How the Hunga Tonga - Hunga Ha’apai water vapor cloud impacts its transport through the stratosphere: Dynamical and radiative effects

The eruption of the Hunga Tonga - Hunga Ha'apai (HTHH) volcano on January 15, 2022 changed the water vapor content of the stratosphere. The eruption injected about 150 Tg of water vapor (H₂O), roughly 10% of the background stratospheric H₂O content, to altitudes above 50 km. Observations with the Aura Microwave Limb Sounder (MLS) detected the transport and distribution of the H2O cloud after the eruption. This provided a great opportunity to compare the simulated transport of the H₂O cloud in the ICON-Seamless model with the MLS observation to see the performance of the stratospheric dynamics in this newly developed model. ICON-Seamless simulations were performed with NWP physics in a low horizontal resolution of about 160 km. A new vertical grid with 130 levels and a maximum grid size of 500 m in the stratosphere allowed the simulation of an internally generated QBO.

The simulated spatial evolution of the H₂O cloud is very close to the MLS observations. In both, model and observation, the vertical transport of the H₂O cloud had three phases: an initial subsidence phase, a stable phase, and a rising phase. Radiative cooling of H₂O clearly affects the transport of the H₂O cloud, as demonstrated with passive tracers. It is the main driver within the subsidence phase. The radiative cooling also counteracts the large-scale rising motion in the tropics, leading to the stable phase, and modulates the large-scale transport of the H₂O cloud for about nine months. This holds for different QBO phases, where the H₂O cloud differs mainly in its vertical extent.