GPS Water Vapour in the UK and
the E-GVAP Project
Jonathan Jones, Upper Air Team, Observations R&D, UK Met Office
jonathan.jones@metoffice.gov.uk
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Contents
• How and Why
• Operational Processing System
• Collaborative Networks
• Monitoring and Validation
• Forecaster Case Studies
• E-GVAP
• Future GPSWV Developments
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GNSS Constellations
Signal is ‘delayed’ by atmosphere.
Signal
is ‘delayed’
by atmosphere.
May
remove
ionospheric
contribution
Can
remove ionospheric
contribution
due
to dispersive
nature of mediumdueoperates
to dispersive
nature
of medium GPS
at dual
frequency.
GPS operates at dual frequency.
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Technique
Non-Neutral Atmosphere (Ionosphere)
•Dispersive, use dual GPS frequencies to
resolve
Neutral Atmosphere
Non-dispersive (troposphere + stratosphere)
•
Too much error in individual slant delay to resolve so
we can use mapping function to we can convert into
a Zenith Total Delay (ZTD)
•
ZTD a combination of dry and wet delays
•
From knowledge of surface pressure and
temperature we can calculate dry component and
resolve vertical Integrated Water Vapour quantity
•
1kg/m2 IWV = ~3-5% RH
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Customers
• NWP
• Shown up to 4% reduction of StdDev by assimilation of GPS
ZTD data
• Forecasters
• Produce IWV field plots to assist forecasters in identifying
atmospheric features/phenomena
• Climate
• Also GPS is potentially a useful tool for climate monitoring
but reprocessing is necessary to ensure common
processing models/standards
• Validation
• Very useful tool for validation of other remote sensing
instruments (radiosonde, WVR, satellite WV etc)
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Cost-Effective GPS Processing
• IESSG, Nottingham University contracted to
develop and deliver operational processing
servers
• Intergovernmental resource sharing
agreements with mapping agencies in UK
• Agreement in place between EUREF and
EUMETNET guaranteeing long term future GPS
data access to Met community. Also providing
positioning community with meteorological
parameters
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GPS Processing at METO
• 4 Servers
• 2 operational in Met Office IT halls, 1 mirror semi-operational
server at IESSG, 1 development server at IESSG
• Hourly Processing Solution
• Bernese v5.0 processing software
• Double Difference Network Approach
• 30-Day sliding window PPP a-priori coordinate
• Absolute Antenna Phase Centres, IGS05 and FES2004
• Process network of ~250 stations in NRT in ~30mins
• ZTD/IWV estimates at 00, 15, 30, 45 and 59 mins
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Data Providers
• 162 EUREF Sites
• 104 OSGB sites
• 86 64 RGP sites
• 20 UK scientific sites (incl. 11 Met Office)
• 19 OSi and OSNI sites
• 2 Icelandic sites
• Attempted hourly ~ 400
• Downloaded hourly ~ 275
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European NRT GPS Network
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METO Products
• ASCII files copied to E-GVAP
server for dissemination and
conversion to BUFR
• BUFR files then put on GTS
for assimilation by National
Met Services
• Plots also copied to E-GVAP
server for dissemination
• Plots downloaded by EGVAP and displayed on EGVAP website
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RS vs. GPS Validation
IWV Comparison Camborne Feb 2008
35
30
25
IWV (kgm-2)
20
15
RS92 TWE
GPS IWV
Diff
10
5
0
-5
-10
31-Jan
05-Feb
10-Feb
15-Feb
Time (Days)
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20-Feb
25-Feb
01-Mar
WVR vs. GPS Validation
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NWP Monitoring
• Data QC’d by AC and gross error check vs. model
• Site specific bias corrections also applied
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Case Studies
• 28th July 2005 – Offset Dry Tongue
• Dry tongue over south west not corresponding with
satellite WV
• By combination of satellite and GPS WV you can infer
vertical structure
• 24th June 2005 – Cold Pool Identification
• Trough progressing over southern UK from westerly
direction with associated high IWV and convective
thunderstorm cells
• GPS IWV identified cold pool behind front which
perpetuated instability and convection
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17.30
Drier Lower Air
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Drier Upper Air
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E-GVAP and E-GVAP II
• E-GVAP I – EUMETNET programme to take GPSWV to
operations in Europe
• Ensure common standards and formats in GNSS data
processing
• European network expansion
• Assist ‘developing’ countries with GPS processing and
assimilation techniques
• Ensure close collaboration with geodetic community (experts)
• E-GVAP II – continuation of E-GVAP I
• Connect to EUCOS and WIS
• Continued network expansion
• Improve homogeneity and quality of GNSS products
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E-GVAP Website and Server
• Website v. useful tool for analysing data/biases etc
• Compiling table of all AC’s GPS processing techniques,
• Recently added Nordic and French regional plots
• E-GVAP Server to be transitioned to more operational
(backed-up) server by end of E-GVAP 1
• Server now hosting met data for EUREF community
• Server soon to be hosting other data (microwave
radiometer, NWP etc)
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QC via the E-GVAP Website
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E-GVAP ‘Supersites’
• 14 sites identified for validation
of GPS IWV
• All sites have collocated
instruments, geodetically
stable, same model layer etc
• Probably will include EKOFISK
site in near future
• Access to data from site at
Inaza Observatory, Tenerife
(GPS, RS, MWR, Infrared
Spectrometer)
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Future GPSWV Developments
• Improved NRT Plots
• Use NWP IWV over data sparse areas (Sea)
• Produce O-B plots
• Add satellite WV to plots to give some vertical structure
• Real-Time GPS Processing (15min/5min)
• Slants/Tomography
• NWP ZTD forecasts
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Questions and answers
Jonathan.jones@metoffice.gov.uk
http://egvap.dmi.dk/
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