Climate simulations with global storm-resolving models from transient states.

Climate models are an important tool to address challenges we face due to the changing climate. The global warming of the atmosphere and ocean leads to an increase in energy accessible to foster more frequent and intense tropical cyclones, extreme precipitation, and heatwaves, causing increasingly larger economic and non-economic damage. Many of those events are caused or influenced by small-scale convective processes that are not resolved in the typical CMIP-style models with resolutions of about 100 km.

Models capable of resolving deep convection and large turbulent eddies in the atmosphere require horizontal resolutions between 1 km and 10 km. Turbulent processes in the atmosphere play a major role in distributing the energy within the atmosphere, and it has been shown that atmospheric models at resolutions of about 10 km or less significantly improve the resemblance to observations, for instance, regarding the magnitude of maximum wind gusts and the statistics and characteristics of tropical cyclones. Similarly, ocean models require resolutions in the order of 10 km or finer to explicitly resolve mesoscale ocean eddies and their contributions to the transport of salinity and heat and their effects upon the global system. 

Before we can make the transient historical simulations from which future projections are typically initialized with climate models, based on a certain emission scenario, the models need to achieve a climate state that is consistent with the boundary conditions at the initial time. For this, the model needs to be gradually spun up to reach a balanced state. Traditional approaches for model tuning and spinup used for coarse resolution models cannot be applied at very high resolutions. Typical spinup times to reach model equilibrium are around 1000 years, which remains unrealistic to achieve for km-scale models until today. 

This work presents the analysis of alternative cost-efficient spinup protocols and evaluates their associated initial shocks and drifts and how efficiently the coupled model approaches the equilibrium. We also contribute to answering the question of how reliable future projections are when initialized from a transient model state. The analysis is based on a series of ensemble simulations performed with the coupled IFS-NEMO climate model at about 25 km atmospheric and ocean resolution, i.e., Tco399/eORCA025 grids, and on different combinations of ocean-only spinup and coupled spinup lengths. Our analysis focuses on spinup designs that are optimized to initialize climate projections and historical simulations of 50 to 100 years with a minimal initial adjustment and “well-behaved” model trends, contrasting them to the existing multi-decadal km-scale simulations from initiatives like Destination Earth and EERIE.