Water HM meeting E. Obligis (CLS) and L. Eymard (LOCEAN) The wet tropospheric correction issue for the WATER HM mission Water HM meeting 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 Water HM meeting Mean differences between Topex/TMR and ECMWF dh for year cm +3cm I. Do we need a microwave radiometer ? Global quality of the meteorological models is still insufficient Radiometer wetter Radiometer dryer Water HM meeting Variance gain when using TMR dh instead of ECMWF dh -3cm +3cm I. Do we need a microwave radiometer ? Water HM meeting Scharroo et al (2004) I. Do we need a microwave radiometer ? Water HM meeting 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). I. Do we need a microwave radiometer ? ECMWF-TMR wet tropo ECMWF change 10/01/2002 Water HM meeting We need a Microwave Radiometer onboard the mission Water HM meeting 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 II. What type of radiometer ? Water HM meeting I. Do we need a microwave radiometer ? Enjolras et al (2007) Water HM meeting A scanning or a fixed radiometer ? Scanning radiometer not compliant with the plateform stability requirements driven by the altimeter II. What type of radiometer ? MeghaTropiques mission Water HM meeting 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 Diffrences between AMSU-A and AMSU-B wet tropo estimations Water HM meeting 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) Water HM meeting II. What type of radiometer ? Nadir track Interferometric swath 80km Interferometric swath 80km 30km Across track Along track Proposition One optimized reflector And several horns to cover the swath with two frequencies (Ka and K for each view) Water HM meeting 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 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 AMSU-TMI-SSMI-AMSR 3H/0.25 2H/0.25 Water HM meeting III. The mixed pixels issue Over open ocean: Accuracy: 1 cm Horizontal resolution: 50 km Mixed zones: the surrounding land surfaces contaminate the signal and make the humidity retrieval method unsuitable. Water HM meeting Topex/TMR illustration TOPEX/Poseidon coverage: blue tracks are further than 50 km to the coasts (Mercier, 2005) And not contaminated Water HM meeting We developed a dedicated simulator to better understand and quantify the difficulty Water HM meeting Evaluation of some current methods Error maps 1. without correction 2. using ECMWF path delay near coasts 3. propagating the last uncontaminated path delay cm 2 -2 cm +2 cm -2 cm +2 cm -2 cm +2 cm Water HM meeting corr ( p, f ) = [TBland( f ) TBsea ( f )] p ( f ) Desportes et al, IEEE TGRS, vol. 45, n7, pp , p : Gaussian smoothing of a land/sea 0.01 mask that takes into account the antenna pattern for each channel RMS error on contaminated PD RMS error on propagated PD RMS error on corrected PD 16 March 12.4 cm5.2 cm2.3 cm 15 April 10.9 cm4.6 cm2.6 cm A correction method based on the proportion of land in the pixel Water HM meeting The one-dimensional variational method (1D-Var) X : Control vector containing parameters to be adjusted X b : Background vector H(X) : Simulated TBs B, E, F : error covariance matrices Y 0 Measured TBs H : radiative transfer model The quality and stability of meteorological models will certainly increase in the next few years so we shall probably combine radiometer and model estimations RTTOV Radiometer ECMWF Water HM meeting 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 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