Page 1© Crown copyright 2004 Development of a Ground Based GPS Network for the Near Real Time Measurement of Integrated Water Vapour (IWV) Jonathan Jones Met Office UK

Page 2© Crown copyright 2004 Why…?  Assimilation of IWV to model (trials have seen 4% improvement of surface temperatures)  Nowcasting applications – extreme weather events etc  Moisture and latent heat are transmitted through the water vapour phase  Validation of other remote sensing instruments (radiosonde, microwave radiometer)  Also complements satellite IWV which only available over open water

Page 3© Crown copyright 2004 Advantages / Disadvantages  Advantages of GPS IWV  All weather operational (vs. MWR)  Continuous, high rate of measurement  Relatively Low cost  (Near) Real-time  Disadvantages  ZTD is an integrated value (vs. radiosonde)  Can’t solve directly slant delay (vs. MWR)

Page 4© Crown copyright 2004 Ground Based GPS Meteorology Ionosphere Dispersive, use L3 to remove Troposphere + Stratosphere Using mapping function we solve to the vertical (Zenith) Zenith Total Delay (ZTD)= ZHD+ZWD ~90% Dry delay (Zenith Hydrostatic Delay ZHD) ~10% Wet Delay (Zenith Wet Delay ZWD)

Page 5© Crown copyright 2004 ZTD to IWV  ZHD mitigated using surface barometric measurements and atmospheric models  ZWD more difficult to predict, varies greatly both spatially and temporally  With further calculation (using surface temperature) we can convert ZWD to IWV  1mm IWV ≈ 3% Relative Humidity

Page 6© Crown copyright 2004 GPS Data Flow

Page 7© Crown copyright 2004 GPS Processing System  2 dual-processor PCs running Bernese v5 GPS processing software  ZTD Processing Solutions  Near Real Time solution, ~25min latency  2 Quality check solutions using more accurate orbits  Quality Analysis  Compare solutions (NRT vs. PP)  Compare against other processing centres  Compare against other obs. methods

Page 8© Crown copyright 2004 Equipment Ashtech Dual Frequency GPS Receiver Choke ring antenna, carbon fibre pole Data collected by local PC and sent to central processing system for conversion to ZTD and IWV

Page 9© Crown copyright 2004 GPS IWV time series vs. RS80 Radiosonde

Page 10© Crown copyright 2004 Approximate Site Installation costs (EURO)  GPS Equipment ≈ 20,000  PC and UPS ≈ 2,500  Works Services ≈ 5,000  Communication connection ≈ 500  1 Euro ≈ $1.29  ~50km spatial resolution required for operational meteorology (COST716)  In UK 50km resolution ≈ 150 station network  150 station network ≈ 4.2M EURO ($5.4M)

Page 11© Crown copyright 2004 Partnerships  Therefore….Met Office must work with other national agencies to reduce network costs  2003 to Present – UK National mapping agency (Ordnance Survey GB) ≈ 120 sites  Others (incl. offshore?) ≈ 50 sites ?  Met Office Network = 10 significant locations

Page 12© Crown copyright 2004 Current UK NRT GPS Network (April ‘05)

Page 13© Crown copyright 2004 Output of Data  COST716 format files disseminated to EU scientific community via UK Met Office  COST716 file converted to BUFR (binary format) for dissemination on the GTS  NRT animated 2D plot

Page 14© Crown copyright 2004 Example NRT Animated Plot

Page 15© Crown copyright 2004 The Future  Operational network and processing  Sub-hour processing (sub-hour RINEX)  Increasing network density  Take forward EU action through E-GVAP (EUMETNET Project)

Page 16© Crown copyright 2004 Questions?