"Speckle" Wind Speed from ERS-1 SAR imagettes

The European Space Agency ERS-1 C-band VV-polarization instrument (AMI) offers the unique ability to combine interlaced scatterometer and SAR wave modes. Since a SAR only relies on its platform displacement to achieve fine azimuth resolution, the motion of the surface reduces its nominal resolution. As the wind speed increases, the SAR scene contrast seems to increase, to give important peak-to-background ratio of its power spectrum. This is mainly due to the decrease in the speckle noise level induces by migrations in the azimuth direction. In this presentation we use wind estimates from the ERS-1 scatterometer IFREMER retrieval algorithm in conjunction with observed cutoff wavelengths of ERS-1 SAR imagettes to derive a 'speckel' wind algorithm. Such a relationship will help for the definition of quasi-linear mapping transforms currently used in most of SAR spectral inversion scheme

To improve our knowledge of the behavior of the SAR mapping process, we also sought cases of extreme wind conditions. Our database includes some famous typhoon cases. It allows us to select the desired extreme cases, either near the eye of the typhoon or hundreds of kilometers away from it. To illustrate our selection we consider ERS-1 SAR imagettes from November 5, 1992 in the region 125E-140E and 5N-25N. Figure 1 gives a general overview of the wind field as estimated from ERS-1 scatterometer data by the IFREMER retrieval algorithm. In the same figure, white dots indicate position of SAR imagettes.

The situation presented corresponds to the typhoon Elsie. Elsie borned from a tropical disturbance near 6N-151E on October 18, 1992. It became a tropical storm on October 30 which developed into typhoon strength the following day. Elsie reached super typhoon intensity on November 4. A maximum intensity of 145kt was reached on November 5. Elsie was moving northwestward under influence of a mid-level steering flow of the subtropical ridge. Along the descending track crossing the cyclone, and only in this particular case, we selected five imagettes (400 km apart) starting from the north to the south Figure 2. The first two imagettes are associated with high wind speeds, while the last two ones are obtained in a calm region see Table 1 . Wind was almost in the range direction on the first imagette while, in contrast, over the last imagette the wind was almost azimuthal. In other words, wind was veering over the chosen region because the wind field pattern converges toward the cyclone eye

D spectral analysis of the five imagettes were performed and plotted in Figure 3. The first four spectra show mixed sea conditions in wavelength and/or in direction, while the last one, which corresponds to a calmer region, shows a single modulation in almost azinuthal direction. In table 1, one can note that the last column varies proportionnely to the wind speed infered from ERS-1 scatterometer.

Figure 2 (left): Five imagettes passing by the typhoon's eye region. They are 400km apart along the swath from north to south. Wind direction changes along the track.

Figure 3 (right): 2D spectra of the given imagettes.