1. Climate applications of Ground-Based GPS KNMI 1.12 2003 Professor Lennart Bengtsson ESSC, University of Reading MPI for Meteorology, Hamburg

2. Why do we need to monitor water vapour? Water vapour is the dominant greenhouse gas and enhances climate warming significantly Substantial change (+40%) is expected during this century

3. The role of the water cycle in the climate system Precipitation is crucial for life on the planet The largest warming factor of the atmosphere is through the relaease of latent heat amounting to 80-90 WM -2 The net transport of water from ocean to the land surfaces amounts to some 40000 km 3 /year Precipitation over land is about 3 times as high Water vapour is the dominating greenhouse gas. Removing the effect of water vapour in long wave radiation reduces climate warming at 2 x CO 2 by a factor of more than 3. (For the GFDL model from 3.38 K to 1.05 K).

5. Annual mean global values of relative humidity f (in %) vertically averaged for 850-300 hPa and vertically integrated absolute humidity q (in kg/m 2 ).

6. Onsala, SwedenE  = 0.13 mm Elgered

7. SummerWinter 1995-2000 Trends

10. Integrated Water vapour 1978-1999 ECHAM5: T106/L31 using AMIP2 boundary conditions Preliminary results: Globally averaged results vary between 25.10 mm (1985) and 26.42 mm (1998) Mean value for the 1990s is 1% higher than in the 1980s Interannual variations are similar as in ERA-40 Variations follow broadly temperature observations from MSU (tropospheric channel) under unchanged relative humidity (1°C is equivalent to some 6%).

11. How has atm. water vapour varied over the last 50 years? Objective: to extend estimates of IWV for longer time periods (Re-analyses exists now for some 55 years)

12. 1. How well can we determine IWV from GCM forced by observed SST? 2. How well can we determine it from analyses with observations typical of the pre-satellite period? 3. How well can we determine it from the present observing and assimilation systems? Questions:

13. Methodology We have mimicked earlier observing systems by redoing the ERA-40 assimilation for limited periods. This has been done at the ECMWF computer system from ESSC at Reading University

14. IWV Decadal trend 1979-2001 ERA-40: + 0362 mm IWV/TLT: 3.15 mm/C ( presumably only half as much) ECHAM 5 (1979-1999): + 0.290 mm IWV/TLT: 1.54 mm/C NCEP/NCAR: - 0.056 mm

15. We have done four main experiments 1. ERA 40 - all humidity observations 2. Exp 1 - all space observations 3. Exp 2 - all upper air observations 4. Exp 1 - all upper air observations

16. Experimental periods DJF 1990/1991 JJA 2000 DJF 2000/2001

17. We call the experiments: 1. ERA-40 dry (b02d) 2. No space observations (b03v) (NOSAT) 3. Surface observations only (b046) 4. Space observations only (b040)

18. Integrated water vapor DJF 1990/91

21. IWV Decadal trends 1958-2001 ERA-40 IWV/TLT : 2.85 mm/C ( 4.05 mm/C) ERA 40 (corr) IWV/TLT : 1.55 mm/C

22. IWV Decadal trends 1958-2001 ERA-40: + 0.405 mm ERA 40 (corr) : + 0.155 mm ( ERA-40(24.9mm) - NOSAT (23.8mm) = -4.3%) NCEP/NCAR: - 0.238 mm

24. GPS data from global IGS network 1997 - present

25. The IGS network of ground-based GPS stations

27. How well does the ECMWF operational forecasting system analyse IWV as compared to those retrieved from GPS measurements? To what extent is it possible to separate errors in model analysed IWV from IWV obtained from GPS measurements? What are the most likely sources of errors? What are the long term requirements for a GPS-based water vapour monitoring system?

28. Comparing operationally analysed IWV and IWV calculated from surface based GPS measurements 1. Calculate IWV from GPS signal delay (temperatures from ECMWF operational analyses and pressure from the GPS station or if not available interpolate from met stations) (Care must be taken in the vertical interpolation of moisture due to the fact that the model height differs from station height)

29. Determination of integrated water vapour, IWV Methodology Zenith path delay, ZPD, from GPS measurements Obtain temperature from operational weather prediction model Surface pressure from met. observations Calculate IWV

30. Error estimates Measurement error: co-located GPS stations (mid- latitudes) indicate very small errors (<0.7 mm IWV) Horizontal interpolation error: (<100 km) and small vertical interpolation (<200 m) also indicate small errors (<0.6mm)

31. Results We undertook intercomparison between IWV analyses and GPS calculated GPS for Jan. and Jul. 2000 and 2001. For all stations we calculated the standard deviation of the daily differences (SD) and the monthly average difference (bias). 1. When both SD and bias were small we concluded that both GPS and the analyzed fields were reliable with an error of the same order as the difference between the estimates.

32. Results 2. When both SD and bias are large it is generally not possible to conclude whether the GPS measurements or the analyzed values are incorrect or both. However, a detailed inspection of the daily differences can be helpful. 3. When SD is large and the bias is small then we may conclude that it is more likely that GPS is more reliable than the analyzed value. 4. When SD is small and the bias large then we may conclude that it is more likely that the analyzed values are more accurate then the GPS.

33. January 2001: IWV [mm] for station METS (Finland)

34. July 2000: IWV [mm] for station LPGS (Argentina)

35. July 2000: IWV [mm] for station CEDU (South Australia)

36. January 2001: IWV [mm] for station Gough Island (South Atlantic)

37. July 2000: IWV [mm] for station Diego Garcia (Indian Ocean)

38. July 2000: IWV [mm] for station Guam (West Pacific )

39. July 2000: IWV [mm] for station MALI (Kenya)

40. July 2000: IWV [mm] for station NKLG (Gabon)

41. July 2001: IWV [mm] for station NLIB (Iowa, USA)

42. January 2001: IWV [mm] for station NLIB (Iowa, USA)

43. January 2001: Vertical humidity profile from radiosonde measurements and OA at station Quad City (Iowa, USA) OA

44. July 2001: Vertical humidity profile from radiosonde measurements and OA at station Quad City (Iowa, USA) OA

45. January 2001: IWV [mm] for station HOFN (Iceland)

46. July 2001: IWV [mm] for station HOFN (Iceland)

47. January 2000: IWV [mm] for station Ascension (Trop. Atlantic)

48. July 2000: IWV [mm] for station Ascension (Tropical Atlantic)

49. Regional station averages of normal. bias (GPS- OA) (IWV bias divided by GPS derived IWV) in %

50. Regional station averages of normal. bias (GPS- ERA 40) (IWV bias divided by GPS derived IWV) in %

53. Concluding Remarks Atmospheric water vapour is a key climate parameter Indications are that water vapour mixing ratio is conserved in the atmosphere so IWV will follow Clausius-Clapeyrons relation ( near exponential increase by temperature) Long-term climate monitoring of IWV is essential GPS observations are here very useful. It is strongly recommended to establish a long-term operational network based on an extended world-wide GPS-network

54. END Thanks to Stefan Hagemann, MPI for Meteorology, Hamburg Gert Gendt, GeoForschungsZentrum, Potsdam