Cloud and Rain Effects on AltiKa/SARAL Ka-Band Radar Altimeter—Part II: Definition of a Rain/Cloud Flag

The main instrument of the French-Indian AltiKa/SARAL mission scheduled for launch in 2010 is the Ka-band AltiKa altimeter. The high attenuation due to atmospheric water (liquid or vapor) at this frequency band is the major drawback of the use of Ka-band. In part I of this paper, the impact of rain/clouds on Ka-band data and on the accuracy of the estimates of the geophysical parameters has been analyzed and quantified using an analytical model of waveform. Waveform distortion and errors on the geophysical parameters can be significant particularly for small dense clouds and rain cells. It is thus necessary to flag the data potentially affected by rain and clouds. The use of a single channel for AltiKa prevents the use of the classical dual-frequency rain flag used for Topex or Jason altimeters, and requires the definition of a new flag based on the altimeter signal alone. Past studies showed that clouds and rain are characterized by sharp coherent along-track fluctuations of the off-nadir angle estimates. The new flagging algorithm is based on the analysis of the variations of this parameter by the Matching Pursuit (MP) algorithm. MP allows the decomposition of a signal into a few salient features or atoms chosen from a dictionary of elementary functions. The dictionary is here defined by the wavelet decomposition of the signal. The method has been tested on an ensemble of AltiKa passes simulated for cloudy, rainy, and cloud/rain-free situations. The false alarm rate is almost nil while the detection performances are better than 90% at a range error of 5 cm and significant wave height error of 20 cm. The flag can easily be adapted to other altimeters' data and has been used to flag several Jason-1 passes. The comparison to the operational dual-frequency flag shows that the MP flag performs better in detecting range errors and waveforms distortion, while its performances are inferior in detecting samples attenuated by rain.

[1]  Jean Tournadre,et al.  The effects of rain on TOPEX/Poseidon altimeter data , 1997, IEEE Trans. Geosci. Remote. Sens..

[2]  W. Paul Menzel,et al.  CLOUD TOP PROPERTIES AND CLOUD PHASE ALGORITHM THEORETICAL BASIS DOCUMENT , 2002 .

[3]  Praveen Kumar,et al.  Role of coherent structures in the stochastic-dynamic variability of precipitation , 1996 .

[4]  Jean Tournadre,et al.  Validation of Jason and Envisat Altimeter Dual Frequency Rain Flags , 2004 .

[5]  Graham D. Quartly,et al.  The Effects of Rain on Topex Radar Altimeter Data , 1996 .

[6]  L. Amarouche,et al.  Improving the Jason-1 Ground Retracking to Better Account for Attitude Effects , 2004 .

[7]  David L. Donoho,et al.  WaveLab and Reproducible Research , 1995 .

[8]  Bertrand Chapron,et al.  A satellite altimeter model for ocean slick detection , 2006 .

[9]  Ronald R. Coifman,et al.  Wavelet analysis and signal processing , 1990 .

[10]  S. Mallat,et al.  Adaptive greedy approximations , 1997 .

[11]  Stéphane Mallat,et al.  Matching pursuits with time-frequency dictionaries , 1993, IEEE Trans. Signal Process..

[12]  Jean Tournadre,et al.  Determination of Rain Cell Characteristics from the Analysis of TOPEX Altimeter Echo Waveforms , 1998 .

[13]  Piotr J. Durka,et al.  Stochastic time-frequency dictionaries for matching pursuit , 2001, IEEE Trans. Signal Process..

[14]  E. Obligis,et al.  Comparison of Two Jason-1 Altimeter Precipitation Detection Algorithms with Rain Estimates from the TRMM Microwave Imager , 2005 .

[15]  Jean-Michel Poggi,et al.  Wavelets and their applications , 2007 .

[16]  E. Foufoula‐Georgiou,et al.  Wavelet analysis for geophysical applications , 1997 .

[17]  Adelino R. Ferreira da Silva,et al.  Approximations with evolutionary pursuit , 2003, Signal Process..

[18]  Christopher Ruf,et al.  TOPEX/Poseidon microwave radiometer (TMR). III. Wet troposphere range correction algorithm and pre-launch error budget , 1995, IEEE Trans. Geosci. Remote. Sens..

[19]  Michael Ghil,et al.  ADVANCED SPECTRAL METHODS FOR CLIMATIC TIME SERIES , 2002 .

[20]  Jean Tournadre,et al.  Cloud and Rain Effects on AltiKa/SARAL Ka-Band Radar Altimeter—Part I: Modeling and Mean Annual Data Availability , 2009, IEEE Transactions on Geoscience and Remote Sensing.

[21]  D. Percival,et al.  Characterizing the European Sub-Arctic Winter Climate since 1500 Using Ice, Temperature, and Atmospheric Circulation Time Series , 2007 .