Lee-waves and eddies behind isolated islands

[janm1] [janm1]

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Studies of three-dimensional lee-wave pattern in the air flow over islands in the Norwegian and the Barents Sea started around 1976 when high resolution satellite images first became available. Under certain atmospheric conditions lee-waves and regular vortex streets develope in the wake of Jan Mayen (71.0 N, 8.5 W). The lee-wave pattern resemble the classical ship-wave pattern. Eddies drifting away organize in a Karman vortex street. The lee-waves and eddies may extend several hundred kilometer downstream in the wake. Wave length ranges from 1-15 km. The diameters of the eddies are of the order 25 km.

The lee-waves and eddies in the wake of Jan Mayen are caused when airflow is diverted mainly by the island isolated conical mountain, Beerenberg (B), 2277 m high .

Eddies are normally generated in situation with a shallow inversion layer intersecting the mountain below about 1800 m. The period of the vortex shedding depends on wind speed and stratification and ranges from 25-100 minutes. The eddy shedding has also been recorded on the island as periodic oscillations in air pressure and shift in the wind direction.

Similar wave pattern were found also behind Bear Island and Hopen in the Barents Sea

Under certain conditions a remarkable ship-wave like pattern also forms in the airflow over Stad, on the west coast of Norway. The waves are clearly a corner effect due to the topography of the coastline and the waves appear mainly over the sea. Tom Marthinsen examined this case in his doctor thesis (University of Oslo,1982).

Publications

B. Gjevik and T. Marthinsen, (1978) ``Three-dimensional lee-wave pattern", Quart. J. Roy. Met. Soc., 104 , 947--957.

B. Gjevik, (1978) ``Le-bølger og le-hvirvler", Været, 2, 14-23, Universitetsforlaget, Oslo.

T. Marthinsen, (1980) ``Three-dimensional lee waves", Quart. J. Roy. Met. Soc., 106 , 569--580.

B. Gjevik, (1980) ``Orographic effects revealed by satellite pictures: Mesoscale flow phenomena", In Orographic effects in planetary flows. GARP Publications Series, No 23, p. 301-316. World Meteorological Organization, (WMO).

author: Bjørn Gjevik <bjorng@math.uio.no>