# Overview and requirements for the COST-716 near real time demonstration project for Numerical Weather Prediction applications 

## Hans van der Marel

Delft University of Technology, Department of Geodesy

Thijsseweg 11, 2629 JA Delft, The Netherlands

e-mail: [H.vanderMarel@geo.tudelft.nl](mailto:K.deJong@geo.tudelft.nl)

## Summary

In the framework of the European COST 716 action \"Exploitation of
Ground-Based GPS for Climate and Numerical Weather Prediction
applications \"a near real-time demonstration project is scheduled for
2001. The current plans foresee in two one-month test datasets, in the
summer and winter of this year, preceding the more continuous
demonstration phase starting in 2001.

The two datasets in 2000 will be used to test the dataflow from GPS
receiver, to GPS analysis center(s) and into the regular meteorological
datastream for numerical weather prediction (NWP). The main objective of
these tests is to make available two GPS datasets, in a format well
suited for meteorology and archived like all other meteorological data,
that can be used to study and further develop assimilation of GPS data
into NWP models. These datasets are intended to assist in preparing the
assimilation process. The assimilation itself will not be tested - in
near real-time - until 2001, when the second phase of the demonstration
project starts.

## 1. Introduction

In 1998 a European COST action was started for the exploitation of
ground based GPS for climate and numerical weather prediction
applications (COST-716). The primary objective of the action is "*The
assessment of the operational potential on an international scale of the
exploitation of a ground based GPS system to provide near real time
observations for numerical weather prediction and climate
applications".* At the beginning of 1999 fourteen countries had signed
the memorandum of understanding (MoU): Austria, Belgium, Denmark,
Finland, France, Germany, Hungary, Italy, Netherlands, Norway, Spain,
Sweden, Switzerland and the United Kingdom. The action is planned to
last until 2004. The COST action 716 is divided into four projects, each
run by a working group:

(1) State of the art and product requirement,

(2) A demonstration project,

(3) Applications,

(4) Planning for the operational phase. This project has not yet started

For working group 1, the participating countries have reviewed their
status and the final report will be presented at the COST 716 workshop
in Oslo. It will contain a review of the state of the art, equipment
specification, possible modifications needed to Geodetic systems, error
sources, recommended software, recommended data format and preliminary
user specification.

Working group 2 started working in September 1999, and consists of 11
members. Working group 2 will build upon the results of working group 1
(State of the Art). The main task of working group 2 is to develop a
near real time demonstration system by March 2001, including all the
steps from data acquisition to assimilation into an NWP model. Further,
the operational reliability of the hardware and software codes has to be
verified, and the sensitivity to meteorological and site variables must
be assessed. Finally, a quality control and validation scheme, which
will concentrate on site-specific issues, has to be developed. Also,
within this project, the production of a reference dataset for
benchmarking purposes is foreseen. The deliverables for the project are
a trial report, the demonstration system and a near-real time data set
on regional/continental scale.

The specifications for the real time demonstration system will be
defined by working group 3 (Assimilation into NWP). This includes the
specifications from the operational meteorology, climate research and
climatology communities. Working group 3 will also develop an approach
for assimilation in Numerical Weather Prediction (NWP) models, use in
climate research and prediction and use in climate monitoring
(climatology). Working group 3 will make an assessment of the impact and
sensitivity of GPS data.

**2. Requirements for GPS networks**

In order for the GPS network(s) to be useful for meteorology and climate
applications in Europe, working group 3 has formulated a number of
requirements:

1)  The data expected from the network(s) should cover (at least) Europe
    and the Northern Atlantic as much as possible (island stations!).

2)  Two data qualities are necessary:

    a.  As needed for near-real-time data assimilation (operational
        meteorology)

    b.  For climate use (i.e. post-processed)

3)  The WAVEFRONT processing recommendations should be followed, and the
    CLIMAP formats, possibly amended, should be used for the
    distribution of data.

The data qualities are further specified in tables 1 and 2, taken from a
working group 3 report. Table 1 summarises the generic requirements for
operational meteorology, adapted from the general User Requirement
Statement for Meteorological Data, drafted by WMO.

