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Mediterranean Sea Mediterranean Sea

Latitude: 35° 52' N - Longitude: 05° 14' W

This image shows in the upper part sea surface manifestations of an oceanic internal wave packet generated in the Strait of Gibraltar by the interaction of the tidal flow with underwater bottom features and in the lower part sea surface manifestations of atmospheric lee waves generated by an eastward wind blowing with 7m/s over the 600 m high mountain range Sierra del Hauz in Morocco.
Image intensity scan - Strait of Gibraltar An image intensity scan from west to east through the internal lee wave pattern east of the Maroccan coast is shown in the figure above. On the left-hand vertical coordinate axis the normalized radar cross section (NRCS) is plotted and on the right-hand vertical coordinate axis the wind speed at a height of 10 m above sea level as calculated from the wind scatterometer model CMOD4 (see section "introduction" to "atmospheric phenomena").
Strait of Gibraltar Strait of Gibraltar

Latitude: 35° 38' N - Longitude: 06° 02' W

Gravity waves generated by a easterly wind blowing through the Strait of Gibraltar and over northern Morocco.
The wave train to the north is generated by a horizontal shear in the wind field and the other wave trains are typically lee waves generated by the interaction of the wind with topographic features at Moroccan Atlantic coast.
Caspian Sea Caspian Sea

Latitude: 40° 57' N - Longitude: 50° 42' E

The wavelike pattern visible in the central part of the Caspian Sea is very likely a sea surface manifestation of a nonlinear atmospheric internal wave packet or of an atmospheric undular bore. Usually such nonlinear wave disturbances in the marine boundary layer are generated by the intrusion of colder, denser air into a stable atmosphere. The modulation structure of this sea surface pattern very strongly resembles a modulation signature detected on an ERS-1 SAR image of the North Sea. By comparison with in-situ data, the latter was identified unambiguously as being caused by a nonlinear wave pertubration in the marine boundary layer. Such wavelike disturbances in the atmosphere are frequently observed phenomena (Vachon et al. (1995), Alpers and Stilke (1996)). The dark patches in the lower part of the image are due to oil pollution.
Taiwan Strait Taiwan Strait

Latitude: 25° 12' N - Longitude: 119° 43' E

Atmospheric lee waves generated by a wind blowing over the mountainous area south of Fuzhou (Fukien, China). The crests of the lee waves are oriented nearly parallel to the coastal mountain ridges.
# Orbit Frame(s) Satellite Date Time Location
1 11168 0711 ERS-1 03-Sep-1993 22:39 Mediterranean Sea
2 25499 2889 ERS-2 06-Jun-2000 11:05 Strait of Gibraltar
3 25230 2781 ERS-1 12-May-1996 07:23 Caspian Sea
4 17773 0423-0441 ERS-1 08-Dec-1994 14:25 Taiwan Strait

If you have any comments on these images please write an e-mail to alpers@ifm.uni-hamburg.de.

Introduction

Atmospheric gravity waves, often also called internal waves, exist in layered atmospheres. They either occur as quasi-periodic waves, solitary waves or undular bores. They are often generated behind mountain ranges in which case they are called lee waves. In the steady state lee waves are stationary with respect to the terrain feature, but they are propagating relative to the mean air flow above the earth surface. Lee waves are very common in visible remote sensing imagery where they manifest themselves as wave-like cloud patterns.

However, they also can manifest themselves on the sea surface since they are associated with varying wind speed at the sea surface and thus a varying short-scale sea surface roughness.

Fig. 1: A linear gravity wave propagating from left to right in a three-layer atmosphere. (left) Assumed height profile of potential temperature for a stably stratified three-layer atmosphere. (middle) isolines of potential temperature (dotted lines), streamlines (dashed lines), and direction of the wind velocity fluctuations at the sea surface (arrows at the bottom). (right) Amplitudes of the wind speed fluctuations in the direction of the wave propagation and in the vertical direction as a function of height. (Reproduced from Alpers and Stilke, 1996)

References

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