Other Projects: GPS Water Vapour Estimation

Our knowledge of atmospheric water vapour is still limited, albeit this natural greenhouse gas is very important for the vertical energy balance of the atmosphere and precipitation forecasts. Ground-based GPS reference stations measure precise carrier-phases which can be used to estimate integrated water vapour. Several projects dealing with GPS-based water vapour estimation have been carried out by the Institute so far. One of the current activities in collaboration with the Max-Planck-Institute of Meteorology is the verification of reginal climate model REMO with respect to integrated water vapour.

The project BALTIMOS deals with the "Development and Validation of a Coupled Climatic Model for the Baltic Area" and is a contribution to the BALTEX/BRIDGE program. Our main intention lies in validating the coupled climatic model REMO LARSIM BSMO (REMO BSMO - existing coupled regional model system, LARSIM - hydrological model) developed by Max-Planck-Institute for meteorology in Hamburg (MPIfM – HH) concerning the integrated water vapour content under use of different GNSS techniques. This project considers the promotion guidelines of the DEKLIM (Deutsches Klimaforschungsprogramm - German Climatic Research Program) program in a clear way and is closely linked with the BALTEX/BRIDGE program, which investigates the energy and water cycles of the Baltic region. To achieve the objectives the MPIfM will develop the suggested coupled model of this area. The promotion guideline B of this project comprises: (a) existing, operational, ground-based GPS networks, which cover the target area defined by BALTEX/ BRIDGE, in order to filter time series of the integrated water vapour content from the GPS measurements and thus to validate the coupled climatic model. (b) collection of new relevant GPS data - in particular on moving platforms like ships. Existing gaps in regions with only thin data cover like the Baltic Sea will be closed.

Global satellite navigation systems like the NAVSTAR GPS cannot only be used for navigational functions, but also allow estimation of integrated water vapour content, which plays - with a portion of 62% of the natural greenhouse effect - an important role for the energy balance and precipitation forecast in

weather and climatic models. Applications of GPS technology concentrate on static networks, thus reference stations, which do not change their position. This procedure simplifies the parameter estimation and leads to a high accuracy from 1 to 2 kg/m² of the water vapour content. In such a way derived time series are suitable very well for the verification of the coupled climatic model.

For validating, approximately 60 permanent stations of different GPS networks (IGS, EUREF, SWPOS, etc.) were used. The time series of the integrated water vapour derived for these points show a high measure of agreement.

The spatial distribution of the monthly mean differences in the Baltic area can be computed by application of the method of “Collocation with least squares” and is in the left illustration for 1 month of the year 2001 graphically represented.

(b) Kinematic Water Vapour Estimation

While the GPS-based water vapour estimation in static networks can be called in the meantime as an established and practically operationally usable application, the measurement of this value on kinematical platforms represents a large challenge: On moving carriers the coordinates of the GPS antenna cannot be expected to be known, but must be estimated additionally to the water vapour content at every time of the measurement as new parameters. In spite of complicated evaluation methodology the results in this project show that the water vapour content of the atmosphere can be successfully determined with GPS also on ships.

In June as well as in October/November 2001 we accomplished measurements on the research ship Alkor (picture above) which show a good agreement with the time series derived from different sensors like radiosondes and Numerical Weather Prediction Model (NCEP, NOAA).

On the oil catching ship Bottsand operated by the Federal Navy a precise GPS receiver was durably installed at the end of 2001. The ship through-crosses regularly the Baltic Sea between its home port in Warnemuende and Luebeck/Keel.

The next topic in the project is the validation of the meteorological values like surface temperature and pressure of the climate model. For this reason the data from meteorological stations collected at permanent GPS stations of the IGS network in the BALTEX/BRIDGE area are used.