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Effects of Cloud/rain on Ka band altimeter

Effects of Cloud/rain on Ka band altimeter. J. Tournadre et al Ifremer Brest. OSTS Hobart, March 2007. Altika : altimeter in Ka band (see Noubel and Verron presentations)

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Effects of Cloud/rain on Ka band altimeter

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  1. Effects of Cloud/rain on Ka band altimeter J. Tournadre et al Ifremer Brest

  2. OSTS Hobart, March 2007 • Altika : altimeter in Ka band (see Noubel and Verron presentations) • Better performances but one major problem at Ka band (35.75 GHz) rain and cloud can strongly attenuate the signal and distort the echo waveform. • Important to quantify : • Impact on echo waveforms and geophysical parameters estimate (use of MLE4 algo) • Percentage of data possibly lost due to rain and/or clouds

  3. Previous studies (Tournadre 1999). In presence of rain larger than 1-2 mm/hr the distortion of the waveform will inhibit the geophysical parameter retrieval. • At Ka band the attenuation by cloud droplet is about 1.1 dB/km par g/m3 (10 time larger than Ku band), not negligible. Cloud at Ka = rain at Ku . • Cloud more frequent than rain. Necessity to analyze in detail the effect of cloud on the signal • Waveform modeling in presence of cloud • Estimation of attenuation, off-nadir angle, leading edge slope at 1Hz and 20 Hz as a function of cloud parameters (IWC, height, diameter,..) • Estimation of the impact of cloud on the geophysical parameters (ssh,s0, SWH) retrieval: waveform modeling and MLE4 (Stenou et al). Determination of data availibility • Rain/cloud flag definition based on the signal analysis (Altika is single-frequency)

  4. Waveform modeling • Analytical model of waveform based on Brown model + an attenuation term • A attenuation by cloud or rain • k attenuation coefficient in dB/km • rain • cloud • R rain rate, mv liquid water content (g/m3)

  5. For clouds • Attenuation depends on Integratedd cloud liquid water (ICLW) • Simpler than the rain case

  6. Cloud liquid water Typical values for the properties of clouds. The values are merely modal-means. The range of observed values is quite large. The radius of cloud droplets is r (microns), the effective optical radius optically is r', N is the number of droplets per cubic centimetre, L is the liquid water content of the cloud (g/m3). For all clouds, the level of observation is just below the freezing level, except for fog and cirrus. (Hess et al 1998)

  7. Cloud : water droplet and ice cristalsIce has a much lower attenuation coefficient and can be neglected in the computation.

  8. Cloud at a given time!!

  9. Cloud liquid water content data • Several satellite sensors gives estimates of the integrated cloud liquid water content such as SSM/I, AIRS or MODIS on Aqua/Terra. • However to have a good modeling we need high resolution data.

  10. Waveform distortion by rain and cloud 0.1 1 10 Ka band : strong attenuation by rain and cloud But more important: strong distortions of the waveform shape : modification of leading edge and plateau slope Return power Attenuation Altika WF over a 10 km 2.5 mm/hr rain cell

  11. Altika unavailability by rain : Use of JASON ENVISAT,TOPEX rain climatology: same sampling as Altika; Probability of rain greater than 1-1.5 mm/hr

  12. Waveform modeling: cloud with gaussian distribution of ICLW • ICLW. • Cloud radius • Distance nadir/cloud • Attenuation • Off-nadir angle Stronger distortion for small high ICLW cloud

  13. Comparison 1Hz 20Hz Small difference on attenuation for cloud radius larger than the footprint (~8km) For ICLW <0.5kg.m² weak impact of cloud (att<1 dB z<0.01°²)

  14. Quantification of the impact of cloud on geophysical parameter see Desjonquéres Presentation Other approach : definition of a rain and cloud flag • Altika : single frequency altimeter : implies a definition of a flag based on the analysis of the signal itself (sigma0 and or waveform). • Critical for light to very light rain and cloud with high water content. • Radiometer liquid water estimate are not sufficient for cloud/rain flagging (see past experience with Topex and Jason) • Flag based on sigma0 and off-nadir angle estimate variations. • Test on modeled waveform using satellite cloud liquid water measurements.

  15. Analysis of s0 and z² variations within a cloud. Nuage à IWC constant Nuage IWC exponentiel High small scale highly correlated variations of s0 et z

  16. Off-nadir angle variations attenuation Modeled waveforms using (10 km resolution 10 km) AIRS liquid water data from AIRS on AQUA

  17. Detection of s0 et z² variations • Method: Wavelet decomposition (here symmlet 8) of s0 et z • Use « matching pursuit » algorithm (Mallat et al, 1997) • Determination of the most significant wavelet from an energy point of view (named atoms) • Each event is characterized by its amplitude and scale (possibility to discriminate by scales)

  18. Exemple AIRS modeled waveforms White noise added to attenuation and z Reconstructedd signal from atoms

  19. Synthesis • IWC • Attenation • Off_nadir • + attenuation atoms • O z atoms

  20. MODIS05 (1km)

  21. IWC • Attenation • Off_nadir • + attenuation atoms • O z atoms

  22. Deformation can be important even for low IWC (~0.2kg/m²) • For IWC >0.6 kg/m2 rain most probable • Below 0.2 kg/m² few data will be contaminated • Distortion depends more strongly n the variability of IWC within the footprint than mean value

  23. To be done : Availability maps Test of the feasibility of an operational rain/cloud flag based on s0 and z variations • Modis06 level3 daily products clouds. • Cloud fraction, cloud liquid water histograms

  24. Conclusion • Model shows that the mean impact of cloud is weak for CLWC lower than 0.2 kg/m² • For higher CLWC it will be necessary to flag the data • Wavelet analysis of the sigma0 and off-nadir angle variations can be used to flag the data affected by rain and clouds..

  25. Problem: Definition of a rain flag • As Altika certainly mono-frequency the rain flag can not be similar to the JASON or ENVISAT one (dual frequency). • Critical for light to very light rain (<1 mm/hr) and some non-raining clouds where microwave radiometer data can not be used for rain detection • New rain flag to be defined : modelisation show that it can be based on variation of sigma0 and off-nadir (plateau) angle Example variation of off-nadir angle induced by rain Off_nadir as function of distance from rain cell and rain cell diameter for 0.5 mm/hr Off-nadir as a function of rain celle diameter

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