High Acccuracy Ocean Surface Winds from SAR Imagery

Wind speed ambiguity problem occurs for wind retrieval from SAR images because of the fact that for most of the incident angles, the NRCS on the ocean surface is dampened under high wind conditions. The problem is especially notable at the near range side of the image, or when the radar-relative wind direction is close to the radar range direction, when both ambiguous solutions are plausible. Although, the ambiguity can be easily removed by taking the smaller of the possible wind solutions for low-to-moderate winds, this can lead to large errors for hurricane-force winds.

To remove the wind speed ambiguity in hurricane wind retrievals, we suggest an approach based on the dominant U10 hurricane wind structure. This approach was evaluated with a simulated SAR image of Hurricane Rita and an actual Envisat ASAR image of Hurricane Rita, in comparison with corresponding HRD winds, Application of the ambiguity removal method is capable of improving the accuracy of the radius of maximum hurricane winds estimated from SAR images, which is a key parameter for parametrical hurricane forecast models [Kurihara et al., 1998]. If the ambiguity removal method is not applied, no clear eyewall may be evident in the retrieved winds.

For actual hurricanes, wind speed does not always monotonically decrease in the storm's outer regions as the radius increases from the maximum wind speed radius. A fluctuating wind speed often exists, associated with storm bands. In these cases, our wind speed ambiguity removal approach is still valid, and involves additional pairs of maxima threshold radii beyond the hurricane's radius of maximum winds. Thus, the ambiguity can be removed, based on pairs of threshold radii.

It must be noted that although CMOD5 was used in this study, validity of the proposed speed ambiguity removal method is not dependent on CMOD5. This is evident because the method is based on the fact that for a large range of SAR incident angles, the NRCS for strong ocean surface winds is dampened [Donelan et al., 2004; Fernandez et al., 2006]. The method is also based on the characteristic structure of hurricane winds (U10) which has a radius of maximum wind [Holland, 1980]. Both of these facts are independent of the GMF form. Our study can be repeated for any GMF which realistically exhibit the dampening behaviour in very strong winds. For example, another newly developed GMF model HWGMF_V [Fernandez et al., 2006; Shen et al., 2006, 2007]. Moreover, the suggested wind speed ambiguity method can be adopted to any GMF models that exhibit damping of the SAR NRCS under very strong winds. Thus, this methodology can potentially increase the accuracy of hurricane wind retrieval from SAR, contributing to improved hurricane forecast analysis.

Shen H., W. Perrie, Y. He (2007), On SAR wind speed ambiguities. In Proc. Int. Geoscience and Remote Sensing Symp. Barcelona, Spain. 4pp.

Shen, H., W. Perrie, and Y. He (2006), A new hurricane wind retrieval algorithm for SAR images, Geophys. Res. Lett., 33, L21812, doi:10.1029/2006GL027087.