Research papers The effect of the spatially inhomogeneous wind field on the wave spectra employing an ERS-2 SAR PRI image Nelson Violante-Carvalho a,n , Ian Robinson b , Christine Gommenginger b , Luiz Mariano Carvalho c , Francisco Ocampo-Torres d a Rio de Janeiro State University (UERJ), Faculty of Oceanography—FAOC, R. S ~ ao Francisco Xavier, Rio de Janeiro 20550-900, Brasil b National Oceanography Centre, Southampton (NOCS), European Way, Southampton SO14 3ZH, United Kingdom c Rio de Janeiro State University (UERJ), Institute of Mathematics and Statistics—IME, Brasil d Centro de Investigacio ´n Cientı ´fica y Educacio ´n Superior de Ensenada (CICESE), Km 107 Carretera Tijuana-Ensenada, Ensenada 22860, Mexico article info Article history: Received 24 February 2011 Received in revised form 10 December 2011 Accepted 12 December 2011 Available online 2 February 2012 Keywords: SAR wave spectra ERS-1&2 SAR precision image Interaction between wind sea and swell abstract Using wave spectra extracted from image mode ERS-2 SAR scenes, the spatial homogeneity of the wave field in deep water is investigated. From the 100  100 km image, several small images of 6.4  6.4 km are selected and the wave spectra are computed. The locally disturbed wind velocity pattern, caused by the sheltering effect of large mountains near the coast, translates into the selected SAR image as regions of higher and lower wind speed. Assuming that the swell field is uniform over the whole image, SAR derived swell spectra retrieved from the sheltered and non-sheltered areas are intercompared. Any difference between them could be related to a possible modification associated with the sheltering effect on the wind speed and/or a possible interaction between wind sea and swell, since the wind sea part of the spectrum would be slightly different due to the different wind speeds. The results show that there is no significant modification, and apparently there is no clear difference in the swell spectra despite the different wind sea components and wind speeds. & 2012 Elsevier Ltd. All rights reserved. 1. Introduction Most of the studies that have examined the effects of long waves (swell) on shorter (wind sea) were carried out in laboratory tanks (for example, Phillips and Banner, 1974; Donelan, 1987), or through numerical simulations (Masson, 1993; Young et al., 1995). Several works seem to indicate that wind waves are attenuated in the presence of long waves, however, there is no consensus regarding the exact causes of the suppression and to what extent. The nonlinear wave–wave interaction term (S nl ) in the wave energy equation has been shown to play an important role in the coupling between swell and wind sea causing the attenuation of the shorter waves in the presence of the longer waves in the frequency range just below the peak frequency of the wind sea. The coupling causes the swell to grow at the expense of the wind waves, but it seems to be only significant if swell and wind waves are close in frequency space. The suppression of wind waves by the swell depends on the swell’s steepness and also on the relative direction of propagation between swell and wind sea, where the coupling is, according to numerical experiments, maximum at about 401 (Masson, 1993). The work by Phillips and Banner (1974) pointed out that the suppression of wind waves is due to enhanced dissipation (S ds term), which is caused by modulation of the short waves by the long waves. Moreover, their theoretical results predict that wind waves are attenuated by opposing swell, which were not corro- borated by other authors. The results presented by Mitsuyasu and Yoshida (2005), for example, point in the opposite direction, with wind waves intensified by an opposing swell. Chen and Belcher (2000), on the other hand, suggested that the attenuation of wind waves by swell is related to reduction in the magnitude of the wind input source term S in . They proposed that the swell reduces the momentum from the wind, therefore hampering the development of the wind sea. More recently, the results reported in Garcı ´a-Nava et al. (2009) suggested that in low wind conditions swell increases drag, whereas at higher winds, swell reduces drag from the wind. Therefore, the presence of swell, depending on the wind speed, could slow down or speed up the development of the short waves. The matter is yet open to discussion, and the interaction is not well investigated in the open ocean where swell is ubiquitous. It is not clear whether or how its presence would change the mechanics of wave growth (Dobson et al., 1989; Hanson and Phillips, 1999; Violante-Carvalho et al., 2004). The main difficulty Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/csr Continental Shelf Research 0278-4343/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.csr.2011.12.006 n Corresponding author. Tel.: þ55 21 2334 0903. E-mail addresses: n_violante@uerj.br, violante_carvalho@yahoo.co.uk (N. Violante-Carvalho). Continental Shelf Research 36 (2012) 1–7