Validation of MERIS Near IR water vapour retrievals using MWR and GPS measurements. ENVISAT Project: 853 (HAZARDMAP)

Transcription

1 Validation of MERIS Near IR water vapour retrievals using MWR and GPS measurements ENVISAT Project: 853 (HAZARDMAP) Jan-Peter Muller, Zhenhong Li, Paul Cross, Peter Albert*, Robert Watson**, and Tim Hewison*** *Free University Berlin, ** University of Bath ***UK Met. Office, Exeter Research Project co-funded by the Department of Geomatic Engineering 10 November 2003

2 Contents Introduction to CWAVE 03 Intercomparison of MERIS, MWR and GPS PWVs for CWAVE03 for 10 July 2003 Spatio-temporal intercomparison of MERIS and GFZ GPS PWV over Germany Spring vs. Autumn Winter vs. Summer Intercomparison of WAVEs and RS profiles Intercomparison of MERIS and 3D WAVEs from GPS Conclusions Research Project co-funded by the

3 Introduction to CWAVE 03 Joint EU-CLOUDMAP2 and CLOUDNET project for intercomparison of instruments and Cloud Resolving Models: CWAVE03 (Clouds, Water Vapour Experiment 2003) Period: 14 June to 11 July 2003 Instruments used here: 3 GPS receivers 1 Microwave Radiometer (MWR) Research Project co-funded by the

4 CWAVE 03: GPS and MWR displayed on SRTM DEM N.B. Triangles show GPS locations whilst square shows location of MWR

5 Intercomparison of MERIS, MWR and GPS PWVs (CWAVE 03, 10 July 2003) GPS( ) MERIS HILL NTL SPAR Chilbolton 31.2 MWR (g) MERIS 33.3 N.B. Unit: mm

6 GASP* GPS Network *GPS Atmosphere Sounding Project Research Project co-funded by the (

7 GFZ GPS PWV Products Software: GFZ EPOS Accuracy: 1-2 mm (Gendt et al., 2001) Processing Strategy (Reigber et al. 2002; Li et al., 2003) Elevation cut-off angle: 7 12 hour data window Sampling rate: 2.5 min ZWD with resolution of 30 min Near Real Time: 1.5h Research Project co-funded by the

8 MERIS vs. GPS PWV over Germany (1) Research Project co-funded by the Period: Oct 2002 Sep 2003

9 MERIS vs. GPS PWV over Germany (2) Spring Autumn

10 MERIS vs. GPS PWV over Germany (3) Summer Winter

11 WAVEs: : GPS 3D PWV Model WAter Vapour Extraction Software developed by Z Li (UCL) for deriving 3D tomographic PWV fields from multiple GPS stations Advantages of WAVEs 3D/4D Model: Reasonable spatial resolution (1-3km) Covariance matrix of ZWD available Full refractivity profile can be recovered Continuous (for 4D) Research Project co-funded by the

12 WAVEs & Assumptions Refractivity Assumptions: 1. x(i,j,k) is constant 2. x(k>h 0 ) =0 3. Horizontal constraints: Weighted average of neighbouring voxels 4. Vertical constraints: Exponential function of the height to scale height ratio (from MacDonald et al., 2002)

13 WAVEs: : Applications to CWAVE 03 Model Extent: 17 x 27 x 9 km Model Resolution: 1 x 1 x 1 km Features: MWR NOT used to construct 3D PWV model MWR used to define the weights of constraints MWR used to check the model Not optimal but results still encouraging Research Project co-funded by the

14 Refractivity Profiles of RS and WAVEs N.B. refractivity ~ PWV/6.2 Distance: 26.8 km

15 MWR vs. WAVEs 3D PWV Date: 10 July 2003 Time: 2:00 ~ 22:00 Interval: 10 min N.B. Red square show the pair for the MERIS Overpass time

16 MERIS vs. WAVEs 3D PWV (Diff = MERIS WAVEs) N.B. 1. Grey means no MERIS data available because of clouds 2. 27% of MERIS-WAVEs are within the 1.6mm MERIS accuracy limits; 3. 83% of MERIS-WAVEs are within 2-sigma; 4. Overall, the closer to GPS sites, the smaller the differences.

17 Research Project co-funded by the Conclusions Intercomparison of MERIS and GPS PWV: High correlation: 0.99 Small Standard deviation: 1.1 mm Slope: 1.03 In cold conditions, further research is required: GPS drier? or MERIS wetter? WAVEs and GPS 3D model in CWAVE 03 Profiles of WAVEs and RS are comparable Good correlation between WAVEs and MWR Spatial comparison of WAVEs and MERIS: Satisfactory for such a small GPS network.

18 Acknowledgements We would like to express our appreciation to Prof. Christoph Reigber, Dr. Gerd Gendt and Dr. Yanxiong Liu for providing us with the GFZ GPS products, and to Charles Wrench and Elizabeth Slack for providing us with surface pressure and temperature at Sparsholt. Research Project co-funded by the