Gravity waves near the Heligoland Island (Germany)

This ERS-1 SAR image (100Km x 100Km) has been acquired over the German Bight of the North Sea on the 8th of March of 1992 at 10:25 UT (orbit 3373, frame 2511).
The image shows a wave-like pattern east of the island of Heligoland which is an imprint on the sea surface of the atmospheric undulant bore generated by an atmospheric cold front propagating eastwards. The bright streaks near the coast are sea surface manifestations of underwater bottom topographic features.

The identification of this feature as an atmospheric undular bore has been possible because simultaneous in-situ meteorological measurements were carried out at the island of Heligoland.
The German Weather service had erected a 80 m high meteorological mast at which wind and temperature measurements were performed every minute at height levels of 10 m, 30 m, 50 m, and 80 m.
A wind with a height-averaged mean wind speed of 3.0 m/s was blowing at the time of this ERS-1 overflight from 170 deg N.

Twenty-nine minutes before the ERS-1 SAR image was taken, a pronounced periodic wind fluctuations in east-west direction were observed at Heligoland. The period of the wind fluctuations in the reference frame moving with the wind was 9.5 minutes and the wavelength 2600 m. The wind speed fluctuations at 10 m height ranged from -1.2 m/s to 1.7 m/s.

Atmospheric undular bores are solitary wave disturbances propagating on a stable-stratified layer in the lower atmosphere.
There exist several reports by pilots of small aircraft and crew members of small sailing boats who encountered a suddenly occurring succession of strong wind squalls or wind gusts of varying direction, the period of which is typically 5-10 minutes.
These unexpectedly occurring strong wind squalls associated with atmospheric undular bores can become dangerous to small landing aircrafts.
The generation of atmospheric undular bores is usually linked with the intrusion of colder. denser air into a stable or indifferently layered atmosphere.
On March 8, 1992, a cold front was moving from west to east over the German Bight. Like atmospheric lee waves, atmospheric undular bores are associated with a varying wind stress at the sea surface and thus with a varying small-scale sea surface roughness which is detectable by radar.

(From Measurements of mesoscale oceanic and atmosphere phenomena by ERS-1 SAR - Werner Alpers)

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry