To be able to use altimetry measurements, we first have to correct them for the effects of atmospheric water – either rain or vapour. One of the side benefits of altimetry satellites is therefore acquiring information about these meteorological parameters, especially rain, for the entire ocean where there are very few meteorological stations. Such studies enable us to gain a better understanding of rain mechanisms and improve altimeter corrections, thus providing increasingly precise data.
Intense storms developing off the coast of West Africa, as seen by several instruments onboard Envisat: background colour map from the AATSR infrared sensor (initially for measuring sea surface temperature), rain rate from the RA-2 altimeter (black along-track curves) and liquid water content from the MWR radiometer (red along-track curves). The optical depth is a measure of cloud thickness. With all three instruments operating simultaneously, it can be noted that rain occurs where the optical depth is greatest, although the spread of active ‘rain cells’ is narrower than the expanse of dense clouds.
(Credits National Oceanography Centre, Southampton/Rutherford Appleton Laboratory)
Each frequency of a dual-frequency altimeter responds differently to rain. This not only makes it possible to accurately detect rain events, but can also be used to yield quantitative values [Tournadre and Morland 1997; Quartly et al. 1999; Chen et al. 1997, 2003; Cailliau and Zlotnicki 2000; McMillan et al. 2002]. However, there are two main problems with these techniques: one is the limited time and space sampling of nadir-pointing instruments, and the other is uncertainty about the height of the melting layer (freezing level) needed to infer the surface rain rate from the measured Ku-band attenuation. Sampling can be enhanced by merging several altimeter measurements, while the microwave radiometer carried by most altimetry satellites enables the latter to be estimated [Tournadre, 2006].
Monthly mean rain rate computed from Topex/Poseidon, for November 1997 (top, at the height of the 1997-1998 El Niño) and November 1999 (bottom, during La Niña). During El Niño rains are more abundant over the warm water pushed toward the South American coasts, whereas during La Niña most rains are over Indonesia and the ‘Warm Pool’.
– Cailliau, D. and V. Zlotnicki, Precipitation detection by the TOPEX/Poseidon dual-frequency radar altimeter, TOPEX microwave radiometer, Special Sensor Microwave/ Imager and climatological ship reports, IEEE Trans. Geosci. Remote Sens., 38, 205–213, 2000.
– Chen, G., B. Chapron, J. Tournadre, K. Katsaros, and D. Vandemark, Global oceanic precipitations: a joint view by Topex and the TopexMicrowave Radiometer, J. Geophys. Res., 102, 10, 457–10471, 1997.
– McMillan, A.C., G.D. Quartly, M.A. Srokosz and J. Tournadre, 2002. Validation of TOPEX rain algorithm: Comparison with ground-based radar, J.Geophys. Res., 107 (D4), 3.1-3.10. (DOI10.1029/2001JD000872).
– Quartly, G., M. Srokosz, and T. Guymer, Global precipitation statistics from dual-frequency TOPEX altimetry. J. Geophys. Res., 31,489–31,516, 1999.
– Tournadre, J. and J. Morland, The effect of rain on Topex/Poseidon altimeter data: a new rain flag based on Ku and C band backscatter coefficients, IEEE Trans. Geosci. Remote Sens., 1117–1135, 1997.
– Tournadre, J., Improved Level-3 oceanic rainfall retrieval from dual frequency spaceborne radar altimeter systems, J. Atmos. Ocean. Tech., (submitted).