Gravity waves near the Island of Hopen in the Barents Sea

This image shows an ERS-1 SAR image which includes an atmospheric lee-wave pattern associated with the island of Hopen. The image clearly shows the island surrounded by loosely packed ice floes, with a large ice-free area on the western side which was created by easterly winds which had persisted during the previous 24 hour period. Click here to better identify the feature.

Six well-defined wave crests of 7.6 km wavelength and oriented nearly parallel to the island are readily apparent. Using the calibrated ERS-1 SAR image and a wind-retrieval model, we can estimate the amplitude of these waves, in addition to their wavelength scale, directly from the SAR image.

Thus, these SAR observations are unique as compared with visible image remote sensing studies of atmospheric lee waves, in which only the wavelength scale and the arrangements of the wave crests, based upon the cloud pattern, could be analyzed.

Using an ERS-1 scatterometer wind retrieval model, the wind speed associated with the first two wind speed maximum was estimated to be 12±3 m/s, and the intervening minimum wind speed was estimated to be 3±3 m/s.

Furthermore, it was shown that the observed wavelength and horizontal wind speed modulation are consistent with a simple lee-wave model which includes an exponential decay of the Scorer parameter (the ratio between the buoyancy frequency and the wind speed) in the lower troposphere, and a bell-shaped barrier. This model includes forced motion over the barrier plus a resonant oscillation downwind.

In this case, the lee waves were trapped by a thin surface-based inversion layer and accompanying wind shear aloft. The lee wave model was configured based upon atmospheric sounding taken at Bear Island, some 250 km to the south-west of Hopen.

Lee waves are a special case of atmospheric gravity waves in which the wave motion is forced over a terrain obstacle, that is, they are generated by orographic interaction. In the steady state, lee waves are stationary with respect to the terrain feature, but they propagate relative to the main airflow above the Earth's surface.
Observations of lee waves are rather common in visible remote sensing imagery: they manifest themselves as wave-like patterns. Spatially periodic uplifting of moist air results in condensation and forms cloud patterns associated with the lee-wave crests.

The main advantages of analyzing lee waves rather than more generic atmospheric gravity waves are that the wave pattern may be taken as steady-state (not propagating) and that the vertical and horizontal motion associated with the lee wave may be estimated since the lower boundary condition must follow the known terrain profile.

NERSC, Bergen, Norway

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