Tropical storm Prapiroon and a von Karman vortex street


NASA’s Terra satellite captures a von Karman vortex street (left) and tropical storm Prapiroon (right) early in the day, October 18, 2012. (NASA)

The decaying typhoon, tropical storm Prapiroon, is no threat to land, safely positioned 325 nautical miles south of Yokosuka, Japan, headed out to sea.

That’s not the interesting feature. Rather it’s the von Karman vortex street spied by NASA’s Terra satellite, less commonly observed in the atmosphere’s turbulent flow.

NASA describes how this von Karman vortex street may have formed:

...[W]hen fluids encounter obstacles, they can form spiral eddies. The air-flow obstacle responsible for these vortices was likely Cheju (Jeju) Island. A volcanic peak on Cheju, Halla Mountain, rises to 1,950 meters (6, 398 feet) above sea level. The spiral eddies extended about 720 kilometers (450 miles) south of the island.


A von Karman vortex street captured by NASA’s Modis sensor on October 30, 2010. (NASA)

As the winds (clouds) reach the island, they are split by the volcano. As the flow moves around the volcano, it creates an off-centered area of low pressure in its wake, in effect a whirlpool or vortex.

As this vortex moves off, another vortex is created, alternately off-centered. This vortex again moves off repeating the process and creating these remarkable patterns in the island’s wake as long as the wind speed and direction will support it.

Jason is currently the Washington Post’s weather editor. A native Washingtonian, Jason has been a weather enthusiast since age 10.
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