge ensembles at this scale. HENS precisely samples the tails of the forecast distribution and presents a detailed sampling of internal variability. HENS has two primary applications: (1) as a large dataset with which to study the statistics and drivers of extreme weather and (2) as a weather forecasting system. For extreme climate statistics, HENS samples events 4$\sigma$ away from the ensemble mean. At each grid cell, HENS increases the skill of the most accurate ensemble member and enhances coverage of possible future trajectories. As a weather forecasting model, HENS issues extreme weather forecasts with better uncertainty quantification. It also reduces the probability of outlier events, in which the verification value lies outside the ensemble forecast distribution.