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Name: Arvid G.
Status: other
Age: 60s
Location: N/A
Country: N/A
Date: 2000-2001


Question:
I am fascinated by jet streams, and have read http://146.139.100.40/webpages/askasci/wea00/wea00065.htm

Questions - 1. Please describe the mechanisms of the jetstream terms divergence and convergence, and the consequences of these.

2. I understand that the jetstream is driven by the effects of the sun. Please explain how.

Arvid,


Replies:
The jetstream is not really driven by the sun. The jetstream forms at breaks in the tropopause, in other words, where the tropopause changes height dramatically. Over North America there can be as little as one tropopause break (and therefore one jet) or three breaks (thus three jets). Sometimes two jets can split from one or combine into one. The northernmost jet is called the polarfront jet (it is usually located over the northern states), the southernmost one is called the subtropical jet (the latter usually hangs around the southern border of the United States). Sometimes there is another jet inbetween. The height of each jet (and the break in the tropopause height) depends on latitude. The subtropical jet is higher (often much higher, 10-16 km) than the polarfront jet (7-12 km).

Divergence and convergence are terms used to describe how air is flowing into or out of a weather system of any kind and is not specific to jetstream analyses. Convergence implies that air is flowing into the system and therefore upwards at the center (cooling as it rises), as in a low pressure system. Divergence implies that air is sinking (and therefore warming) as air flows out of the system (and the system expands) as in a high pressure area. Jetstreams normally result in convergence beneath them. They are often involved in the genesis of weak low pressure systems at the surface, especially in the wintertime. The low pressure system in the central part of the country today (Dec. 18, 2000) is a good example of this. The low pressure system normally is found just north and west of a jet that is oriented northeast-southwest and bulges to the north, as today. I often say to people that the low pressure area rides the jetstream bubble, west of this bulge. Compare the upper level map and the surface map on Intellicast weather to see this.

Another important aspect of the jetstream is the turbulence at the edge (clear air turbulence, or CAT) that results from the wind shear between the high jetstream speeds inside and the slower wind speeds outside it. This has damaged many aircraft and shaken up many thousands of passengers as commercial airlines use it to decrease travel times.

David R. Cook
meteorologist
Argonne National Laboratory


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