Can MIPAS HDO observations be used to determine the role of convection for moistening the stratosphere?
Joerg Steinwagner(1), Thomas Roeckmann(1), Gabriele Stiller(2), Thomas von Clarmann(2) and Stephan Fueglistaler(3)
(1) University Utrecht, Princetonplein 5, 3584 CC Utrecht, Netherlands
(2) Forschungszentrum Karlsruhe, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany
(3) University Cambridge, , Cambridge, United Kingdom
Water is the most important trace species in the atmosphere. It plays a crucial role in the upper
troposphere/lower stratosphere (UT/LS) region both for the radiative budget of the stratosphere and
its chemistry. However, still a lot of details about the mechanisms that transport water from the UT
to the LS are unrevealed. Also the pathways of water in the TTL remain not fully understood. One reason
that lead to this situation is that we lacked continous and reliable global observations
of suitable tracers.
In recent years high precision satellite based remote sensing isotope data has become available and
matured to enhance available in situ data. Together with modelling approaches these data constitute
a new tool for the examination of atmospheric processes on a global scale. We have used vertical profiles
of HDO inferred from spectral IR measurements, recorded by the Michelson Interferometer for Passive
Atmospheric Sounding (MIPAS) between September 2002 and March 2004, to study transport processes of
water in the tropical UT/LS. Physical processes like phase changes leave a fingerprint in the isotopic
composition of water. Thus, the analysis of naturally occuring isotope abundance has the potential to
reveal the processes water has undergone along its atmospheric transport.
A clear annual cycle in the isotopic composition has been identified at the tropopause for the first
time in this clarity. This signal propagates upward into the stratosphere similar to the well-known
water tape recorder and is mainly in pahse with the respective H2O signal. Quantitative evaluation
of the relative HDO and H2O variations reveals that the seasonality is close to, but not exactly equal
to the one expected from Rayleigh fractionation. In our study we examine the question whether
convection can be the process relevant for this deviation.