Observation of a nonlinear wave disturbance in the marine atmosphere by the synthetic aperture radar aboard the ERS 1 satellite

A nonlinear wave disturbance in the marine atmosphere associated with an atmospheric cold front has been identified on a spaceborne radar image of the North Sea by its imprint on the sea surface. This nonlinear wave disturbance is either a solitary wave disturbance or an undular bore propagating in the lower marine atmosphere. The radar image showing sea surface manifestations of this atmospheric wave disturbance was acquired over the North Sea by the synthetic aperture radar (SAR) aboard the first European Remote Sensing Satellite ERS 1 on March 8, 1992. Atmospheric waves become visible on radar images of the sea surface because they are associated with a varying wind field at the sea surface which modulates the sea surface roughness and thus the backscattered radar power. The identification of the wave pattern visible on the ERS 1 SAR image as a sea surface manifestation of an atmospheric nonlinear wave disturbance has been possible because this disturbance was also captured by meteorological sensors mounted on an 80-m high meteorological mast located on the island of Heligoland. Geophysical parameters characterizing the atmospheric wave disturbance are derived from the ERS 1 SAR image and from the in situ meteorological data. The wave period in the reference frame moving with the average wind is 9.5 min, the average wavelength is 2300 m, and the propagation velocity is 4.5-4.9 m s(-1). Estimates of the amplitude of the wind speed fluctuations in the wave propagation direction derived from the meteorological measurements at Heligoland and from the gray level variations in the SAR image yield values between 1.3 and 3.0 m s(-1). The experimental data are interpreted in terms of theories of atmospheric nonlinear gravity waves.