Lagrangian Simulation of Stratospheric Water Vapour: Impact of Large-Scale Circulation and Small-Scale Transport Processes
2020
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.
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