Validation of Different Algorithms to Retrieve Complex Wind Fields over the Black Sea from Envisat SAR Images
Alpers, Werner¹; Mouche, Alexis²; Horstmann, Jochen³; Ivanov, Andrei⁴; Barabanov, Vladyslav⁵; Alpers, Werner^1
1University of Hamburg, GERMANY; ²CLS Radar Applications, FRANCE; ³NURC, ITALY; ⁴P.P. Shirshov Institute of Oceanology, Russian Academy of Sciences, RUSSIAN FEDERATION; ⁵Marine Hydrophysical Institute, National Academy of Sciences of Ukraine, UKRAINE

In recent years, several algorithms have been developed to retrieve near-sea surface winds from synthetic aperture radar (SAR) images acquired over the ocean. They include algorithms in which the wind direction is taken from (1) atmospheric models, like the global NCEP model (Monaldo et al., 2001) and (2) from linear features visible on SAR images using the Local Gradient Method (LG-method) (Horstmann and Koch, 2005). Recently, a new wind retrieval algorithm has been proposed by Mouche et al. (2012) which exploits also the Doppler shift induced by the motion of the sea surface on the backscattered SAR signal. The Doppler shift depends on the radial components of the surface current, the wind drift, and on sea state. An empirical relationship has been established which relates the Doppler shift to the near-surface wind speed and the surface current in look direction of the antenna. The extraction of the Doppler shift from SAR data requires a special analysis of the single-look complex (SLC) SAR data. This new wind retrieval algorithm uses all three sources of information on wind direction (atmospheric model, linear features on SAR images, and Doppler shift) and combines them using the Bayesian method.
In this paper we apply different wind retrieval algorithms to four complex wind fields that were captured by the C-band SAR onboard the Envisat satellite over the Black Sea and compare the SAR-derived wind fields with in-situ data and with wind fields calculated by a regional atmospheric model of the Black Sea based on the Weather Research and Forecasting (WRF) model. The Black Sea it is an ideal test site to validate algorithms to retrieve near-surface wind fields in complex wind environments, since this sea is surrounded by mountains of differing heights with valleys opening to the sea. Local winds blowing onto the sea over coastal mountains or through valleys as well as meso-scale atmospheric eddies are often encountered over the Black Sea (Alpers et al., 2011) These local winds, like katabatic and gap winds, foehn and bora, interact with synoptic-scale winds or with each other.
It is shown that the new algorithm yields, in general, better wind fields than the other wind retrieval algorithms. In particular, the strong dependency of the Doppler shift anomaly enables to better constrain the wind inversion in terms of wind direction. However, we present one wind event where the new algorithm, when applied blindly, does not give a realistic wind field in some areas of the imaged scene. We conclude that, although the new algorithm is highly promising, it still needs some amendments for yielding correct wind fields in a complex environment. Challenging tasks are also to properly separate the contributions from wind and waves from the contribution on surface currents and to decrease the weight of the ancillary data in the wind inversion scheme.

References

Alpers, W., Ivanov, A.Yu., & Dagestad, K.-F. (2011). Encounter of foehn wind with an atmospheric eddy over the Black Sea as observed by the synthetic aperture radar onboard Envisat. Mon. Wea. Rev., 139, 3992-4000, doi:10.1175/MWR-D-11-00074.

Monaldo, F. M., Thompson , D. R., Beal R. C., Pichel, W. G., & Clemente-Colon, P. (2001). Comparison of SAR derived wind speed with model predictions nd ocean buoy measurements. IEEE Trans. Geosci. Rem. Sens., 39, 2587-2600.

Horstmann, J. & Koch, W. (2005). Comparison of SAR wind field retrieval algorithms to a numerical model utilizing ENVISAT ASAR data. IEEE J. Oceanic Eng., 30, 508-515.

Mouche, A. A., Collard, F., Chapron, B., Dagestad, K.-F., Guitton, G., Johannessen, J. A., Kerbaol, V., & Hansen, M. W. (2012). On the use of Doppler shift for sea surface wind retrieval from SAR, IEEE Trans. Geosci. Rem. Sens., 50, 2901-2909, doi:10.1109/TGRS.2011.2174998.