Posted at 03:06 PM ET, 10/18/2012

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)
A massive swirl and a “street” of mini-swirls. These vortices in the western Pacific adjacent to one another are examples of a decaying typhoon and a phenomenon known as a von Karman vortex street.

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)
CWG’s Brian Jackson wrote about this phenomenon in November, 2010, when a vortex street formed in the Southern Ocean between Madagascar and Antarctica, also in the vicinity of a volcanic island peak. He described the process, very similar to what happened in the western Pacific earlier today:

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.

By  |  03:06 PM ET, 10/18/2012

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