1. Annette Eicker GRACE and geophysical applications Annette Eicker Institute of Geodesy and Geoinformation University of Bonn TexPoint fonts used in EMF. Read the TexPoint manual before you delete this box.: AAAAAAAAA TexPoint fonts used in EMF. Read the TexPoint manual before you delete this box.: AAAAAAAAA

2. Annette Eicker Outline 2 The GRACE mission - observation principle What are the challenges? Applications (examples)

3. Annette Eicker The GRACE mission 3

4. Annette Eicker Gravity Recovery and Climate Experiment GRACE: launch: March 2002 altitude: ~450 km distance: ~250 km orbit period: 94 min polar orbit GRACE: launch: March 2002 altitude: ~450 km distance: ~250 km orbit period: 94 min polar orbit JPL 4

5. Annette Eicker GRACE Observations 5

6. Annette Eicker GRACE Observations 6 distance: ca. 250 000 000 000 μm K-band microwave ranging instrumentaccuracy: < 1 μm GPS receiver:accuracy: 2-3 cm K-band microwave ranging instrumentaccuracy: < 1 μm GPS receiver:accuracy: 2-3 cm Comparison: average human hair: 0.05 mm => 50 μm (keep in mind: speed: 27.500 km/h) accuracy: 12 digits

7. Annette Eicker GRACE results: gravity field 7 Long-term mean gravity field (differences of gravity compared to ellipsoid) Long-term mean gravity field (differences of gravity compared to ellipsoid) gravity anomalies [mGal]

8. Annette Eicker GRACE results: gravity field 8 Long-term mean gravity field (differences of gravity compared to ellipsoid) Long-term mean gravity field (differences of gravity compared to ellipsoid) Temporal variations e.g. monthly models Temporal variations e.g. monthly models

9. Annette Eicker GRACE results: gravity field 9 Temporal variations e.g. monthly models Temporal variations e.g. monthly models First time observation of temporal gravity field variations on the global scale! download GRACE solutions: http://icgem.gfz-potsdam.de/ICGEM/ New measurement type for hydrology, oceanography, glaciology, geophysics…

10. Annette Eicker 10 But there are also some difficulties….

11. Annette Eicker Outline 11 The GRACE mission - observation principle What are the challenges? Applications (examples)

12. Annette Eicker Challenges 12 Why can GRACE data be a little difficult? GRACE observes the gravity field from far away => Downward continuation 1)

13. Annette Eicker Upward/downward continuation 13 450 km Ground level [ mgal ] Gravitational potential in spherical harmonics 6378 km 6378 + 450 km (10.000 km) (160 km) (110 km) Dampening factors Satellite altitude [ mgal ]

14. Annette Eicker Upward/downward continuation 14 Ground level Satellite altitude dampening of small features (high frequencies) GRACE sees only smoothed version of gravity field c c observation noise Amplification of (high frequency) noise noise [ mgal ]

15. Annette Eicker Challenges 15 Why can GRACE data be a little difficult? GRACE observes the gravity field from far away => Downward continuation 1) The gravity field changes continuously, but it takes time to collect the data => Aliasing 2)

16. Annette Eicker Ground tracks 16

17. Annette Eicker It takes time to collect satellite data, but the gravity field changes continuously It takes time to collect satellite data, but the gravity field changes continuously GRACE 30 days 15 days 1 day 17

18. Annette Eicker GRACE 18 tides:  tidal forces (sun, moon, planets)  Earth tides  ocean tides atmospheric variations and the reaction of the ocean pole tides Short-periodic gravity changes: It takes time to collect satellite data, but the gravity field changes continuously It takes time to collect satellite data, but the gravity field changes continuously atmospheric variations

19. Annette Eicker GRACE 19 tides:  tidal forces (sun, moon, planets)  Earth tides  ocean tides atmospheric variations and the reaction of the ocean pole tides Short-periodic gravity changes: It takes time to collect satellite data, but the gravity field changes continuously It takes time to collect satellite data, but the gravity field changes continuously ocean tides

20. Annette Eicker GRACE 20 tides:  tidal forces (sun, moon, planets)  Earth tides  ocean tides atmospheric variations and the reaction of the ocean pole tides Short-periodic gravity changes: Short-term gravity variations have to be reduced BUT: every model has errors Models: JPL DE405 IERS2003 EOT11a AOD1B OMCT IERS2003 It takes time to collect satellite data, but the gravity field changes continuously It takes time to collect satellite data, but the gravity field changes continuously

21. Annette Eicker Aliasing 21 unmodelled variations residual signal (after reduction of models) monthly models Undersampling of the short-term variations => „Aliasing“ This results in …

