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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

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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.

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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

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neutral atmosphere

Errors to geodesists

Signalsto meteorologists

Measuring water vapor using GPS

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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)

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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

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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

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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)

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Sensitivity of the trends to different time periods

21 Nov. 1996 – 20 Nov. 2009 21 Nov. 1997 – 20 Nov. 2010

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Summer and winter trends

Summer (April – September) Winter (October – March)

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Trend comparisons: GPS vs. radiosonde

• Analysis period: November 21, 1996 – November 20, 2010

• 13 GPS sites (bold font) vs. 7 radiosonde sites (italic font)

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Trend comparisons: GPS vs. radiosonde (cont.)

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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.

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Chalmers University of Technology

Thank you for your attention!