1. OSTST, March 12-15, 2007 – 1 Charles Desportes (CLS)& Estelle Obligis (CLS), Laurence Eymard (LOCEAN) On the Wet Tropospheric Correction for Altimetry in Coastal Regions

2. OSTST, March 12-15, 2007 – 2 Introduction The exploitation of altimetric measurements over ocean relies on the possibility to correct the altimeter range from atmospheric humidity perturbation. A dedicated instrument, a microwave radiometer, is added to the mission Over open ocean: Accuracy: 1 cm Horizontal resolution: 50 km Coastal zones: the surrounding land surfaces contaminate the signal and make the humidity retrieval method unsuitable.

3. OSTST, March 12-15, 2007 – 3 Introduction TOPEX/Poseidon coverage: blue tracks are further than 50 km to the coasts (Mercier, 2005)

4. OSTST, March 12-15, 2007 – 4 1 – Data used ALADIN analysis (0.1° resolution meteorological model, analysis every 12 hours) validated by many in-situ measurements during the FETCH experiment (March – April 1998) in the northwestern Mediterranean Sea 10 meter windspeed, March 16: Mistral case, strong northern wind, dry atmosphere Wet tropospheric correction (Path Delay), April 15: wet atmosphere Collocated TOPEX radiometer (TMR) measurements: TBs at 18, 21, 37 GHz

5. OSTST, March 12-15, 2007 – 5 2 – Radiometer simulator Objective : evaluate the current tropospheric correction methods near coasts in any type of geographical configuration met on Earth  It allows the generalization to any configuration Track 187, cycles 202 (March 16) and 205 (April 15) We simulate the radiometer behavior

6. OSTST, March 12-15, 2007 – 6 ALADIN model : Surface P, T Windspeed 10 m profiles P, T, H (15 P levels) Radiative transfer modelling (Prigent, Boukabara, Guillou) :  Sea surface : geometric optic theory (Cox and Munk), foam (Monahan, Droppleman), dielectric constant (Ellison)  Atmosphere : oxygen and water vapor absorption (Liebe 93), cloud liquid water absorption (Mie) SEALAND Oceanic surface emissivity model (wind & SST) F. Karbou emissivity Land surface emissivity atlas (from AMSU-A) at 23.8 GHz: 0.25° resolution F. Karbou, C. Prigent, L. Eymard, and J.R. Pardo, “Microwave land emissivity calculations using AMSU measurements”, IEEE Trans. Geosci. and Remote Sensing, vol. 43, no 5, pp. 948-959, May 2005 2 – Radiometer simulator

7. OSTST, March 12-15, 2007 – 7 Simulated TBs at 18, 21, 37 GHz Antenna lobe: simulations are weighted by a Gaussian function Main lobes simulated by a Gaussian function. ALADIN model : Surface P, T Windspeed 10 m profiles P, T, H (15 P levels) SEALAND Oceanic surface emissivity model (wind & SST) F. Karbou emissivity 2 – Radiometer simulator

8. OSTST, March 12-15, 2007 – 8 Comparison between measures and simulations along track :  Validated for 2 different characteristic configurations  2D maps are simulated: allow the evaluation in any possible configuration 2 – Radiometer simulator

9. OSTST, March 12-15, 2007 – 9 3 – Evaluation of some current methods Error maps: difference between ALADIN wet path delay & retrieved path delay from simulated TBs 1. without correction 2. using ECMWF path delay near coasts 3. propagating the last uncontaminated path delay 1 3 +2 cm 2 -2 cm +2 cm -2 cm +2 cm -2 cm +2 cm

10. OSTST, March 12-15, 2007 – 10 4 – Using the proportion of land in the footprint corr ( p, f ) = [TBland( f ) – TBsea ( f )] × p ( f ) R. Bennartz, “On the use of SSM/I measurements in coastal regions,” J. Atmos. Oceanic Tech., vol. 16, pp. 417-431, April 1999 p : Gaussian smoothing of a land/sea 0.01° mask No θ information Any configuration TBsea / TBland estimation at the nearest pixel with p=0 / p=1 But the assumption of a linear dependency is not always valid

11. OSTST, March 12-15, 2007 – 11 Performance analysis, comparison with the propagation method: TBs from simulator along real TOPEX tracks, we add translated tracks TBs correction Path delay retrieval For each transition case (Sea → Land, Land → Sea, island, tangent track), we calculate the RMS error between each PD and ALADIN PD. Then we calculate the quadratic mean of these RMS errors (in this way, longer transitions (smaller θ) are not favored): 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 4 – Using the proportion of land in the footprint

12. OSTST, March 12-15, 2007 – 12 5 – Application to simulated TBs  Propagation method introduces huge gaps  Proportion method gives the best results Creus Cape Ibiza Island Algeria France

13. OSTST, March 12-15, 2007 – 13 6. Conclusions The objective of the study was:  to analyze in detail the contamination of the brightness temperatures by land,  to evaluate the methods used up to now to provide the wet tropospheric correction in transition areas,  to propose a new approach, more accurate and adapted for an operational processing. The proposed method is robust and results obtained on simulations are satisfactory. Desportes et al, IEEE Transaction on Geoscience and Remote Sensing, in press

14. OSTST, March 12-15, 2007 – 14 6. Perspectives (1/3) 1. Evaluation of the potential of high frequency (AMSU-B config.) radiometers to provide better wet tropospheric correction near coasts : higher resolution, weaker secondary lobe Weak land contamination effect Africa France

15. OSTST, March 12-15, 2007 – 15 6. Perspectives (3/3) 2. Feasibility of a retrieval with a one-dimensional variational method (1D-var) based on the adjustment, near coasts, of surface emissivity, temperature and wet tropospheric correction Operational Met Office Tool from Satellite Application Facility for Numerical Weather Prediction Minimization of the J cost function X : Control vector containing parameters to be adjusted X b : Background vector H(X) : Simulated TBs at TOA B, E, F, error covariance matrices Y0 Measured TBs