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Altimetric rain detection: Past, Present and Future PowerPoint Presentation
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Altimetric rain detection: Past, Present and Future

Altimetric rain detection: Past, Present and Future

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Altimetric rain detection: Past, Present and Future

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  1. Altimetric rain detection: Past, Present and Future Graham Quartly & Trevor Guymer

  2. Seasat Ex. 3 AGC (dB) Ex. 5 AGC (dB) Hs (m) Srokosz & Guymer (1988)

  3. ERS-1 Mechanisms for s0 change: reflection from rain clouds change in surface roughness attenuation by rain Ds0 = 2 H a Rb Guymer et al. (1995)

  4. Quantify Ds0 (1)

  5. Quantify Ds0 (2) With dual-frequency e.g. TOPEX, have a clear reference s0

  6. Quantify Ds0 (3) Ku-band s0 responds to wind and rain fields. Rescaled C-band s0 shows change due to wind alone.

  7. Fine tuning (1) TOPEX Improved s0 resolution Jan. 29th 1998. Software patch to improve s0 resolution from 0.25 dB to 0.0625 dB. [ Grateful thanks to folks at Wallops. ] NASA/Wallops web page

  8. Fine tuning (2) — Hs effect Wave height has a subtle effect, affecting the equilibrium state of roughness at the two scales (2.3 cm and 5.7 cm radiation). To get accurate estimate of rain’s effect, need to include Hs. Elfouhaily et al. (1998); Quartly et al. (1999)

  9. Envisat ...

  10. Envisat - Jason comparison Plotted as fn ( u10 ) rather than separately-scaled s0 Arbitrary vertical offset (uncertainty in absolute cal.) Increased scatter at S-band only for winds < 2m s-1

  11. S-band anomalies Sea-ice Rain Surface slicks Why use rain flags? Two purposes for rain flagging: i) remove contaminated data Lillibridge et al. (2005)

  12. Effect of DF data-editing Compare r.m.s. variability plots before and after editing Result is to reduce estimates of oceanic variability (i.e. it has removed anomalous height measurements). Lillibridge et al. (2005)

  13. Why use rain flags? Two purposes for rain flagging: ii) study rainfall at sea Quartly et al. (1996); Quartly et al. (2000)

  14. 1) All rain climatologies have large uncertainties Why study rain with altimetry? (1) Not ideal — narrow swath, long gap between repeats — in many ways SSM/I, AVHRR, TRMM are better 2) Stable algorithm — good for decadal studies

  15. Why study rain with altimetry? (2) 3) Relatively small footprint — reduce beamfilling errors, and enables fine along-track sampling 4) Can record high rain rates 5) Simultaneous wind, wave and rain info McMillan et al. (2002); Quartly et al. (1999)

  16. Jean Tournadre’s web server http://www.ifremer.fr/cersat/facilities/browse/rain/rain.htm

  17. Incorporating MWR data Altimetric rain rates require knowledge of MLH — can be derived from MWR data Ds0 = 2 H a Rb Tournadre (2006)

  18. Combining with IR data AATSR —> Optical depth MWR-2 —> Liq. Water Content RA-2 —> Rain rate Envisat Quartly & Poulsen. (2005)

  19. Summary Dual-frequency flagging effective at editing data Altimetry can give a useful contribution to rain studies Maximise potential by combining with other (simultaneous) data

  20. References Elfouhaily et al.. 1998, Estimation of wind stress using dual-frequency TOPEX data. J. Geophys. Res. 103, 25101-25108. Gulf of Alaska, 1979, ref?? Guymer et al., 1995, The effects of rain on ERS-1 radar altimeter data. J. Atmos. Oceanic Tech. 12, 1229-1247. Lillibridge et al. 2005, Rain and ice flagging of Envisat altimeter and MWR data, Envisat & ERS symposium, Salzburg, 6-10th Oct. 2004, SP-572 (8pp). McMillan et al., 2002, Validation of TOPEX rain algorithm: Comparison with ground-based radar. J. Geophys. Res. 107 (D4), 3.1-3.10. (DOI 10.1029/2001JD000872) Quartly et al., 1996, The effects of rain on Topex radar altimeter data. J. Atmos. Oceanic Tech. 13, 1209-1229. Quartly et al.. 1999, Global precipitation statistics from dual-frequency TOPEX altimetry. J. Geophys. Res. 104, 31489-31516. Quartly et al., 2000, Changes in oceanic precipitation during the 1997-98 El Niño. Geophys. Res. Lett. 27, 2293-2296. Quartly and Poulsen, 2005, Coincident cloud observations by altimetry and radiometry, Envisat & ERS symposium, Salzburg, 6-10th Oct. 2004, SP-572 (5pp). Srokosz and Guymer, 1988, A study of the effect of rain on Seasat radar altimeter data. Proc. of IGARSS '88, 651-654. Tournadre, 1998, Determination of rain cell characteristics from the analysis of TOPEX altimeter echo waveforms, J. Atmos. Oceanic Tech. 15, 387-406. Tournadre 2006, Improved Level-3 oceanic rainfall retrieval from dual frequency spaceborne radar altimeter systems, to appear in J. Atmos. Oceanic Tech. References

  21. Indian Ocean rain regimes Principally swell (calm) conditions Principally freshening seas (storm) conditions Quartly et al. (1999)

  22. Envisat - Jason comparison Plotted as fn(Ku-band s0) Arbitrary vertical offset (uncertainty in absolute cal.) Increased scatter at S-band only for winds < 2m s-1

  23. Rescaled power in wavebins Modelled, according to derived rain cells Waveform analysis Higher spatial resolution studies possible using waveform data. Tournadre (1998)