Gravity Waves and Vortices at Heard Island

Gravity waves and Von Karman vortices at Heard Island, May 1, 2015.  Resolution is 250 m/pixel.  Image credit: NASA Aqua/MODIS.
Gravity waves and Von Karman vortices at Heard Island, May 1, 2015. Resolution is 250 m/pixel. Image credit: NASA Aqua/MODIS.

Last week at Heard Island, a pair of interesting atmospheric phenomena occurred and were nicely captured in the image above: gravity waves, and Von Karman vortices. Von Karman vortices have been mentioned here previously, and we will explore them in a little more depth later in this post.

Gravity waves are phenomena which occur when a parcel of air is moved out of equilibrium (e.g. lofted too high by a mountain) and then moves back toward equilibrium. The momentum of the air parcel will cause it to overshoot equilibrium (on both sides), oscillating back and forth across the equilibrium level until the energy is dampened and dissipated. This is similar to the wake of a boat, which will bring the water up and down until eventually it restores itself to an equilibrium level.

In the image, you can see the gravity waves formed by Mt. Dixon on the Laurens Peninsula, on the northwest corner of Heard Island (you won’t see the mountain, but that is where the waves begin). In the atmosphere, if the waves happen to take water through condensing/evaporating levels, clouds will form at the peaks and disappear in the troughs. The very nearest waves to Mt. Dixon are punctuated by these clear troughs, while further downwind there are still clouds in the troughs.

Another striking feature of the image is the Von Karman vortex street downwind of Big Ben, the volcano on Heard Island. Von Karman vortices are formed when the flow on the leeward side of the obstruction (here the volcano) becomes turbulent. The turbulence leads to eddy formation. Here, the eddies are particularly visible because, like with the gravity waves, some areas are evaporative and have no clouds, while others are condensing and do have clouds. As the vortex moves downwind from the island, gradually the eddies are slowed by viscosity and dissipate. Equilibrium moisture levels are also reached further downwind from the island, visible in the increased cloudcover.

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