1. WG3: GNSS for climate monitoring
COST Action ES GNSS4SWEC
Advanced GNSS Tropospheric Products for
monitoring Severe Weather Events and Climate
3nd workshop, Reykjavik, 8-10 March 2016
WG3: GNSS for climate monitoring
Olivier Bock (IGN, France)
Rosa Pacione (e-geos, ASI/CGS, Italy)

2. WG3 activities : summary
WP3.1: GNSS data processing and validation
Inventory of available reprocessed GNSS ZTD/IWV datasets (Europe, Globe, other regions/projects), incl. DORIS, VLBI
ZTD processing methodology: study of impact of stochastic model settings
WP3.2 Standards and methods
ZTD screening: develop and standardize methods (GNSS info + model QC)
ZTD to IWV conversion: assess datasets and methods => standardize
ZTD/IWV homogenisation: assess methods
WP3.3 Assessment of GNSS IWV accuracy
Studies on IWV intercomparisons using IGS repro1 and other reprocessed GNSS data (EUREF repro2)
Established contact with GRUAN for intercomparisons => assess absolute IWV accuracy
WP3.4 GNSS and climate research
Studies on IWV trends & variability from GPS/DORIS, ERA-Interim
Studies with climate model simulations
WP3.5 Database and formats
GOP tropo Data Base for coordinated activities
Tropo SINEX formats upgrade (collab. w. IGS, EUREF)
WG3 : 18 countries / 60 participants

3. WP3.1: GNSS data reprocessing
WG3 activities
WP3.1: GNSS data reprocessing
Inventory of 26 reprocessed GNSS ZTD/IWV datasets (Europe, Globe, other regions/projects), incl. DORIS, VLBI.
To update inventory & share data => contact O. Bock & J. Dousa
Set up a community dataset for global climate studies:
IGS repro1 + trop_new (screened and converted to IWV) and ERA-Interim IWV, global dataset, 1995/ /12.
Access to dataset => contact O. Bock
Next: EPN repro2 combined tropo solution => 2nd dataset
More stations over EU (for regional climate studies & model validations)
For assessment of EPN repro2 => contact R. Pacione
Reprocessing : many studies on impact of GNSS processing options on ZTD estimates (and their information content):
Mapping Functions
Stochastic tropo model (RW, GM…)
Cutoff angle => strong impact on ZTD bias
Baseline strategy in DD processing
Session length (3-day vs. 1-day)
Software (Bernese, Gamit, Gipsy)
Processing mode (PPP vs. DD)
=> Need to synthesise the results and provide recommendations for future reprocessing… Contributions welcome as papers to Special Issue
=> contact O. Bock, J. Dousa, E. Brockmann

4. WP3.2 Standards and methods
WG3 activities
WP3.2 Standards and methods
ZTD screening:
Several methods have been tested
A standardized method has been proposed based on 2 steps (Bock and Bosser):
1. Range check & outlier check using GNSS processing outputs (ztd & formal errors, percentage of ambiguities, number of observations…)
2. Model departure QC : normality test on GPS – model differences (e.g. ERA-Interim)
Method validated with Gipsy and Bernese GNSS solutions and also with DORIS
=> To participate to further testing of this and other methods => contact O. Bock
ZTD to IWV conversion:
1) assessement of Ps, Tm datasets:
SYNOP surface pressure obs => issues with inhomogeneity and position errors
=> Need to homogenise SYNOP Ps data with climate and/or NWP community…
Reanalyses (ECMWF/ERAI) => OK but evidenced representativeness issues.
=> Test new, higher resolution reanalyses…
2) uncertainty in refractivity constants = limit to absolute accuracy (issue also raised by GNSS RO community)
Status quo ! => New refractivity measurements are required…
=> To participate to further work on conversion methods => contact O. Bock
Homogenisation of ZTD/IWV time series:
Several segmentation methods and datasets used so far in the community.
=> Agreed on testing methods on a common dataset (IGS repro1 & ERAI)
=> Homogenisation workshop (Brussels, April), contact R. Van Malderen & E. Pottiaux

5. WP3.3 IWV intercomparisons
WG3 activities
WP3.3 IWV intercomparisons
A literature review (45 papers) highlighted poor consensus in the results/conclusions of IWV intercomparisons (GNSS vs. radiosondes, MWR, photometers, satellite data, NWP models…).
=> To enhance this bibliographic data base contact R. Van Malderen
An inventory listed 29 high quality instrumented sites from GRUAN and NDACC networks. They have established standardized procedures to operate instruments, process & QC data, record metadata, do intercomparisons campaigns.
GRUAN leaders are keen to cooperate with WG3
WG3 provides expertise on GNSS processing and QC
GRUAN provides access to high IWV data and metadata
An intercomparison exercise is being setup to assess absolute accuracy of IWV measurements:
GRUAN sites: Sodankyla and Ny Alesund
GNSS ZTD data: GFZ product
Apply WG3 screening and conversion methods…
Intercompate IWV data from GNSS, ref. radiosonde data, VLBI, DORIS, MWR…
=> To participate to the exercise contact R. Van Malderen

6. WP3.4 GNSS and climate research
WG3 activities
WP3.4 GNSS and climate research
ZTD and IWV trends & variability intercomparisons:
Several studies estimated ZTD and/or IWV trends from long GPS times series and compared them to other datasets: NWP reanalyses (ERA-Interim, NCEP…) satellite products (GOME/SCIAMACHI, SSMI…), DORIS & VLBI
=> Inconsistencies in results due to different GPS processings, screening, conversion methods & data, and analysis methods
=> Use newly available community dataset to harmonize
Climate model assessment
A few studies reported on use of GNSS IWV data to assess climate climate model simulations (EC-Earth, CMIP5, CORDEX…)
=> Contact established with the climate community (EC-Earth, CMIP5, CORDEX)
Study representation of atmospheric processes in climate model simulations (ENSO, NAO, African Monsoon…)
=> More studies to come with the newly available community dataset
Further studies on atmopsheric water cycle
Link with reanalysis community
Many studies used ERA-Interim => strong experience in the GNSS community, knowledge on quality of reanalysis, evidenced limitations, expressed needs…
=> Link with ECMWF established and looking forward to collaborate on ERAC-CLIM2 reanalysis project / Operational GNSS assimilation

7. WP3.5 Database and formats
WG3 activities
WP3.5 Database and formats
GOP tropDB as a community tool and database
Collaborative developments of methods (screening, conversion, interpolation, analysis…) between WG1 & WG3.
=> Contact J. Dousa
Tropo-SNX format standardization
Cooperation with IGS Troposphere WG, EUREF Technical WG, E-GVAP expert team.
Revised format and addition of new block for slant delays => useful for assimilation of slants in NWP models
Prospective: handling of information for time series analysis
=> contact R. Pacione