The Impact of Volcanic Aerosol Heating on Stratospheric Water Vapor and Convection

Event Type: Hybrid (Zoom Access)

Speaker: Clarissa Kroll, Max Planck Institute for Meteorology

Abstract:

Volcanic aerosol heating perturbs the tropical tropopause layer (TTL) and with it the stratospheric water budget. Whereas the effect of increased cold point temperatures on the water slowly ascending into the TTL can be studied using general circulation models (GCMs), the robustness of changes in overshooting convection after volcanic eruptions in these models is unclear as the TTL is tuned to unperturbed conditions and the simulations highly rely on parametrizations. Estimating the changes in the contribution of large-scale slowly ascending water vapour and small-scale overshoots or turbulent mixing to the stratospheric water vapour budget after a volcanic eruption or in a geoengineering study remains a challenge in GCM simulations. The emerging storm-resolving simulations, however, offer the unprecedented possibility to gain insight into the sensitivity of a TTL, which is not strongly constrained by parameterized convection, to external forcings. They allow in particular the study of potential changes of convection, where two processes counteract each other after volcanic eruptions: the downwards shift of the lapse rate tropopause favoring overshooting convection, and the increased stability in the TTL region suppressing overshooting convection. These effects are analyzed employing convection-resolving simulations for the atmosphere with the Icosahedral Nonhydrostatic Weather and Climate Model (ICON-A) at 10 km horizontal resolution in two scenarios: a control run and a volcanically perturbed run. The perturbed run has an aerosol layer in the lower stratosphere corresponding to the peak loading of an injection of 20 Tg sulfur. It is found that the level of neutral buoyancy, based on the temperature difference in convective areas and their surroundings, moves downwards less than the lapse rate tropopause in the volcanically perturbed simulations. This allows – contrary to previous assumptions – for more overshoots into the region above the tropical lapse rate tropopause revealing that even under strong perturbations the partitioning of moisture fluxes into large-scale and small-scale processes remains constant.