The wet tropospheric correction issue for the WATER HM mission

1. The wet tropospheric correction issuefor the WATER HM mission E. Obligis (CLS) and L. Eymard (LOCEAN)

2. The presence of water vapor in the troposphere induces an additional delay in the altimeter range measurement that has to be corrected for. • This term is between 0 and 50 cm, and characterized by a high variability in space and time • A microwave radiometer is usually added to altimetry missions (ERS1, Topex-Poseidon, ERS2, Jason1, Envisat, Jason2, AltiKa, Sentinel topography mission…) • Do we need a radiometer on the WATER HM mission ? • What type of radiometer ? • The mixed « land-sea » pixel issue

3. I. Do we need a microwave radiometer ? Global quality of the meteorological models is still insufficient Mean differences between Topex/TMR and ECMWF dh for year 2003 Radiometer wetter Radiometer dryer +3cm -3cm

4. I. Do we need a microwave radiometer ? Variance gain when using TMR dh instead of ECMWF dh -3cm +3cm

5. I. Do we need a microwave radiometer ? Scharroo et al (2004)

6. I. Do we need a microwave radiometer ? The spatial and temporal resolutions are insufficient • The actual grid is half a degree but the resolved scales are not better than 1 degree. • Operationnal outputs every 6 hours (problem of localisation and temporal evolution of the meteorological structures) The model stability is not assured • Radiometers do not provide a constant quality product (all experienced drifts and jumps, more or less accurately corrected for) • On the contrary, meteorological models assimilate different satellite and in-situ measurements so they are less liable to drift with time • Nevertheless they regularly change due to new assimilation schemes or new assimilated data). ECMWF-TMR wet tropo ECMWF change 10/01/2002

7. We need a Microwave Radiometer onboard the mission

8. II. What type of radiometer ? • Assumption that  is not impacted by the wet tropo (same along r1 and r2) • But absolute dh is necessary to determine h • We need dh into the swath

9. I. Do we need a microwave radiometer ? Enjolras et al (2007)

10. II. What type of radiometer ? A scanning or a fixed radiometer ? • Scanning radiometer not compliant with the plateform stability requirements driven by the altimeter MeghaTropiques mission

11. II. What type of radiometer ? • Standard or high frequency radiometer ? • HF: • Much better resolution • Easier accomodation • But clouds opacity and saturation for very wet atmospheres Différences between AMSU-A and AMSU-B wet tropo estimations

12. II. What type of radiometer ? • Standard or high frequency radiometer ? • HF: • Much better resolution • Easier accomodation • But clouds opacity and saturation for very wet atmospheres • Maybe not mature enough • BF: • Worse resolution • But consolidated performances (retrieval) • New generation of antennas (15km of resolution at 800km  24km at 1300km)

13. Nadir track Across track Along track Interferometric swath 80km Interferometric swath 80km 30km II. What type of radiometer ? • Proposition • One optimized reflector • And several horns to cover the swath with two frequencies (Ka and K for each view)

14. II. What type of radiometer ? • In case of no radiometer to characterize the wet tropo into the swath, • an optimal combination of the available satellite water vapor products is possible AMSU-TMI-SSMI-AMSR 3H/0.25° 2H/0.25° 94,9% of the Jason1 pixels can benfit from an estimation at less than 2hours/0.25° Difference JMR-ECMWF over 1 cycle :1.3 cm rms Difference JMR-MERGED over 1 cycle:1.05 cm rms

15. III. The mixed pixels issue Mixed zones: the surrounding land surfaces contaminate the signal and make the humidity retrieval method unsuitable. Over open ocean: Accuracy: 1 cm Horizontal resolution: 50 km

16. Topex/TMR illustration TOPEX/Poseidon coverage: blue tracks are further than 50 km to the coasts (Mercier, 2005) And not contaminated

17. We developed a dedicated simulator to better understand and quantify the difficulty…

18. Evaluation of some current methods Error maps without correction using ECMWF path delay near coasts propagating the last uncontaminated path delay 1 +2 cm -2 cm 2 3 -2 cm +2 cm +2 cm +2 cm -2 cm

19. corr ( p , f ) = [TBland( f ) – TBsea ( f )] × p ( f ) A correction method based on the proportion of land in the pixel p : Gaussian smoothing of a land/sea 0.01° mask that takes into account the antenna pattern for each channel Desportes et al, IEEE TGRS, vol. 45, n°7, pp 2139-2149, 2007.

20. The quality and stability of meteorological models will certainly increase in the next few years so we shall probably combine radiometer and model estimations The one-dimensional variational method (1D-Var) RTTOV H : radiative transfer model Y0 Measured TBs X : Control vector containing parameters to be adjusted Radiometer H(X) : Simulated TBs Xb : Background vector ECMWF B, E, F : error covariance matrices

21. CONCLUSIONS • Meteorological models are not yet accurate enough (resolution, accuracy, stability) to provide the wet tropo with the required accuracy (1cm for current missions) • Wet tropo is needed in the entire interferometric swath. The use of a nadir estimation implies an error higher than 2 cm rms depending on the position into the swath • The nadir estimation could be provided by a bi-frequency radiometer • one around the 22.235 GHz water vapor absorption line • one around 37 GHz for cloud liquid water content • Surface roughness may be provided by the altimeter backs. coefficient • A pushbroom radiometer with a big reflector and several horns to cover the swath could be a solution • On going activities to improve the wet tropo in case of mixed pixels…