GPS total water vapour – potential for future operational observations
John Nash
Met Office, Upper Air Technology Centre,
Beaufort Park, Easthampstead, Wokingham, Berks RG40 3DN
Phone 44-1344-855649, Fax. 44-1344-855897: e-mail john.nash@metoffice.com
Introduction
The purpose of this paper is to outline the potential for
improving knowledge of the distribution of water
vapour in the lower troposphere provided by the
measurements that can be derived from ground-based
measurements of navigation signals from the Global
Positioning System (GPS) satellites. This type of
measurements would appear to fit well into a future
design of the Global Observing system since it is able
to fill gaps in observations over the land where satellite
based measurements of water vapour prove difficult in
the lower troposphere.
As a new system, the potential costs of running
operations need to be assessed so that sensible choices
about the scales of deployment can be made.
The principles of operation of the system and the
challenges to be met in establishing regional networks
will be discussed.
Examples of current measurement skills will be
illustrated with observations in Europe from the COST
716 real time demonstration network.
GPS sensing technique
The time delay of a GPS signal from transmission to
reception at the ground is influenced by the state of the
ionosphere, surface pressure, vertical temperature
structure and the total water vapour in the atmosphere.
The influence of the ionosphere is usually cancelled
out by observing the differences in phase delay
between the L1 and L2 GPS frequencies. Then, if the
position of the satellite is known very accurately,
possible errors in the satellite clocks are identified and
the height of the ground based sensor is identified
accurately, then the total delay along the slant path to
the satellite caused by the atmosphere can be
computed. The GPS satellites change position
relatively slowly with respect to the ground sensor and
signals from a given satellite can be received for many
hours at a time. It is usually possible to receive reliable
signals from between 8 and 12 satellites distributed
over a variety of elevations and azimuth from the
sensor. In order to solve for the height of the sensor
and the total zenith delay it is necessary to observe
from several satellites along different slant paths (to
reduce height errors to acceptable values). A network
of sensors with some ground stations distributed along
baselines larger than about 500 km from the centre of
the network is also required.
Once the zenith delays are estimated along a variety of
slant paths from a given site, the values are mapped
into the vertical to yield a total zenith delay. This can
then be provided to the potential user to assimilate into
a numerical weather prediction model. The total zenith
delay can be converted into total water vapour if the
surface pressure and temperature are known. It is not
clear whether the numerical model background
temperature fields are sufficiently accurate to provide a
significant advantage in assimilating total zenith delay
or whether it is better to use total water vapour
estimates derived independently of the model fields.
A considerable research effort is attempting to derive
additional information about the distribution of water
vapour in the vertical utilising the individual zenith
delays along slant paths, particularly at low elevations.
This is most likely to work if many GPS ground–based
sensors are operated in a network with very small
network spacing (5km or less). Whether the results
obtained can justify the expense of all the sensors is yet
to be established, and I will not consider this area any
further in this presentation.
Network demonstrations
The GPS method of measuring water vapour was
discussed in Bevis et.al [1], and results from the first
proof of concept experiment GPS/Storm were first
reported in 1995, by Rocken et al [2]. Subsequently the
NOAA-GPS PW network was established by NOAA’s
Forecast System Laboratory in the USA. Japan has
undertaken extensive research and development using
the JMA GPS network, established originally for
seismological observations, Naito et al [3] and also see
the Report of N. Mannoji, CIMO Rapporteur on this
technique for CIMO-XIII. Within Europe various
research projects have contributed to the EC supported
COST 716 project “ Exploitation of Ground –based
GPS for Climate and Numerical Weather Prediction
Applications”. National research and development of
the technique continues in many other countries.
In late 2000 I was invited to chair the Working Group
of COST 716 tasked with establishing options for
future operational networks in Europe. This paper will
show the results that have been achieved in Europe and
review some of the problems associated with
establishing operational networks.
