[D66] Proefschrift Van Aalst, klimaatprofessor KNMI

[Dr. Marc-Alexander Fluks]

[Van de week presenteerde Maarten van Aalst een KNMI-rapport over 
klimaatscenario's in Nederland. Even gekeken naar wie hij is... 
Hieronder bla bla uit diens proefschrift]


Bron:  Universiteit Utrecht
Datum: 20 januari 2005
URL:   https://dspace.library.uu.nl/bitstream/handle/1874/1483/index.htm
        https://dspace.library.uu.nl/bitstream/handle/1874/1483/full.pdf


Dynamics and Transport in the Stratosphere: Simulations with a General
Circulation Mode
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Maarten Krispijn van Aalst
- Institute for Marine and Atmospheric Research Utrecht, Utrecht 
University
   E-mail: maarten.vanaalst at xs4all.nl


Summary

The middle atmosphere is strongly affected by two of the world's most 
important environmental problems: global climate change and 
stratospheric ozone depletion, caused by anthropogenic emissions of 
greenhouse gases and chlorofluorocarbons (CFCs), respectively. General 
circulation models with coupled chemistry are a key tool to advance our 
understanding of the complex interplay between dynamics, chemistry and 
radiation in the middle atmosphere. A key problem of such models is that 
they generate their own meteorology, and thus cannot be used for 
comparisons with instantaneous measurements.

This thesis presents the first application of a simple data assimilation 
method, Newtonian relaxation, to reproduce realistic synoptical 
conditions in a state-of-the-art middle atmosphere general circulation 
model, MA-ECHAM. By nudging the model's meteorology slightly towards 
analyzed observations from a weather forecasting system (ECMWF), we have 
simulated specific atmospheric processes during particular 
meteorological episodes, such as the 1999/2000 Arctic winter.

The nudging technique is intended to interfere as little as possible 
with the model's own dynamics. In fact, we found that we could even 
limit the nudging to the troposphere, leaving the middle atmosphere 
entirely free. In that setup, the model realistically reproduced many 
aspects of the instantaneous meteorology of the middle atmosphere, such 
as the unusually early major warming and breakup of the 2002 Antarctic 
vortex. However, we found that this required careful interpolation of 
the nudging data, and a correct choice of nudging parameters. We 
obtained the best results when we first projected the nudging data onto 
the model's normal modes so that we could filter out the (spurious) fast 
components. In a four-year simulation, for which we also introduced an 
additional nudging of the stratospheric quasi-biennial oscillation, we 
found that the model reproduced much of the interannual variability 
throughout the stratosphere, including the Antarctic temperature minima 
crucial for polar ozone chemistry, but failed to capture the precise 
timing and evolution of Arctic stratospheric warmings. We also 
identified an important model deficiency regarding tracer transport in 
the lower polar stratosphere.

The success of the runs with tropospheric nudging in simulating the 
right stratospheric conditions, including the model capability to 
forecast major stratospheric warming events, bodes well for the model's 
representation of the dynamic coupling between the troposphere and the 
stratosphere, an important element of realistic simulation of the future 
climate of the middle atmosphere (which will partly depend on a changing 
wave forcing from the troposphere). However, for some aspects of 
stratospheric dynamics, such as the quasi-biennial oscillation, a higher 
vertical resolution is required, which might also help to reduce some of 
the transport problems identified in the lower polar vortex.

The nudging technique applied and developed in this thesis offers 
excellent prospects for applications in coupled-chemistry simulations of 
the middle atmosphere, including for the interpretation of instantaneous 
measurements. In particular, it can be used to test and improve the new 
MA-ECHAM5/MESSy/MECCA coupled chemistry climate model system, in 
preparation for more reliable simulations of past and future climates.

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(c) 2005 Universiteit Utrecht