22. Annette Eicker Monthly solution 22 Why north-south stripes? very high measurement accuracy along-track sampling along the orbit => gravity field (unmodelled short-periodic effects) might have changed completely between neighboring arcs 2007 - 04

23. Annette Eicker Challenges 23 Why can GRACE data be a little difficult? GRACE observes the gravity field from far away => Downward continuation 1) The gravity field changes continuously, but it takes time to collect the data => Aliasing 2) We have to do something about the noise => Filtering, Leakage 3)

24. Annette Eicker Filtering 24 Gaussian filter 2007 - 04 water height [cm]

25. Annette Eicker Filtering 25 2007 - 04 Filter: 200 km Gaussian filter water height [cm]

26. Annette Eicker Filtering 26 2007 - 04 Filter: 250 km Gaussian filter water height [cm]

27. Annette Eicker Filtering 27 Filter: 300 km Gaussian filter 2007 - 04 water height [cm]

28. Annette Eicker Filtering 28 Filter: 400 km Gaussian filter 2007 - 04 water height [cm]

29. Annette Eicker Filtering 29 Filter: 500 km Gaussian filter 2007 - 04 water height [cm] stronger filtering => less noise But: filtering implies spatial averaging also of the signal => „Leakage effect“

30. Annette Eicker Leakage 30 modelled signal (unfiltered, without noise) signal (500 km Gauss filter) stronger filtering => less noise But: filtering implies spatial averaging also of the signal => „Leakage effect“

31. Annette Eicker modelled signal (unfiltered, without noise) signal (500 km Gauss filter) Leakage 31 average: 7.4 cm/year average: 4.0 cm/year stronger filtering => less noise But: filtering implies spatial averaging also of the signal => „Leakage effect“ river basin

32. Annette Eicker Filter Leakage 32 Leakage Signal original Leakage-Out Leakage-In Region of interest stronger filtering => less noise But: filtering implies spatial averaging also of the signal => „Leakage effect“

33. Annette Eicker Filter Leakage 33 Leakage Signal original Leakage-Out Leakage-In Region of interest Generally damping of signal in region of interest => underestimation of amplitude => Estimation of re-scaling factor to obtain full signal. (Can be difficult!)

34. Annette Eicker Challenges 34 Why can GRACE data be a little difficult? GRACE observes the gravity field from far away => Downward continuation 1) The gravity field changes continuously, but it takes time to collect the data => Aliasing 2) We have to do something about the noise => Filtering, Leakage 3) GRACE observes the integral mass signal => Loading, Signal separation 4)

35. Annette Eicker Loading 35 Mantle Crust Mass GRACE measures gravitational potential => conversion to mass But: GRACE has no depth perception

36. Annette Eicker Loading 36 GRACE measures gravitational potential => conversion to mass Mantle Crust Mass mass gain mass loss equivalent water height elastic response of the Earth (load love numbers) gravity field coefficients Signal separation of variations at surface and in mantle using loading theory But: GRACE has no depth perception

37. Annette Eicker Signal Separation 37 Atmosphere Ocean GIA Ice Mantle and Crust Separation of integral mass signal: Reduction of unwanted signals using models => Model errors included in mass estimate statistical / mathematical approaches (e.g. PCA, ICA) Hydrology

38. Annette Eicker Outline 38 The GRACE mission - observation principle What are the challenges? Applications (examples)

39. Annette Eicker GRACE results 39 (ITG-Grace03) Already reduced: tides (ocean, Earth, …), atmosphere & ocean variations

40. Annette Eicker Annual amplitude water height [cm] Trend and amplitude Trend water height [cm/year] 40

41. Annette Eicker water height [cm] Annual amplitude 41

42. Annette Eicker Hydrology 42 1gt = 1km³ water! [giga tons] Amazon

43. Annette Eicker Hydrology 43 1gt = 1km³ water! [giga tons] Amazon Orinoco equator

44. Annette Eicker Hydrology 44 GRACE time series provide valuable information to hydrologists WHY? Improvement of global hydrological models canopy snow soil groundwater local lakes local wetlands river global wetlands WaterGAP: models water storages and flows on 0.5° x 0.5 ° grid global lakes (Döll et al 2003) Problems: model physics, insufficient data coverage (e.g. percipitation)

45. Annette Eicker Hydrology 45 1gt = 1km³ water! [giga tons] Amazon GRACE

46. Annette Eicker Hydrology 46 1gt = 1km³ water! [giga tons] Amazon GRACE WaterGAP Underestimation of amplitude in the model Possible solution: model calibration

47. Annette Eicker Hydrology 47 Underestimation of amplitude in the model! Possible solution: model calibration GRACE WaterGAP WaterGAP calibrated (Werth et al. 2009)

