April 20, 2012 - Ship-wave-shaped wave clouds and vortices induced by Aleutian Islands
The combination of clouds and wind created a spectacular canopy across the Bering Sea and northern Pacific Ocean in early spring, 2012. The Moderate Resolution Imaging Spectroradiometer (MODIS) aboard NASA’s Terra satellite captured this true-color image on April 17 as it passed over the region at 22:40 UTC (1:20 p.m. local time).
Marine stratocumulus clouds cover the Bering Sea in a typical mottled pattern in the north of this image, roughly aligned from north to south by the prevailing wind flow. As the moving air hits the tall volcanic peaks of the Aleutian Islands, turbulence is created. The bouncing and swirling of the air as it passes the immobile islands leaves a tell-tale trail in the on the leeward side, sculpting its motion in the clouds that cover the Pacific Ocean.
Near the center of this image, three long swirling patterns can be seen extending on the leeward side of the islands. These are von Karman vortex streets, formed by a mathematically predictable – yet quite stunning – response to moving air hitting a “bluff” body. A bluff body is an object in which the length in the direction of flow is close to the length perpendicular to the flow direction. In simpler terms, to be a bluff body, an island should be just about as wide as it is tall where the wind flows across it.
On the lee side of several other islands, the turbulent air forms patterns that look like the waves that trail behind a ship as travels through water. These are called ship-wave-shaped waves, and are caused by the rising and falling of air as it passes over the stationary islands. Disturbed air rises and falls causing peaks and troughs. Rising air cools, and because the air is moist, clouds form on the peaks. As the air falls, it warms and the clouds dissipate. It is this formation and dissipation of clouds which creates the striking cloudy-and-clear pattern of ship-wave-shaped waves.