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Tong Ning and Gunnar Elgered Department of Earth and Space Sciences

Monitoring Long Term Variability in the Atmospheric Water Vapor Content Using Ground-Based GPS Receiver Networks. Tong Ning and Gunnar Elgered Department of Earth and Space Sciences Chalmers University of Technology Onsala Space Observatory, Sweden. Motivation.

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Tong Ning and Gunnar Elgered Department of Earth and Space Sciences

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  1. Monitoring Long Term Variability in the Atmospheric Water Vapor Content Using Ground-Based GPS Receiver Networks Tong Ning and Gunnar Elgered Department of Earth and Space Sciences Chalmers University of Technology Onsala Space Observatory, Sweden

  2. Motivation • Water vapor is a very important greenhouse gas. • Water vapor is one of the most important climate feedback process. • Long-term trends of the atmospheric water vapor content can be used as an independent data source to detect global warming. • Accurate observations with long-term stability is important for trend estimations. • A high spatial density of measurements is desired.

  3. GPS networks • GPS can work under in principle all weather conditions with increasing spatial • resolution locally and globally. • Global:the number of stations from the permanent International Global Navigation Satellite Systems (GNSS) Service (IGS), formerly the International GPS Service, is now (July 2011) globally over 360. • Local network from Sweden: • SWEPOS has been in operation since 1993 with 21 geodetic quality stations (stars). • More than 170 stations, 1200 km from north to south, and 400 km from east to west, with an average site separation of approximately 70 km.

  4. Measuring water vapor using GPS Errors to geodesists Signals to meteorologists neutral atmosphere

  5. Measuring water vapor using GPS (continued) • Use GPS processing software, e.g. GIPSY 5.0 applying antenna phase center corrections and an elevation cut-off angle of 10 degrees • Solve for station coordinates, clock errors, Zenith Total Delay (ZTD), etc. • ZTD=Zenith Hydrostatic Delay (ZHD) +… • Zenith Wet delay (ZWD) • ZHD can be estimated if surface pressure is known. • ZWD is related to the Integrated Water Vapor (IWV) content of the atmosphere: • ZWD (mm) =Q • IWV (kg/m2) • where Q ≈ 6.5 (depending on location and season)

  6. Estimating IWV trends The IWV has been obtained from we make a fit to the model: where t is the time in years and the coefficients I0, A, B, C, D, E are estimated. Both annual and semi-annual terms are used to model the seasonal variations.

  7. IWV trends for some GPS sites Latitude: 66.32o Trend: -0.27 kg/m2/decade Latitude: 62.23o Trend: 0.08 kg/m2/decade Latitude: 56.09o Trend: 0.17 kg/m2/decade

  8. IWV trends over Sweden and Finland • 21 sites from Sweden and 12 sites from Finland • IWV trends in kg/m2/decade. • Analysis period: November 21, 1996 – November 20, 2010. • The uncertainties in the trends are estimated to ~0.35 kg/m2/decade (taking the temporal correlation into account)

  9. Sensitivity of the trends to different time periods 21 Nov. 1997 – 20 Nov. 2010 21 Nov. 1996 – 20 Nov. 2009

  10. Summer and winter trends Summer (April – September) Winter (October – March)

  11. Trend comparisons: GPS vs. radiosonde • Analysis period: November 21, 1996 – November 20, 2010 • 13 GPS sites (bold font) vs. 7 radiosonde sites (italic font)

  12. Trend comparisons: GPS vs. radiosonde (cont.)

  13. Conclusions • IWV trends estimated from GPS vary from –0.3 to +0.5 kg/m2/decade over Sweden and Finland for the last 14 years. • Uncertainties in the trends are ~0.35 kg/m2/decade (taking temporal correlations into account) • Trends are (as expected) sensitive to the specific time period investigated (due to the short period of data available). • Good agreement — correlation coefficient of 0.68 — with the trends from radiosondes launched nearby.

  14. Thank you for your attention!

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