Real time observations
As noted above, it has been possible to demonstrate the
information content of the GPS water vapour
measurements since 1995, but this was usually on
observations processed a significant time after the time
of observation when the estimates of the precise orbit
positions of the satellites became available. It is only
within the last few years that the IGS (International
GPS Service for Geodynamics) has begun to issue
rapid orbital position estimates of sufficient quality that
the total water vapour estimates can be achieved with
time delays of less than 2 hours after observation. In
addition techniques for orbit estimation have now
developed to the stage that individual GPS processing
centres can also attempt to generate their own rapid
orbit estimates.
A second hindrance to real time network operation has
been that in many countries where GPS sensors have
been deployed for geodetic purposes, real time
communications to the sites were poor. Thus, the move
towards real time operations requires some investment
in communications infrastructure. It is not always
clear at the moment whether some GPS networks are
currently running in real time or where the networks
are being evaluated with observing system experiments
based on processing after the event.
Characteristics of total water vapour
The random errors associated with real time total water
vapour measurements over the UK are about 1 mm of
water vapour and systematic bias is of similar
magnitude. It is expected that these errors will not
differ widely with geographic location. Fig.1 indicates
the distribution of water vapour in the vertical for a
saturated pseudoadiabatic atmosphere, and the
dependence of total water vapour on temperature in
saturated conditions. This dependence on temperature
is quite strong, and leads to marked differences in total
water vapour across frontal systems, even when there
are no significant dry layers present.
When temperatures are cold, total water vapour is less
than 10mm even for saturation and a 1-mm error
corresponds to an equivalent relative humidity error of
greater than 10 per cent. However, in tropical
conditions with total water vapour higher than 60 mm
in many conditions, then 1mm random error offers a
random error in equivalent relative humidity of around
2 per cent, as good as can be achieved by the best
radiosonde system. Thus, this is a system, which could
be very useful for water vapour measurements in
equatorial regions given that the sensor site and
communications can be sustained.
Real-time Observations
The COST 716 Demonstration Network has built up
within two years to a situation where more than 100
sites are regularly supplying data for processing in real
time from more than 15 different countries. Most of the
resultant total zenith delays are being supplied to a
Data Assimilation centre within 2 hours of completing
the GPS measurements. All these processing centres
are from the geodetic community and for the future
some form of partnership will have to be organised if
operations are to continue. Fig.2 shows examples of
GPS signals received at Herstmonceux in southern
England and processed by several centres within
Europe. Total zenith delay and integrated water vapour
are plotted as a function of time for two days, with the
last data added within 2 hours of the nominal time.
Plots such as these can be obtained each day from the
relevant COST 716 web page maintained by KNMI,
(http://www.knmi.nl/samenw/cost716/ztd-iwv.html).
During this period the HIRLAM numerical weather
prediction model fields were similar to the
measurements at Herstmonceux.
Fig. 3 contains time series of measurements from 4
sites in early July 2002, when there were large
discrepancies between the model fields and actual
measurements over central France for most of the night
of 8 July. The situation at 12 UTC on this day over the
UK, France, Belgium and the Netherlands is
summarised in this plot of measurements from real
time GPS sensors. Note the very strong gradient in
total water vapour over the UK associated with lower
troposphere temperature change across a frontal
surface. This system gave very heavy rainfall over
parts of the UK that day. In France the low values of
total water vapour rose rapidly later in the day as a line
of thunderstorms moved eastwards from the coast. The
lowest values of total water vapour occurred just in
front of the thunderstorms and the high values once the
thunderstorms were overhead. These data were taken
straight from the web page and were not subjected to
any further editing. Similarly data from the majority of
the COST 716 stations were used to derive the estimate
of the total water vapour field shown in Fig.5 for 06
UTC in the morning on 19 July 2002. On this day, the
total water vapour remained lower than 20 mm over
France and Spain and much of Germany and
Switzerland. High values of total water vapour, 35 to
40 mm were found aligned north – south from Sweden
down to Croatia, associated with a trough, with a
smaller pool of higher values associated with a trough
over the Adriatic Sea. Later in the day thunderstorms
developed close to the regions of high total water
vapour identified.