+-----------------------+----------------------+----------------------+
|                       | ##### Colu           | > **Total Zenith     |
|                       | mn Specific Humidity | > Delay**            |
+-----------------------+----------------------+----------------------+
| > **Horizontal        | > Global / Regional  | > Global / Regional  |
| > Domain**            |                      |                      |
+-----------------------+----------------------+----------------------+
| > **Horizontal        | > \< 300 km / \< 100 | > \< 300 km / \< 100 |
| > Resolution**        | > km                 | > km                 |
+-----------------------+----------------------+----------------------+
| > **Time Resolution** | > 1-6 hrs / 1-2 hrs  | > 1-6 hrs / 1-2 hrs  |
+-----------------------+----------------------+----------------------+
| > **Absolute          | > 5000 g/m^2^ / 1000 | > 30 mm / 6 mm       |
| > Accuracy**          | > g/m^2^             |                      |
+-----------------------+----------------------+----------------------+
| > **Timeliness**      | > 1-3 hrs / \<1 hr   | > 1-3 hrs / \<1 hr   |
+-----------------------+----------------------+----------------------+

Table 1 Generic GPS Meteorology Network Requirements for Operational
Meteorology

The Column Specific Humidity, or Integrated Water Vapour (IWV), is
usually expressed in g/m^2^ or alternatively as the equivalent height of
the water column in mm (1000 g/m^2^ = 1 mm). This is equivalent to about
6-7 mm delay in the vertical (Wet Zenith Delay). The IWV can be computed
from the Total Zenith Delay (TZD) using surface pressure and the mean
temperature of the atmosphere. Even if surface meteorological
measurements are available, it is still difficult to compute the mean
temperature, which is the main uncertainty in the conversion to IWV
(figure 1). Fortunately, the new generation of Numerical Weather
Prediction (NWP) models, the so-called 3D- and 4D-VAR, can directly
assimilate the Total Zenith Delay (TZD) from GPS.

![](media/image1.wmf){width="2.895138888888889in"
height="2.422222222222222in"}

Table 1 explicitly includes users requiring data for regional and/or
mesoscale NWP models. In general, the requirements are very similar
except that horizontal resolution better than 50km and timeliness better
than 1 hour are preferred for mesoscale applications.

+-----------------------------------------+----------------------------+
|                                         | > **Column Specific        |
|                                         | > Humidity**               |
+-----------------------------------------+----------------------------+
| > **Horizontal Domain**                 | > Global                   |
+-----------------------------------------+----------------------------+
| > **Horizontal Sampling**^1)^           | > 2.5° x 2.5°              |
+-----------------------------------------+----------------------------+
| > **Time Domain**                       | > \> 10 years              |
+-----------------------------------------+----------------------------+
| > **Time Resolution**                   | > 1/day 1/hour             |
+-----------------------------------------+----------------------------+
| > **Absolute Accuracy**                 | > \< 1000 g/m^2^           |
+-----------------------------------------+----------------------------+
| > **No. of observations/grid box/day    | > \> 40                    |
| > based on random accuracy**            |                            |
+-----------------------------------------+----------------------------+
| > **Long Term Stability**               | > 200-400 g/m^2^ /decade   |
+-----------------------------------------+----------------------------+
| > **Timeliness**                        | > 1-2 months               |
+-----------------------------------------+----------------------------+

Table 2 Generic GPS Meteorology Requirements for Climate Monitoring and
Prediction

Table 2 illustrates in a similar way the user requirements for the
climate community, in particular those for climate monitoring and
prediction where trends in past and future are analysed. The
requirements for \'climatology\' are very similar, except that long-term
means are taken and stable, and low biases are demanded. Climate users
have expressed no direct interest in using zenith delays; they would
either use time- and spaced-averaged individual water vapour columns, or
use 3D analysed fields, which have assimilated GPS ground-based
meteorology data via NWP systems.

**3. Near real time network demonstration**

The GPS processing is organised around several near real-time networks,
which contribute to the demonstration project. Networks, which already
have expressed an interest, are

-   MAGIC project (Western Mediterranean),

-   GASP network (German Atmospheric Sounding Project),

-   Alpine network (Switzerland, Austria, Italy, Germany, France).

It is expected that other networks will follow. In order to be useful
for meteorology and climate, the networks should cover (at least) Europe
and the Northern Atlantic as much as possible. Therefore, it would be
very useful to add networks covering the British Islands, Iceland and
Scandinavia.

