OBSERVATION OF TROPICAL CYCLONES BY HIGH-RESOLUTION SCATTEROMETRY

Unprecedented views of surface wind fields in tropical cyclones therea fter TCs) are provided by the European Remote Sensing Satellite (ERS) C band scatterometer. Scatterometer measurements at C band are able to penetrate convective storms clouds, observing the surface wind fields with good accuracy. However the resolution of the measurements (50x50 km(2)) limits the interpretation of the scatterometer signals in such mesoscale events. The strong gradients of the surface wind existing a t scales of a few kms are smoothed in the measured features such as th e intensity and location of the wind maxima, and the position of the c enter. Beyond the ERS systems, the scatterometers on-board the ADEOS a nd METOP satellites, designed by the Jet Propulsion Laboratory and by the European Space Agency, respectively, will be able to produce measu rements of the backscattering coefficient at about 25x25 km(2) resolut ion. A few sets of ERS-1 orbits sampling TC events were produced with an experimental 25x25 km(2) resolution. Enhancing the resolution by a factor of 2 allows location of the wind maxima and minima in a TC with a much better accuracy than at 50 km resolution. In addition, a bette r resolution reduces the geophysical noise (variability of wind speed within the cell and effect of rain) that dominates the radiometric noi se and hence improves the definition of the backscattering measurement s. A comprehensive analysis of the backscattering measurements in the case of high winds and high sea states obtained within TCs is proposed in order to refine the interpretation of the wind vector derived from a backscattering model that is currently only calibrated up to modera te winds (< 20 m/s) in neutral conditions. Observations of the TOPEX-P OSEIDON dual-frequency altimeter are also used for that purpose. Patte rns of the surface winds in TCs are described and characteristic featu res concerning asymmetries in the maximum winds and in the divergence field are discussed.