Reply to comment by Q. Zheng on “Can near‐inertial internal waves in the East Sea be observed by synthetic aperture radar?”

Abstract

[1] SAR images of the sea surface are very useful to detect numerous phenomena which are driven by oceanic as well as atmospheric processes [Johannessen et al., 1994]. We think that the SAR images of D. J. Kim et al. [2005] are not exceptions as they show regular patterns of various scales in the East Sea. In particular, we investigated the specific signals denoted by A, B and C in Figure 1, which is the same as Figure 2 by D. J. Kim et al. [2005] except that it is enlarged to show feature clearly. [2] Zheng [2005] argued that the wave phase speed, which was derived by D. J. Kim et al. [2005] from two SAR images, was not a real phase speed but related to a group speed. This argument is based on an assumption that these wave-like patterns (A, B and C) are footprints of atmospheric coastal lee waves. However, it should be noticed that interpretations by D. J. Kim et al. [2005] are based on characteristics of near-inertial internal waves, which were clearly observed by an ocean buoy as well. The relationship between the phase velocity and group velocity of near-inertial internal waves is shown in Figure 2. The phase velocity and group velocity are perpendicular and the horizontal directions of two velocities are the same, but vertical directions are opposite [Thorpe, 1999]. The horizontal directions of the two velocities are shoreward because the crest of A, B and C moves to A0, B0 and C0 toward the coast. The vertical direction of group velocity of near-inertial waves is downward because the phase of the east-west currents measured at the ESROB propagates upward in the depth 5–15 meters during 140– 142 days [see D. J. Kim et al., 2005, Figure 5]. The movement of the wave crests between the A-A0, B-B0 and C-C0 on the two successively acquired SAR images is about 12 km, indicating that the horizontal phase speed (not group speed) of the waves is about 0.3 m/s in average. We can not estimate the group velocity with SAR images only, because the movements of the near-inertial internal wave group can not be detected directly on the SAR image.