InSAR water vapour correction models: GPS, MODIS, MERIS and InSAR integration

Transcription

1 InSAR water vapour correction models: GPS, MODIS, MERIS and InSAR integration Zhenhong Li (1), Eric J. Fielding (2), Paul Cross (1), and Jan-Peter Muller (1) (1): Department of Geomatic Engineering, University College London, UK (2): JPL, California Institute of Technology, Pasadena, California, USA Sponsored by:

2 Contents Introduction to water vapour products Water vapour effects on InSAR measurements InSAR water vapour correction models GPS Topography-dependent Turbulence Model (GTTM) MODIS-based water vapour correction model MERIS water vapour correction model MERIS+MODIS water vapour correction model Discussion and Conclusions

3 Water vapour products used in this study GPS MODIS MERIS (Global Positioning System) (Moderate Resolution Imaging Spectroradiometer) (MEdium Resolution Imaging Spectrometer)

4 MODIS/MERIS Channel Positions Related to PWV MODIS: 2 non-absorbing 3 absorbing MERIS: 1 non-absorbing 1 absorbing PWV retrievals rely on channel ratio techniques (Figure adapted from Gao and Kaufman [1998])

5 Comparisons of GPS, MODIS and MERIS PWV GPS, MODIS and MERIS PWV products are complementary!

6 Water vapour effects on InSAR measurements --Example 1: Southern California (29 Jan Apr 2005) 1 mm of PWV => ~6.2 mm of ZPD ZPDDM: Zenith Path Delay (ZPD) Difference Map ZPDDM Max: 7.8 cm Min: -5.2 cm Mean: -0.9 cm StdDev: 1.0 cm

7 Water vapour effects on InSAR measurements --Example 2: Southern California (07 Aug Apr 2005) ZPDDM: Max: 2.7 cm; Min: -12.8cm; Mean: -2.9 cm; StdDev: 2.9 cm

8 Water vapour effects on InSAR measurements --Example 3: Southern California (11 Sep Oct 2004) ZPDDM: Max: 5.8 cm ; Min: -16.5cm; Mean: -9.0 cm; StdDev: 3.5 cm

9 GPS Topography-dependent Turbulence Model (GTTM) Underlying assumptions of GTTM: Water vapour variations conform temporally and spatially to a statistical turbulent model; Water vapour distributions are correlated with topography. Key Finding: Interpolation methods should be applied to Zenith Path Delay differences (NOT ZPD themselves) (Li et al., 2005, JGR, in press)

10 Case study 1: GTTM and IDW correction Original interferogram: 1996/01/ /01/11 After GTTM Correction After IDW Correction Unwrapped interferogram 2D Spatial Structure Function (Li et al., 2005, JGR, in press)

11 Case study 2: GTTM correction (11 Sep Oct 2004) Left: Before correction 1.94 rad Right: After Correction 1.32 rad (Li et al., 2005, ION) Comparison of range changes: GPS vs. InSAR 0.8cm 0.6cm

12 Case study 2: Water vapour variation + Clouds (11 Sep Oct 2004) Strong water vapour variation (11/09/2004) Presence of clouds (16/10/2004) This highlights the major advantage of the GTTM (over MODIS and MERIS): It can reduce water vapour effects even under cloudy conditions!

13 MODIS-based (or GPS/MODIS integrated) water vapour correction model Basic principles: There is a scale uncertainty in MODIS near-ir water vapour products (Li et al., 2003, JGR) Only one continuous GPS station is required to calibrate MODIS scale uncertainty within a 2,030 km 1,354 km MODIS scene GPS and MODIS data can be integrated to provide regional water vapour fields with a spatial resolution of 1 km 1 km ~60 minute time difference between ENVISAT and MODIS Li, Z, J-P Muller, P Cross and E Fielding, InSAR atmospheric correction: GPS, MODIS and InSAR integration. JGR - Solid Earth, 110, B03410, 2005

14 MODIS case study: 2000/09/ /12/16 Left: Before correction 2.48 rad Right: After Correction 1.47 rad (a) Black rectangles: PWV effects; Black oval: uplift (b) Dashed rectangle and oval: uncertainties due to clouds Comparison of range changes: GPS vs. InSAR 1.1cm 0.5cm (Li et al., 2005, JGR)

15 MERIS water vapour correction model MERIS data can be acquired at the same time as ASAR data (time differences between MODIS and SAR data: ~1 hour) MERIS has better spatial resolution, up to 300 m against 1km for MODIS MERIS near IR water vapour product agrees more closely with GPS than MODIS (Li et al., 2005, IJRS) A paper on MERIS water vapour correction model is under review for Geophysical Research Letters

16 MERIS case study: 2004/08/ /01/29 Left: Before correction 2.38 rad Right: After Correction 1.49 rad (Li et al., 2005, under review for GRL) Comparison of range changes: GPS vs. InSAR 0.9cm 0.5cm (1) black solid triangles represent GPS stations w/o changes after correction (2) white squares with black borders imply improvement after correction (3) red solid circles indicate deterioration after correction

17 Comparison between MERIS and GTTM (2004/08/ /01/29) Original MERIS GTTM InSAR vs. GPS 0.9 cm 0.5 cm 0.7 cm

18 Comparison between MERIS and GTTM (2004/08/ /04/09) InSAR vs. GPS Original MERIS GTTM All GTTM Bottom 0.8 cm 0.6 cm 0.9cm 0.7 cm NB: The density of CGPS stations may be crucial under certain conditions!

19 Comparison between MERIS and MERIS-MODIS (2004/08/ /04/09) InSAR vs. GPS Original MERIS MERIS-MODIS* 0.8 cm 0.6 cm 0.6 cm *: 2004/08/07(MERIS)- 2005/04/09(MODIS): Time diff: 65 min

20 Impact of time differences on MODIS-based water vapour correction models Terra MODIS descending node: 10:30 am (local time) ERS-2 descending node: 10:30 am (local time) ENVISAT descending node: 10:00 am (local time) MODIS-ERS time difference: <60 min (usually) MODIS-ASAR time difference: ~60 min (normally)

21 Impact of time differences on MODIS-based correction model (2003/09/27) Time difference across scene: 65 min Little water vapour variation

22 Impact of time differences on MODIS-based correction model (2004/08/07) Time difference across scene: 50 min Strong local water vapour variation NB: Stripes are due to radiometric calibration errors in MODIS sensors.

23 Impact of time differences on MODIS-based correction model (2005/01/29) Time difference across scene: 5 min Little water vapour variation

24 Impact of time differences on MODIS-based correction model (2005/04/09) Time difference across scene: 65 min Little water vapour variation

25 Seasonal Frequencies of cloud free conditions Global average frequency: ~25% Good News from MODIS: Tibet: 38% SCIGN: 48% Iran: Even higher?! Area: Eastern Tibet (Sep Aug 2004) Highest: Winter Lowest: Summer (Li et al., 2005, IJRS)

26 Conclusions THREE water vapour correction models (GTTM, MODIS & MERIS) have been developed & validated It shows that the order of water vapour effects on interferograms can be reduced from ~10 mm to ~5 mm using water vapour correction model(s) The impacts of time differences on MODIS-based models depend on water vapour distribution Cloud free frequency is a major limitation for MODIS/MERIS models. BUT

27 Current & future work Water vapour field stacking + correction (Encouraging results achieved) Water vapour correction + phase stacking (Ongoing) Water vapour + Time series analysis (Ongoing) Other water vapour products, e.g. infrared

28 Questions?