48. Annette Eicker Trend 48

49. Annette Eicker India 49

50. Annette Eicker India 50 Rodell et al. (2009), Nature Groundwater withdrawal seems to be detectable by GRACE GRACE

51. Annette Eicker India 51 canopy surface waters soil snow groundwater e.g. altimetry e.g. SMOS Groundwater withdrawal seems to be detectable by GRACE Why is this so important? For the first time it is possible to observe groundwater changes from space

52. Annette Eicker Trend Greenland Antarctica Alaska 52

53. Annette Eicker Trend 53

54. Annette Eicker Mass loss in Greenland as observed by GRACE water height trend [m/year] Greenland 54 (GFZ-RL05 time series) water height [m] (ITG-Grace regional solution) Jakobshavn glacier Acceleration?! Leakage is very important (and difficult) in Greenland!! Mass loss: ~ 240 Gt/year

55. Annette Eicker 55 How is the melted ice distributed in the oceans? Sea level change

56. Annette Eicker Ice Sea level 56 What happens when ice is melting in Greenland? Ocean Greenland Water is being attracted 1.Step: Ice generates gravity Simulated sea level change after 40 years FESOM (Brunnabend et al 2012)

57. Annette Eicker Sea level 57 What happens when ice is melting in Greenland? 1.Step: Ice generates gravity Ocean Greenland Sea level is sinking at the Greenland coast Simulated sea level change after 40 years FESOM (Brunnabend et al 2012) At same time: - continents rise (reduced loading) - sea floor deforms (increased loading) - this again changes gravity - …

58. Annette Eicker Sea level 58 Ocean Greenland Sea level is sinking at the Greenland coast At same time: - continents rise (reduced loading) - sea floor deforms (increased loading) - this again changes gravity - … 1.Step: Ice generates gravity What happens when ice is melting in Greenland? Simulated sea level change after 40 years FESOM (Brunnabend et al 2012) „Fingerprint“ of Greenland ice melting

59. Annette Eicker Contributions to sea level change Ice Hydrology (Jensen et al. 2013) Sea level trend as observed by GRACE (Riva et al. 2010) 59

60. Annette Eicker Sea level 60 Ocean Greenland Sea level is sinking at the Greenland coast At same time: - continents rise (reduced loading) - sea floor deforms (increased loading) - this again changes gravity - … 1.Step: Ice generates gravity What happens when ice is melting in Greenland? 1.Step: Ice generates gravity 2.Increase in global temperature heats the ocean (=> volume change) Simulated sea level change after 40 years FESOM (Brunnabend et al 2012) Cannot be observed by GRACE „Fingerprint“ of Greenland ice melting

61. Annette Eicker Altimetry 61 Determination of geometric sea level variations 61

62. Annette Eicker Sea level (NASA) 62 Altimetry observes both mass variations and volume change (steric sea level change) Separation only possible when combining GRACE and altimetry

63. Annette Eicker Sea level Böning et al. (2012) 63 Altimetry

64. Annette Eicker Böning et al. (2012) Sea level 64 Altimetry Sea level drops 6 mm in 2010 ! What happened here?

65. Annette Eicker Böning et al. (2012) Sea level 65 Altimetry

66. Annette Eicker Böning et al. (2012) Sea level 66 Altimetry Water redistribution observed by GRACE

67. Annette Eicker Trend per year 67

68. Annette Eicker Glacial isostatic adjustment (GIA) 68 Mantle Crust Ice Mantle Crust Ice 1 2 Mantle Crust Ice 3 Mantle Crust 4 Viscoelastic response of the Earth

69. Annette Eicker (Sasgen 2011) Glacial isostatic adjustment (GIA) GRACE Model (adjusted to GRACE)

70. Annette Eicker Trend 70

71. Annette Eicker Sumatra-Andaman earthquake (December 2004) Han et al. 2006, Science Earthquakes

72. Annette Eicker Sumatra-Andaman earthquake (December 2004) Han et al. 2006, Science Tohoku earthquake (Fukushima, April 2004) GRACE Model Matsuo and Heki, 2011 Earthquakes

73. Annette Eicker Summary and outlook 73 The GRACE mission - observation principle GRACE offers many interesting applications, e.g. in hydrology, oceanography, glaciology, geophysics, … but there are still enough challenges left, => we are far from completly undertstanding the data Summary: Applications (examples) Outlook: end of GRACE mission lifetime to be expected in the next 1-3 years (batteries are a problem!) GRACE follow-on mission will be launched August 2017 => longer time series, important for climate research GRACE-FO: laser interferometer as technoloy demonstrator => distance between satellites 5-50x more accurate but that does not mean that the gravity field will be equally more accurate