Von Kármán vortex streets to the lee of the Canaries

By Xavier Calbet and Jochen Kerkmann (EUMETSAT)

The ocean and atmosphere are fluids in constant motion and we are aware of such motion when we feel the wind or a strong current. But we rarely have the possibility to observe large scale fluid motion in the ocean and atmosphere. An exception is this feature is, in effect, lines or streets of spiral eddies. They are called von Kármán vortices after Theodore von Kármán (1881–1963, co-founder of NASA's Jet Propulsion Laboratory) who first described the phenomenon in the atmosphere.

Von Kármán vortex streets form at all scales of fluid motion. They consist of two roughly parallel rows of vortices positioned such that a vortex in one row is situated opposite the mid-point of two adjacent vortices in the other row (see Figure 2 below).

Initially, the vortices are comparable in size to the island creating the disturbance. As the vortices move downstream they tend to increase in diameter and they may persist to form a wake 100km wide and several hundred of kilometres in length.

A pair of vortices is created by an island roughly once every eight hours and may have a duration as long as 30 hours. All the vortices in one row have similar circulation, but opposite to that in the other row.

In Figure 1, it can be seen that the vortices diverge as they become larger further downstream in the wake.

Von Kármán vortices will normally form in any region where fluid flow is disturbed by an object. In the atmosphere, the resulting vortex streets are easily identified by the characteristic formations in stratocumulus cloud sheets. Such clouds are often associated with a well-marked temperature inversion in the lower levels of the atmosphere.

The height of the temperature inversion is particularly important because it determines whether the air flows more easily around the island rather than over it. In addition, the low level wind flow should be steady, at around 10m/s, and the surface isobars (lines of equal atmospheric pressure) should be generally straight over a large area.

Figure 2 is the Meteosat-7 visible imagery showing the vortex street on 4 April 2001.

The areas in the world where these conditions are most frequently encountered are the regions of the Trade Winds. Thus places to look out for vortex streets would be the Canary Islands, Madeira Island, Cape Verde Islands, Guadalupe and Socorro Islands (west of Baja California in the Pacific Ocean) and the Juan Fernandez Islands (off the Chilean ast). Some sample images are shown below (Figures 3–5).

Figure 5 shows Von Kármán vortex streets in the lee of the island of Socorro, Mexico (1050m). The island is part of a group known as the Revillagigedo Archipelago, located about 400km off the southern tip of Baja, California.

The Meteosat-8 imagery in Figure 6 shows a nice case of Von Kármán vortex streets to the lee of the Canary Islands. In particular, a nice pair of vortices can be seen to the lee of Gran Canaria (see image interpretation). The animation shows how the vortices form and how they move away from the islands.

Another small Von Kármán vortex can be seen in the upper left part of the image within the large field of stratocumulus clouds. This vortex is coming from Madeira Island, where it was formed some hours before by the same process (Figure 7)

The case from 8 June 2005 also shows the importance of the horizontal and temporal resolution (1km, 15 minutes) of the Meteosat-8 images (Figure 7). In fact, the stratocumulus clouds over the Atlantic are not stationary but undergo rapid changes. For example, in addition to the Von Kármán vortices one can see how the stratocumulus clouds dissolve from east to west due to: 1) the diurnal heating of the Sun and 2) the advection of dry air at low levels (below the inversion at 925hPa) coming from the African continent.

Other interesting features are the gravity waves (waves that typically form in stratocumulus fields with stable stratification, see also Low-level gravity waves within a layer of Bénard cells) that seem to move in opposite directions: from SW to NE in the lower left part of the image and from NW to SE in the upper part of the image (Figure 8).

Additional content

Radiosounding Tenerife (8 June 2005, 12:00 UTC. Source: Univ. of Wisconsin)
Wind vectors and temperature at 925hPa (8 June 2005, 12:00 UTC. Source: NOAA Air Resources Laboratory)
Wind vectors and temperature at 850hPa (8 June 2005, 12:00 UTC. Source: NOAA Air Resources Laboratory)