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 im age 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 showi ng sea surface manifestations of this atmospheric wave disturbance was acquired over the North Sea by the synthetic aperture radar (SAR) abo ard 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 surfa ce which modulates the sea surface roughness and thus the backscattere d radar power. The identification of the wave pattern visible on the E RS 1 SAR image as a sea surface manifestation of an atmospheric nonlin ear wave disturbance has been possible because this disturbance was al so captured by meteorological sensors mounted on an 80-m high meteorol ogical mast located on the island of Heligoland. Geophysical parameter s characterizing the atmospheric wave disturbance are derived from the ERS 1 SAR image and from the in situ meteorological data. The wave pe riod in the reference frame moving with the average wind is 9.5 min, t he 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 meas urements at Heligoland and from the gray level variations in the SAR i mage yield values between 1.3 and 3.0 m s(-1). The experimental data a re interpreted in terms of theories of atmospheric nonlinear gravity w aves.