IGARSS 2011, Vancuver, Canada July 28, 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 Elgered Department of Earth and Space Sciences Chalmers University of Technology Onsala Space Observatory, Sweden

IGARSS 2011, Vancuver, Canada July 28, of 14 Chalmers University of Technology 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.

IGARSS 2011, Vancuver, Canada July 28, 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 networks

IGARSS 2011, Vancuver, Canada July 28, of 14 Chalmers University of Technology neutral atmosphere Errors to geodesists Signals to meteorologists Measuring water vapor using GPS

IGARSS 2011, Vancuver, Canada July 28, 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/m 2 ) where Q ≈ 6.5 (depending on location and season)

IGARSS 2011, Vancuver, Canada July 28, of 14 Chalmers University of Technology 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 I 0, A, B, C, D, E are estimated. Both annual and semi-annual terms are used to model the seasonal variations.

IGARSS 2011, Vancuver, Canada July 28, of 14 Chalmers University of Technology IWV trends for some GPS sites Latitude: o Trend: kg/m 2 /decade Latitude: o Trend: 0.08 kg/m 2 /decade Latitude: o Trend: 0.17 kg/m 2 /decade

IGARSS 2011, Vancuver, Canada July 28, 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/m 2 /decade. Analysis period: November 21, 1996 – November 20, The uncertainties in the trends are estimated to ~0.35 kg/m 2 /decade (taking the temporal correlation into account)

IGARSS 2011, Vancuver, Canada July 28, of 14 Chalmers University of Technology Sensitivity of the trends to different time periods 21 Nov – 20 Nov Nov – 20 Nov. 2010

IGARSS 2011, Vancuver, Canada July 28, of 14 Chalmers University of Technology Summer and winter trends Summer (April – September)Winter (October – March)

IGARSS 2011, Vancuver, Canada July 28, of 14 Chalmers University of Technology Trend comparisons: GPS vs. radiosonde Analysis period: November 21, 1996 – November 20, GPS sites (bold font) vs. 7 radiosonde sites (italic font)

IGARSS 2011, Vancuver, Canada July 28, of 14 Chalmers University of Technology Trend comparisons: GPS vs. radiosonde (cont.)

IGARSS 2011, Vancuver, Canada July 28, of 14 Chalmers University of Technology Conclusions IWV trends estimated from GPS vary from –0.3 to +0.5 kg/m 2 /decade over Sweden and Finland for the last 14 years. Uncertainties in the trends are ~0.35 kg/m 2 /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, of 14 Chalmers University of Technology Thank you for your attention!