Lagrangian Simulation of StratosphericWater Vapour: Impact of Large-Scale Circulation and Small-Scale Transport Processes

The atmospheric global circulation, also referred to as the Brewer-Dobson circulation, controls the composition of the upper troposphere and lower stratosphere (UTLS). The UTLS trace gas composition, in turn, crucially affects climate. In particular, UTLS water vapour (H2O) plays a significant role in the global radiation budget. Therefore, a realistic representation of H2O and Brewer-Dobson circulation, is critical for accurate model predictions of future climate and circulation changes. This thesis is structured in two main parts: focussing on the (i) effect of model uncertainties (due to tropical tropopause temperature, horizontal transport and small-scale mixing) on stratospheric H2O, and on the (ii) uncertainties in estimating Brewer-Dobson circulation trends from the observed H2O trends. The results presented here are based largely on stratospheric H2O studies with the Chemical Lagrangian Model of the Stratosphere (CLaMS). Firstly, to investigate the robustness of simulated H2O with respect to different meteorological datasets, we examine CLaMS driven by the ERA-Interim reanalysis from the European Centre of Medium-RangeWeather Forecasts, and the Japanese 55-year Reanalysis (JRA-55). Secondly, to assess the effects of horizontal transport, we carry out CLaMS simulations, with transport barriers, along latitude circles: at the equator, at 15° N/S and at 35° N/S. To investigate the sensitivity of simulated H2O regarding small-scale atmospheric mixing, we vary the strength of parametrized small-scale mixing in CLaMS. Finally, to assess the reliability of estimated long-term Brewer-Dobson circulation changes from stratospheric H2O, we apply different methods of calculating mean age of air trends involving two approximations: instantaneous entry mixing ratio propagation, and a constant correlation between mean age of air and the fractional release factor of methane. The latter assumption essentially means assuming a constant correlation between the mean age of air and the mixing ratio of long-lived trace gases. The results of this thesis show significant differences in simulated stratospheric H2O (about 0.5 ppmv) due to uncertainties in the tropical tropopause temperatures between the two reanalysis datasets, JRA-55 and ERA-Interim. The JRA-55 based simulation is significantly moister, when compared to ERA-Interim, due to a warmer tropical tropopause of approximately 2 K. Moreover, through introducing artificial transport barriers in CLaMS, we suppress certain horizontal transport pathways. These transport experiments demonstrate that the Northern Hemisphere subtropics have a strong moistening effect on global stratospheric H2O. Interhemispheric exchange shows only a very weak effect on stratospheric H2O. Small-scale mixing mainly increases troposphere-stratosphere exchange, causing an enhancement of stratospheric H2O, particularly, along the subtropical jets in the summer hemisphere and in the Northern hemispheric monsoon regions. In particular, the Asian and American monsoon systems, during boreal summer, turn out as regions especially sensitive to changes in small-scale mixing. The estimated mean age of air trends from stratospheric H2O changes, in general, are strongly determined by the assumed approximations. Depending on the investigated region of the stratosphere, and the considered period, the error of estimated mean age of air trends can be large. Interestingly, depending on the period, the effects from both approximations can also be opposite, and may even cancel out. The results of this thesis provide new insights into the leading processes that control stratospheric H2O and its trends, and are therefore relevant for improving climate model predictions. Furthermore, the results of this work can be used for evaluating the uncertainties of estimated stratospheric circulation changes from global satellite measurements

Impact of groundwater representation on heat events in regional climate simulations over Europe

The representation of groundwater is simplified in most regional climate models (RCMs), potentially leading to biases in the simulations. This study introduces a unique dataset from the regional Terrestrial Systems Modelling Platform (TSMP) driven by the Max Planck Institute Earth System Model at Low Resolution (MPI-ESM-LR) boundary conditions in the context of dynamical downscaling of global climate models (GCMs) for climate change studies. TSMP explicitly simulates full 3D soil and groundwater dynamics together with overland flow, including complete water and energy cycles from the bedrock to the top of the atmosphere. By comparing the statistics of heat events, i.e., a series of consecutive days with a near-surface temperature exceeding the 90th percentile of the reference period, from TSMP and those from GCM–RCM simulations with simplified groundwater dynamics from the COordinated Regional Climate Downscaling EXperiment (CORDEX) for the European domain, we aim to improve the understanding of how groundwater representation affects heat events in Europe. The analysis was carried out using RCM outputs for the summer seasons of 1976–2005 relative to the reference period of 1961–1990. While our results show that TSMP simulates heat events consistently with the CORDEX ensemble, there are some systematic differences that we attribute to the more realistic representation of groundwater in TSMP. Compared to the CORDEX ensemble, TSMP simulates fewer hot days (i.e., days with a near-surface temperature exceeding the 90th percentile of the reference period) and lower interannual variability and decadal change in the number of hot days on average over Europe. TSMP systematically simulates fewer heat waves (i.e., heat events lasting 6 d or more) compared to the CORDEX ensemble; moreover, they are shorter and less intense. The Iberian Peninsula is particularly sensitive with respect to groundwater. Therefore, incorporating an explicit 3D groundwater representation in RCMs may be a key in reducing biases in simulated duration, intensity, and frequency of heat waves in Europe. The results highlight the importance of hydrological processes for the long-term regional climate simulations and provide indications of possible potential implications for climate change projections.</p

How can Brewer–Dobson circulation trends be estimated from changes in stratospheric water vapour and methane?

The stratospheric meridional overturning circulation, also referred to as the Brewer–Dobson circulation (BDC), controls the composition of the stratosphere, which, in turn, affects radiation and climate. As the BDC cannot be directly measured, one has to infer its strength and trends indirectly. For instance, trace gas measurements allow the calculation of average transit times. Satellite measurements provide information on the distributions of trace gases for the entire stratosphere, with measurements of particularly long temporal and dense spatial coverage available for stratospheric water vapour (H2O). Although chemical processes and boundary conditions confound interpretation, the influence of methane (CH4) oxidation on H2O in the stratosphere is relatively straightforward, and thus H2O is an appealing tracer for transport analysis despite these caveats. In this work, we explore how mean age of air trends can be estimated from the combination of stratospheric H2O and CH4 data, by carrying out a proof of concept within the model environment of the Chemical Lagrangian Model of the Stratosphere (CLaMS). In particular, we assess the methodological uncertainties related to the two commonly used approximations of (i) instantaneous stratospheric entry mixing ratio propagation and (ii) constant correlation between mean age and the fractional release factor of CH4. Performing various sensitivity studies with CLaMS, we test different methods of the mean age of air trend estimation, and we aim to provide simple and practical advice on the adjustment of the used approximations for obtaining more reliable mean age of air trends from the measurements of H2O and CH4. Our results show that the estimated mean age of air trends from the combination of stratospheric H2O and CH4 changes may be significantly affected by the assumed approximations. Depending on the investigated stratospheric region and the considered period, the error in estimated mean age of air trends can be large, especially in the lower stratosphere. For particular periods, the errors from the two approximations can lead to opposite effects, which may even cancel out. Finally, for a more reliable estimate of the mean age of air trends, we propose adjusting the approximation method by using an idealized age spectrum to propagate stratospheric entry mixing ratios. The findings of this work can be used for assessing the uncertainties in stratospheric BDC trend estimation from global satellite measurements