Analysis of the Effect of Rain on the Envisat Altimeter: Definition of a Rain Flag

Past experiences with ERS and Topex/Poseidon altimeter data have shown that rain can significantly alter the quality of altimeter measurements (dynamic heights, significant wave heights, wind speed) [1][2][3][4]. Among all the different atmospheric phenomena which can affect the altimeter measurement, rain is one of the less well understood and at present no reliable correction can be made for the whole range of geophysical parameters. To avoid any contamination by rain, the data which might possibly be affected are simply discarded using a flag set by using concurrent passive microwave radiometer measurements. These passive microwave data are also used to calculate an atmospheric water vapour correction to the height and can also give an estimate of atmospheric liquid water [5]. Recent studies [2][3] have analysed the effects of rain on the dual frequency (Ku and C band) Topex altimeter (NRA) data. They showed that the frequency dependence of the attenuation of electromagnetic signal by rain can be used to define a rain flag based on the detection of departures from the normal relationship between the two frequencies backscatter measurements. They also showed that this kind of rain flag based on the altimeter measurements itself performs better than the one based on coincident passive microwave data. The future Envisat altimeter will have a dual frequency capability, and will operate in Ku and S band. Compared with the ERS altimeters, this dual frequency capability can be used to defined a simple rain flag. The aim of the present study is firstly to analyse in some details the influence of rain on the Envisat altimeter waveforms and secondly to propose a rain flag for operational use.