Investigation of stratospheric water vapour by means of the simulation of water isotopologues

This modelling study aims to gain an improved understanding of the
processes that determine the water vapour budget in the
stratosphere by means of the investigation of water isotope ratios.
At first, a separate hydrological cycle has been introduced into
the chemistry-climate model EMAC, including the water isotopologues
HDO and H218O and their physical fractionation processes.
Additionally, an explicit computation of the contribution of
methane oxidation to HDO has been incorporated. EMAC simulates
explicit stratospheric dynamics and a highly resolved tropical
tropopause layer. These model expansions, now allow detailed
analyses of water vapour and its isotope ratio with respect to
deuterium (deltaD(H2O)), throughout the stratosphere and in the
transition region to the troposphere. In order to assure the
correct representation of the water isotopologues in the model's
hydrological cycle, the expanded system has been evaluated in
several steps. The physical fractionation effects have been
evaluated by comparison of the simulated isotopic composition of
precipitation with measurements from a ground-based network (GNIP)
and with the results from an isotopologue-enabled ECHAM5 general
circulation model version. The model's representation of the
chemical HDO precursor CH3D in the stratosphere has been confirmed
by a comparison with chemical transport models (CHEM1D, CHEM2D) and
measurements from radiosonde flights. Finally, the simulated HDO
and deltaD(H2O) have been evaluated in the stratosphere, with
respect to retrievals from three different satellite instruments
(MIPAS, ACE-FTS, SMR). Discrepancies in stratospheric deltaD(H2O)
between two of the three satellite retrievals can now partly be
explained. The simulated seasonal cycle of tropical deltaD(H2O) in
the stratosphere exhibits a weak tape recorder signal, which fades
out at altitudes around 25 km. This result ranges between the
pronounced tape recorder signal in the MIPAS observations and the
missing upward propagation of the seasonal variations in the
ACE-FTS retrieval. Revisions of different insufficencies in the
respective satellite measurements, however, are expected to alter
both observational datasets towards the results of the EMAC model.
Extensive analyses of the water isotope ratios have revealed the
driving mechanisms of the stratospheric deltaD(H2O) tape recorder
signal in the EMAC simulation. A sensitivity study without the
impact of methane oxidation on deltaD(H2O) demonstrates the damping
effect of this chemical process on the tape recorder signal. An
investigation of the origin of the enhanced deltaD(H2O) in the
lower stratosphere during boreal summer, shows isotopically
enriched water vapour, crossing the tropopause over the subtropical
Western Pacic. A correlation analysis confirms this link, and thus
the Asian Summer Monsoon could be identified to be the major
contributing process for the stratospheric deltaD(H2O) tape
recorder. This finding contradicts an analysis of ACE-FTS satellite
data, which assigns the lower stratospheric deltaD(H2O) increase
during boreal summer to the North American Monsoon. A possible
explanation for this discrepancy has been found to be an
underrepresentation of convective ice overshooting in the applied
convection scheme.