E-GVAP
 Input to the EUMENET glossy magasine 

E-GVAP
EUMETNET GPS Water Vapour programme
Responsible member: DMI. Programme manager Henrik Vedel.

Water vapour is a key constituent of the atmosphere. About half of the energy driving atmospheric dynamics is brought there by water vapour. It is the dominant greenhouse gas. And it is obviously closely related to precipitation, the forecast of which is a main product in meteorology. Continuous access to observations water vapour are of great importance, yet measurements are .

E-GVAP was started April 2005 to set up a new system for near real time (NRT) water vapour observations for use in operational meteorology. The system utilises data from ground based Global Positioning System (GPS) receivers, most of which were installed for completely different purposes, e.g. land surveying and tectonic motion studies. A close collaboration between geodesists and meteorologists is required. Luckily exchange of data and sharing of facilities turn out to be beneficial to both sides. From the GPS observations zenith total delays (ZTD´s) can be derived. To geodesists it is a noise term that they remove, but to us it is an atmospheric observation of importance, since the ZTD is closely related to the integrated water vapour (IWV) above the observing site. Compared to other measurements of water vapour the ZTD has high time and spatial resolution and poor vertical resolution.

ZTD can be utilised via assimilation into the numerical weather prediction (NWP) models used to predict the weather. A positive impact has been demonstrated in recent years. Now E-GVAP ZTD´s have started to become used operationally, both for assimilation and verification. The GPS ZTD data are made available via ftp and GTS. Alternatively ZTD may be converted to IWV. Maps of the evolution of the field of IWV can be used to assist in now-casting, when combined with other observations and NWP data, work on this has begun in E-GVAP. The time sequence of IWV evolution over 24 h is available at the project homepage.

The NRT GPS ZTD observing system is gradually being extended. Recently with new GPS sites from the Netherlands and Iceland. Now from Northern Ireland, and from Scandinavia where a centre for processing of all Nordic data is being set up in Sweden. E-GVAP expert teams are guiding on how to best process the GPS data and how to get the most of them in operational meteorology. In 2007 a central issue will be the development of feedback mechanisms that enable automatic detection and flagging of poor data as well as corrective actions, resulting in higher quality and homogeneity of the GPS derived produtcts.

Visit the E-GVAP homepage, http://egvap.dmi.dk, for further information.

The NRT GPS ZTD observing system performance is continuously monitored and results made available at the project homepage. As an example the figure shows a graphical comparison of ZTD from different processing centres and NWP data for a single GPS site.

Integrated water vapour field. Visible as 24 h movie via “products” on E-GVAP homepage. Useful in now-casting. Purple squares indicate lightning.