Fourth Study Conference on BALTEX Scala Cinema Gudhjem

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GPS-Based Integrated Water Vapour Estimation on Static and Moving
Platforms for Verification of Regional Climate Model REMO
Torben Schüler 1 , Andrea Posfay 1 , Eva Krueger 1 , Günter W. Hein 1 and Daniela Jacob 2
1 University FAF Munich, Institute of Geodesy and Navigation, D-85579 Neubiberg, torben.schueler@unibw-muenchen.de
2 Max-Planck-Institute for Meteorology, D-20146 Hamburg, jacob@dkrz.de
1. Introduction
Knowledge of the distribution of water vapour is essential
for our understanding of global and regional climate as this
greenhouse gas is associated with a large latent energy
influencing the vertical stability of the atmosphere
significantly. As a consequence, water vapour measurements
with sufficient temporal and spatial resolution are needed for
the validation of climate models such as the coupled climate
model REMO-LARSIM-BSMO developed for the region of
the Baltic Sea and its catchment.
Figure 1. Results presented within the scope of this paper
were carried out as part of the BALTIMOS consortium and
funded by the German climate research programme
DEKLIM.
Precise carrier-phase measurements of GPS receivers allow
to estimate tropospheric delays which can be easily
converted into integrated water vapour (IWV) contents. This
method has proven to be of great value when static, i.e. nonmoving
GPS receivers on the ground are used, see for
instance Schueler (2001).
On the other hand, very little efforts have been devoted to
apply existing GPS technology to sense integrated water
vapour contents on moving platforms like ships allowing to
obtain measurements in areas which usually suffer from
sparse data coverage. Experiments on board of two vessels
cruising the Baltic Sea were conducted within the scope of
this work representing a new and innovative approach.
This presentation will outline both some verification results
collected using static reference networks in the Baltic region
as well as results obtained with the kinematic processing
technique on the two ships used for this study.
2. Static Water Vapour Estimation
GPS-based water vapour estimation has meanwhile become
a routinely used technique and, consequently, it is applied
to the verification and validation of the REMO climate
model.
GPS data from the IGS as well as the EUREF networks were
processed by using the modified software package TropAC
of the Institute of Geodesy and Navigation (University FAF
Munich) in order to determine the tropospheric propagation
delay in zenith direction (ZTD). For a total of up to 63
stations these ZTD data were converted into IWV via a
separation of the ZTD into the hydrostatic and the wet
component for the validation period 1999-2002 with
further data tracing back to 1993. The surface pressure at
the site and the mean temperature of the troposphere are
needed for the conversion process. These data were
extracted from numerical weather models (NWM).
Additionally radiosonde data of 6 sites were used for the
validation process. Surface meteorological data of selected
IGS stations serve for the validation of REMO pressure
and temperature data.
The technique and the error budget of static GPS water
vapour estimation will be briefly discussed in the
presentation as well as some results of the validation
activities.
3. Kinematic Water Vapour Estimation
The aspects of water vapour estimation will, however,
receive more attention in this presentation since this
application is essentially new and still in the process of
development and thus represents an innovative step
forward in science.
GPS-receivers of type Trimble 4000 SSE were installed on
two vessels for collection of measurements. The first one
on research vessel Alkor of the Institute of Marine
Research (University Kiel) in cooperation with the
Institute of Meteorology (University Hamburg). Two
campaigns within the BRIDGE phase (June 2001 and
Oct./Nov. 2001) were conducted. These field experiments
are particularly valuable since high resolution radiosonde
data were collected by the research crew allowing to
precisely derive the IWV content in order to assess the
quality of the GPS-derived water vapour samples.
Figure 2. Sample ground track of oil recovery vessel
“Bottsand” in the Baltic Sea on 18 August 2002.
Furthermore, a GPS-receiver was operated on the oil
recovery vessel Bottsand from October 2001 till January
2003. This ship is operated by the German Navy and
regularly cruises the Baltic Sea from Warnemuende to
Luebeck and Kiel. It therefore served as a kind of
operational kinematic platform for GPS water vapour
estimation over sea during the BRIDGE period. Figure 2