by Andreas Wirth and Jarno Schipper (ZAMG)Jump to images
Below is a nice example of so-called von Karman vortices that developed to the lee of the island of Madeira. The pattern often forms on the lee side of mountainous islands during a situation with stratiform clouds (e.g. stratocumulus sheets) moving around on the east side of a sub-tropical high pressure system. The circumstances in which these vortices' are formed are characterised by a layered buildup of the troposphere. In the layer from the surface up to 850 hPa there is usually less wind than in the "free atmosphere". Because there is almost no mixing between the two layers an inversion can persist for some time. Such inversions often occur in the vicinity of high pressure systems. The island causing the vortices has to be high enough to penetrate the inversion. In addition, there need to be wind speeds of 5 to 15 meters per second in the lowest layer. With wind speeds less than this threshold the vortices can not develop. Higher wind speeds do not guarantee that the vortices are maintained. Once the vortices have formed they follow the guiding airflow and dissolve when they reach an area where the temperature buildup of the troposphere is different. This pattern was first discovered in the atmosphere in 1962 with the help of the American weather satellite TIROS V. Up till then this feature had not been detectable in the atmosphere.
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