Global precipitation statistics from dual-frequency TOPEX altimetry

Abstract

TOPEX is a nadir-pointing dual-frequency radar altimeter that has been in orbit for more than 6 years. Empirical methods, based on the close correlation between the C band and Ku band backscatter, yield an estimate of the rain rate. The principal rain bands of the Intertropical Convergence Zone (ITCZ) are shown to be bounded by "marine deserts" receiving only one hundredth of the rainfall of the ITCZ. Analysis of the entire data set reveals variations in the frequency of oceanic precipitation over a range of timescales. For most regions the diurnal cycle of precipitation has a minimum likelihood at evening local time, with maximum being early morning for the tropics and northern oceans, but late afternoon for the Southern Ocean. On the seasonal cycle, the Pacific ITCZ reaches its southernmost limit during March-April, and is then nearly always accompanied by a secondary but weaker band an equal distance to the south of the equator. Finally, on the interannual timescale the 1997-1998 El Niño is seen to cause a broadening of the Pacific ITCZ and its translation to the south and east, with increased global levels of rainfall rather than just a geographical redistribution. High-resolution sampling shows that the length scale of rain events varies not only latitudinally, but also longitudinally according to whether they lie in regions of genesis or decay of storms. Simultaneous wind and wave information from TOPEX indicates that the rainfall in midlatitudes occurs preferentially with high winds, but in the tropics is associated with swell. The long-term near-global single-instrument data set from TOPEX is thus complementary to those from other sensors. Copyright 1999 by the American Geophysical Union.