The Sensitivity of Moisture Flux Partitioning in the Cold‐Point Tropopause to External Forcing

• Published in: Geophysical research letters Vol. 50; no. 12
• Main Authors: Kroll, C. A., Fueglistaler, S., Schmidt, H., Kornblueh, L., Timmreck, C.
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 28-06-2023; Wiley
• Subjects: Aerosols; Cold; Crystals; Fluctuations; Fluxes; General circulation models; Geoengineering; Heating; Hydrometeors; Ice crystals; Ice formation; Low temperature; Moisture; moisture budget; Moisture effects; Moisture flux; Parameter sensitivity; Partitioning; Perturbation; Perturbations; Simulation; Storms; Stratosphere; Stratospheric water vapor; Sublimation; Tropical tropopause; Tropopause; Troposphere; Volcanic aerosols; volcano; Water circulation; Water vapor; Water vapour
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2022GL102262

The dryness of the stratosphere is the result of air entering through the cold tropical tropopause layer (TTL). However, our understanding of the moisture flux partitioning into water vapor and frozen hydrometeors is incomplete. This raises concerns regarding the ability of General Circulation Models to accurately predict changes in stratospheric water vapor following perturbations in the radiative budget due to volcanic aerosol or stratospheric geoengineering. We present the first results using a global storm‐resolving model investigating the sensitivity of moisture fluxes within the TTL to an additional heating source. We address the question how the partitioning of moisture fluxes into water vapor and frozen hydrometeors changes under perturbations. The analysis reveals the resilience of the TTL, keeping the flux partitioning constant even at an average cold‐point warming exceeding 8 K. In the control and perturbed simulations, water vapor contributes around 80% of the moisture entering the stratosphere.
Plain Language Summary
The stratosphere is a dry region since moisture entering it from below has to pass the cold‐point, a temperature minimum between troposphere and stratosphere. The low temperatures lead to ice formation and sedimentation of moisture. Frozen moisture within clouds rising above the cold‐point tropopause can pass this temperature barrier and be injected into the stratosphere, where temperatures increase again, promoting the melting and sublimation of ice crystals. However, little is known about the sensitivity of the split of moisture entering the stratosphere into frozen and non‐frozen moisture, especially under external influences, like heating by volcanic aerosol or stratospheric geoengineering efforts. Convective parameterizations in conventional simulations can lead to biases. The emerging km‐scale simulations, which explicitly resolve the physical processes, offer the unique possibility to study moisture fluxes under external forcing while circumventing the downsides of parameterizations. Here, the sensitivity of the moisture flux partitioning into non‐frozen and frozen components to an additional heating source is studied for the first time in global storm‐resolving simulations. The analysis reveals an unaltered flux partitioning even at an average cold‐point warming exceeding 8 K. In the control and perturbed simulations, water vapor contributes around 80% of the moisture entering the stratosphere.
Key Points
Water vapor dominates the stratospheric moisture budget with a contribution of around 80% in global storm‐resolving simulations
The partitioning of stratospheric moisture fluxes into vapor and frozen hydrometeors remains stable under large temperature perturbations