Hunga Tonga-Hunga Ha’apai Volcano Impact Model Observation Comparison (HTHH-MOC) Project: Experiment Protocol and Model Descriptions

Abstract. The 2022 Hunga volcanic eruption injected a significant amount of water vapor and a moderate amount of sulfur dioxide into the stratosphere causing observable responses in the climate system. We have developed a model-observation comparison project to investigate the evolution of volcanic water and aerosols, and their impacts on atmospheric dynamics, chemistry, and climate, using several state-of-the-art chemistry climate models. The project goals are: 1. Evaluate the current chemistry-climate models to quantify their performance in comparison to observations; and 2. Understand atmospheric responses in the Earth system after this exceptional event and investigate the potential impacts in the projected future. To achieve these goals, we designed specific experiments for direct comparisons to observations, for example from balloons and the Microwave Limb Sounder satellite instrument. Experiment 1 is a free-running ensemble experiment from 2022 to 2031. Experiment 2 is a nudged-run experiment from 2022 to 2023 using observed meteorology. To allow participation of more climate models with varying complexities of aerosol simulation, we include two sets of simulations in Experiment 2: Experiment 2a is designed for models with internally-generated aerosol while Experiment 2b is designed for models using prescribed aerosol surface area density. We take model results from the previously developed Tonga-MIP to fulfill Experiment 3, which focuses on the initial dispersion and microphysical evolution of aerosol and water plumes. Experiment 4 is designed to understand the climate impact on the mesosphere from 2022–2027, for which the experiment design is the same as Experiment 1 but for models that resolve the upper stratosphere and mesosphere.