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Abstract:
Moisture enters the tropical tropopause layer (TTL) and lower stratosphere via different pathways; be it as large-scale slowly ascending water vapor or by way of small-scale processes such as convective overshoots, in-cloud upwelling, or turbulent mixing. The partitioning between these different pathways is highly influenced by theTTL’s character as a transition region between radiative-dynamically and convective-radiative-dynamically controlled atmospheric regions. Volcanic eruptions or geoengi-neering interventions based on stratospheric aerosol injection have the potential todisturb the heating balance by injecting sulfur compounds into the tropopause layerand stratosphere. The consecutively formed sulfate aerosol interacts with terrestrial and solar near infrared radiation, heating up its surroundings. In this work the impact of the corresponding temperature perturbation on the stratospheric water vaporbudget in the tropics is investigated.As sulfate aerosol often interferes with water vapor measurements and the last strong eruption occurred in a time with less observational data coverage in general, very little data on the changes of stratospheric water vapor (SWV) after volcanic eruptions exists. My first study therefore focuses on a direct quantification of changes in SWV based on a simulation set of 100 ensemble members for volcanic eruption magnitudes ranging from 2.5 Tg S to 40 Tg S. The huge number of different realizations allows for a deduction of those parameters which are important in determining the impact of the radiative forcing on the SWV levels. A comparison between the 100 ensemble member simulations for the Mt. Pinatubo eruption and 5 Tg S to 10 Tg S eruptions scenarios shows that the profile shape and height with respect to the tropopause are of crucial importance. This finding highlights the pitfalls of prescribing aerosol at fixed height levels in models differing with respect to their tropopause height. The study also identifies 10 Tg S as the volcanic eruption strength at which the volcanic signal dominates over internal variability of the SWV. At this eruption magnitude the SWV forcing counteracts up to 4 % of the aerosol forcing in the tropics. The dependence of important variables, such as the cold-point temperature and the radiative forcing of SWV, on aerosol optical depth values is determined via curve fitting. In the second study the role of small-scale processes transporting frozen moisture into the stratosphere is investigated. Here I present the first budget analysis based on global convection resolving simulations for SWV changes after perturbations by sulfate aerosol in the TTL. A moisture flux analysis relying on online budget calculations reveals an exceptionally robust 80:20 partitioning of water vapor and frozen hydrometeor fluxes into the stratosphere - even under the substantial perturbations the aerosol heating entails, such as a 9 K warming of the cold-point temperatures and a downward shift of the tropopause by 1 km. The constant flux partitioning suggests that the Clausius Clapeyron scaling for the water vapor fluxes can be extended to the frozen fluxes as well. This greatly simplifies estimates of changes in moisture fluxes: The frozen moisture flux is easily determined by the partitioning ratio and the slope of the Clausius Clapeyron equation at the lowest cold-point temperatures
