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IGARSS 2011, Vancuver, Canada July 28, 2011 1 of 14
Chalmers University of Technology
Monitoring Long Term Variability in the Atmospheric Water Vapor
Content Using Ground-Based GPS Receiver Networks
Tong Ning and Gunnar ElgeredDepartment of Earth and Space Sciences
Chalmers University of TechnologyOnsala Space Observatory, Sweden
IGARSS 2011, Vancuver, Canada July 28, 2011 2 of 14
Chalmers University of Technology
MotivationMotivation
• 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.
IGARSS 2011, Vancuver, Canada July 28, 2011 3 of 14
Chalmers University of Technology
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.
GPS networksGPS networks
IGARSS 2011, Vancuver, Canada July 28, 2011 4 of 14
Chalmers University of Technology
neutral atmosphere
Errors to geodesists
Signalsto meteorologists
Measuring water vapor using GPS
IGARSS 2011, Vancuver, Canada July 28, 2011 5 of 14
Chalmers University of Technology
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)
IGARSS 2011, Vancuver, Canada July 28, 2011 6 of 14
Chalmers University of Technology
Estimating IWV trendsThe 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.
)4cos()4sin()2cos()2sin(0 tEtDtCtBAtIIWV
IGARSS 2011, Vancuver, Canada July 28, 2011 7 of 14
Chalmers University of Technology
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
IGARSS 2011, Vancuver, Canada July 28, 2011 8 of 14
Chalmers University of Technology
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)
IGARSS 2011, Vancuver, Canada July 28, 2011 9 of 14
Chalmers University of Technology
Sensitivity of the trends to different time periods
21 Nov. 1996 – 20 Nov. 2009 21 Nov. 1997 – 20 Nov. 2010
IGARSS 2011, Vancuver, Canada July 28, 2011 10 of 14
Chalmers University of Technology
Summer and winter trends
Summer (April – September) Winter (October – March)
IGARSS 2011, Vancuver, Canada July 28, 2011 11 of 14
Chalmers University of Technology
Trend comparisons: GPS vs. radiosonde
• Analysis period: November 21, 1996 – November 20, 2010
• 13 GPS sites (bold font) vs. 7 radiosonde sites (italic font)
IGARSS 2011, Vancuver, Canada July 28, 2011 12 of 14
Chalmers University of Technology
Trend comparisons: GPS vs. radiosonde (cont.)
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Chalmers University of Technology
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.
IGARSS 2011, Vancuver, Canada July 28, 2011 14 of 14
Chalmers University of Technology
Thank you for your attention!