1. Intercomparison of GlobVapour total column water vapour (TCWV) and Envisat RA2-MWR measurements (preliminary results) Bruno Picard (CLS), Jean-Rémi Deboer (CLS) Martin Stengle (DWD), Marc Schröder (DWD) Estelle Obligis (CLS), Marie-Laure Frery (CLS) bpicard@cls.fr (+33)5 6139 3737 bpicard@cls.fr

2. ISRO-CNES Meeting Ahmedabad, sept 7th and 8th Gewax /GlobVapour WS ESRIN, 08/03/2010 Objectives A comparison of GlobVapour TWVC against RA2-MWR product (microwave radiometer on Envisat) The present study is focusing on 4 monthly SSMI/Meris combined products The results are preliminary and show what metrics could be used when more data will be available Page 2

3. ISRO-CNES Meeting Ahmedabad, sept 7th and 8th Gewax /GlobVapour WS ESRIN, 08/03/2010 Context Altimetry missions usually embed a microwave radiometer in charge of providing the wet tropospheric correction ([cm]) to the radar signal MWR are 2-bands (23.8/36.5 GHz, ERS-1/2, RA2-Envisat [ESA]) or 3-bands (+18.7GHz, Topex, Jasons [NASA/CNES]) Page 3

4. ISRO-CNES Meeting Ahmedabad, sept 7th and 8th Gewax /GlobVapour WS ESRIN, 08/03/2010 Context Altimetry missions usually embed a microwave radiometer in charge of providing the wet tropospheric correction ([cm]) to the radar signal MWR are nadir 2-bands (23.8/36.5 GHz, ERS- 1/2, RA2-Envisat [ESA]) or 3-bands (+18.7GHz, Topex, Jasons [NASA/CNES]) Page 4

5. ISRO-CNES Meeting Ahmedabad, sept 7th and 8th Gewax /GlobVapour WS ESRIN, 08/03/2010 Wv for altimetry The wet tropospheric correction  6 cm of wet tropo. corr = 1 g/cm 2 TWVC Page 5 « dry » term (  2m) provided by meteorological models with a good accuracy « wet » term, almost proportional to the integrated water vapor (1cm = 1.66 kg/m 2 ), highly variable in space and time, ranging from a few cm to about 50 cm, not provided with enough accuracy by meteorological models (actual resolution of 50km/6hours)

6. ISRO-CNES Meeting Ahmedabad, sept 7th and 8th Gewax /GlobVapour WS ESRIN, 08/03/2010 Altimetry requirements To provide the wet tropospheric corrrection : with a good accuracy (≈ 1 cm rms) with a good stability (≈ 1 mm/year) with no geographically correlated errors GCOS recommandation: < 0.05mm/yr ! Page 6

7. ISRO-CNES Meeting Ahmedabad, sept 7th and 8th Gewax /GlobVapour WS ESRIN, 08/03/2010 RA2-MWR RA2-MWR is a 2-bands (23.8/36.5 GHz) nadir Dicke radiometer (resolution of about 20 km) Wet tropo. corr and TWVC are retrieved by a neural network algorithm developed by CLS E. Obligis, L. Eymard, et al, “First three years of the microwave radiometer aboard ENVISAT: In-flight calibration, processing, and validation of the geophysical products,” J. Atmos. Ocean. Technol., vol. 23, no. 6, pp. 802–814, Jun. 2006. Data are available since 2002 Page 7

8. ISRO-CNES Meeting Ahmedabad, sept 7th and 8th Gewax /GlobVapour WS ESRIN, 08/03/2010 Reminder BUT, when using RA2-MWR products, precaution should be taken on the coherency of the dataset Algorithms have been constantly improved leading to discontinuities  some RA2-MWR events since BOM Oct.24, 2005, v5.02, 1st day of cycle 42, L1b upgrade:  MWR Side Lobe correction upgrade Jan.12, 2010, v6.02, 1st day of cycle 86, L1b/L2 upgrade:  new in-flight calibration / new NN algorithm Page 8

9. ISRO-CNES Meeting Ahmedabad, sept 7th and 8th Gewax /GlobVapour WS ESRIN, 08/03/2010 Reprocessing Page 9 Envisat RA2 whole mission reprocessing in on-going The present RA2-MWR data have been reprocessed using the CLS ESL MWR L1B reference prototype applying a consistent set of algorithms

10. ISRO-CNES Meeting Ahmedabad, sept 7th and 8th Gewax /GlobVapour WS ESRIN, 08/03/2010 SSMI/Meris combined product 4 monthly mean 0.5°x0.5° SSMI/Meris combined TWVC product (prototype): July 2007, August 2007, January 2008, August 2008 Page 10 January 2008 (missing data ?)

11. ISRO-CNES Meeting Ahmedabad, sept 7th and 8th Gewax /GlobVapour WS ESRIN, 08/03/2010 RA2-MWR (reproc) product Along-track nadir TWVC over a month has been averaged on a 0.5°x0.5° map Altimetry validity flag has been used to edit data of poor quality Page 11 January 2008

12. ISRO-CNES Meeting Ahmedabad, sept 7th and 8th Gewax /GlobVapour WS ESRIN, 08/03/2010 RA2-MWR (reproc) - GlobVapour mean bias = -0.74 kg/m 2 mean rms = 7.11 kg/m 2 no geographical pattern Page 12 January 2008

13. ISRO-CNES Meeting Ahmedabad, sept 7th and 8th Gewax /GlobVapour WS ESRIN, 08/03/2010 RA2-MWR (reproc) - GlobVapour mean bias = -0.74 kg/m 2 mean rms = 7.11 kg/m 2 larger dispersion around 15 kg/m 2 Page 13

14. ISRO-CNES Meeting Ahmedabad, sept 7th and 8th Gewax /GlobVapour WS ESRIN, 08/03/2010 RA2-MWR (reproc) - GlobVapour Impact of the reprocessing –official product –reprocessed product larger bias with the reprocessed product BUT, more stable in time Page 14 mean of diff [kg/m 2 ] std of diff [kg/m 2 ] months since 07/2008

15. ISRO-CNES Meeting Ahmedabad, sept 7th and 8th Gewax /GlobVapour WS ESRIN, 08/03/2010 Conclusion Comparison of SSMI/Meris with RA2-MWR shows good agreement a bias of about 0.8 kg/m 2, constant in time (?) no geographical pattern Page 15

16. ISRO-CNES Meeting Ahmedabad, sept 7th and 8th Gewax /GlobVapour WS ESRIN, 08/03/2010 Further study Expected improvement for this study – > 15 kg/m 2 dispersion ? –more data from GlobVapour ! SSMI/Meris but also MWR 1D-Var daily product ? –validation against: model: ECMWF TWVC in-situ measurements: radiosonde Page 16

17. ISRO-CNES Meeting Ahmedabad, sept 7th and 8th Gewax /GlobVapour WS ESRIN, 08/03/2010 THANK YOU! bpicard@cls.fr bpicard@cls.fr (+33)5 6139 3737 Page 17