Network development
Within Europe there are known to be large numbers of
GPS sensors that are not connected up in real time to
the current demonstration network. For instance, within
the UK there are more than 40 sites used for lighthouse
navigation, tide gauges and sea level measurements,
maintenance of the national grid reference for surveys
and university experiments associated with
communications infrastructure. To install 40 GPS sites
for operational use would cost far too much for the Met
Office in the current economic climate. However, if
some form of partnership agreement can be made with
these other institutions then it should be possible to
build a high resolution-observing network for water
vapour for the future. Fig.6 shows the distribution of
water vapour around a severe thunderstorm that moved
northwards over the UK, with the water vapour
produced using observations from a combination of
Met office and other institution sites. The air to the
north of the storm was saturated so that the gradients in
water vapour in this area represent temperature
gradients in the horizontal in the lower troposphere.
Analyses such as these have led Met Office forecasters
to request that data be delivered with a time lag of less
than 1 hour if possible with as high a resolution in the
horizontal that can be achieved without large scale
investment. Authorisation to develop the basis of an
UK GPS observing network has been given.
Acknowledgements: Real time data for the UK has
been made possible by the processing centre of the
Observatory of Pecny, Czech Republic. Internally
within the UK, IESSG, Nottingham University has
made available and processed data from sensors within
the UK for the detailed post processing studies.
Summary
Development of GPS water vapour sensing has
progressed to the stage where measurements of suitable
quality can be delivered in a timely fashion to users.
References:
[1] Bevis. M., Businger, S., Herring, T., A, Rocken,
C., Anthes, A., and Ware, R..1992 Remote Sensing of
the atmospheric water vapour using the Global
Positioning System, J. Geophys. Res.,97, 15,787-15801
[2] Rocken, C., T. Van Hove, J. Johnson, F. Solheim,
R. Ware, M. Bevis, S. Chiswell, S. Businger, 1995
GPS/Storm-GPS Sensing of Atmospheric Water
Vapour for Meteorology, J. Atmos & Ocean . Tech, 12
468-478
[3] Naito, I., Y. Hatanaka, R. Ichikawa, S. Shimada, T.
Yabuki, H. Tsuji, and T. Tanaka, Global positioning
System project to improve Japanese Weather,
Earthquake Predictions, EOS, Transactions, Am.
Geophys. Union, 79,301.308.311, June 1998
.
Effective exploitation of the GPS technique requires
working arrangements to be established with other
agencies that deploy suitable GPS sensors, e g. for
geodesy, seismology, or land survey purposes.
Collaboration in processing observations needs to be
established, whether the processing centre is hosted by
a Met Service for a given region, or by a national
geodetic centre
Wet bulb
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temperature[ºC]
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Total water
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vapour [mm]
Fig.1 Distribution in the vertical of contributions to total water vapour for a saturated pseudoadiabatic atmosphere
Fig.2 Total zenith delay and associated integrated water vapour measured at Herstmonceux, south-east England, as
displayed on the COST 716 web page in real time for two days in February 2002, data processed by three different
processing centres and compared to HIRLAM model fields (grey line labelled NWP).
Fig.3 Near real time GPS IWV plots from COST 716 for 7 to 9 July 2002 from Herstmonceux and Camborne in
southern UK and two sites in central France see Fig.4
Fig.4 Water vapour around the UK, COST 716 only
Fig.5. Water vapour contours derived from COST716
Fig. 6 Water vapour distributions superimposed by hand on weather radar observations of a severe storm progressing
northwards into southern England. Grid for radar displays has spacing of about 23 km. Air to the north and west of the
storm is much colder than air to the east. Relatively strong winds were feeding into the storm from the south east, with
light winds at low levels to the north and west of the storm.