The networks are free to organise the processing as they seem fit, in
order to get the best possible results. Several aspects of real time
estimation, estimation procedures, software codes, etc., which are of
interest for COST-716 have already been investigated. Therefore, it is
expected that many of the guidelines of projects like WAVEFRONT, CLIMAP,
MAGIC can be followed, and that COST716 can build further upon the
expertise gained in GPS networks like IGS and EUREF. Further expertise
will be provided by near real time networks outside Europe (SuomiNet,
Japanese permanent network). The only requirements from the point of
view of COST 716 are, that each GPS processing center computes properly
validated Total Zenith Delays (TZD) with a well defined quality
indicator, in one of the agreed formats, to be made available within 3
hours.

A significant amount of work will be needed in order to derive a good
quality indicator and to understand the statistical properties of the
TZD data. A quality control/validation scheme, that will concentrate on
site specific issues, has to be developed. Firstly, an accurate station
history must be maintained. Routine plots of the residuals versus
azimuth and elevation could play a role in this. Secondly, a q/c
indicator for the TZD data itself is needed. A q/c flag should be
included in the CLIMAP format.

Each NRT network will handle the data issues within its own area. The
density and size of the networks primarily depends on the number of
stations, within each area, which can provide NRT data, and can be
handled by each network. Thereby, the spacing between the stations can
vary significantly among the networks. In addition, each NRT network
will also need data outside its area in order to

-   give absolute estimates of TZD

-   improve orbits in NRT

Data for several stations is required on a continental/global scale,
which will form a kind of reference network. Currently IGS and EUREF
provide hourly data for a number of stations from their data centers,
but there are still large gaps in the hourly data network, and
reliability and latency are poor. Nevertheless, it is expected that in
future the daily data deliveries will be phased out, and replaced by
hourly data deliveries. At the end of each day the hourly files will be
combined into daily files for postprocessing purposes. Also, the
reliability must be improved. Recently, EUREF has appointed a data flow
coordinator to address these issues.

If necessary, the reference network could be processed by a separate
processing center, prior to the analysis of the subnetworks. This could
be useful for orbit improvement, or to provide estimates of TZD, or to
check the quality of the data of the reference network. Within this
hierarchical processing scheme the subnetworks could be smaller.
However, considering IGS already delivers a combined ultra rapid orbit
prediction twice daily, and some IGS analysis centers do short term
predictions every hour, such an hierarchical processing scheme is not
needed. Especially, because every subnetwork has to check the orbit
quality, and probably will have to do some orbit relaxation anyhow.

Presently, it is not foreseen to combine the TZD estimates of individual
networks, but to make results available as soon as possible. This means
that for some stations two or more estimates of TZD will be available. A
combination step would only delay the results, and add an additional
layer of complexity. In addition, TZD will not be converted to IWV, as
the TZD is assimilated directly into the NWP models. The proposed
exchange format is the CLIMAP or BUFR format. However, since these
formats do not contain any co-variance matrix information the processing
centers should also archive the SINEX files as used in the IGS
combination.

A related, yet important, deliverable of this part of the work is an in
report assessment of equipment and expertise needed to run the
processing at meteorological institutes, without requiring too much
geodetic expertise.

**4. Post-processed network for climate research and climatology**

The requirements for climate research and climatology are basically the
same as for the NRT network, but more precise and especially smaller
biases, and less strict on timeliness. This means that results can be
achieved using post-processing with the final IGS orbits.

The starting point for the post-processed network could be the existing
EUREF network. The EUREF network consists of over 90 stations,
distributed over Europe. Each station is processed by at least 3
analysis centers. The analysis centers estimate a TZD parameter for each
station every 2 hours. EUREF recently has appointed a troposphere
coordinator, who's tasks it is to create a combined troposphere product
from the EUREF analysis centers (as is done within IGS). The EUREF
troposphere product can be enhanced by adding a few stations to the
EUREF permanent GPS network, e.g. stations collocated with a radiosonde,
and by adding post-processed results of the "NRT-networks" to the
combination product. Special attention should be given to the long term
stability, biases, archival of data and documentation of site related
changes. Pressure data, when measured at the GPS site, should be made
available for climate research and monitoring purposes.

## 5. Conclusion

Two NRT datasets, one in the summer, and one in winter, will be produced
from the networks. The main purpose is to provide realistic data for
assimilation tests, simulating the NRT environment. The data must be
available within 1-2 weeks, so that it can be inserted in the regular
dataflow to other meteorological institutes. In 2001, these experiments
will be continued on a more continuous basis, with stricter requirement
on the timeliness.

The requirements for climate research and climatology are basically the
same as for the NRT network, but more precise and especially smaller
biases, and less strict on timeliness.