Sunday, April 11, 2010

Jet Streams and Tropical Cloudbands

Jet streams

Early in the twentieth century, meteorologists predicted that there would be high-speed rivers of air in certain areas of both hemispheres at around the area of the tropopause, approximately 10 to 15 km above the surface. They posited that these would be a consequence of the physics of the rotating Earth–atmosphere system.

The German meteorologist H. Seilkopf is believed to have first used the term “strahlstromung” (jet stream) in a treatise on upper atmospheric flow in 1939. However, it was not until World War Two that anyone directly encountered these ‘jet streams’. Aircraft on high-level bombing missions over Europe and Japan encountered tremendous headwinds on occasion, up to 350 km/h, which was almost as fast as the aircraft of the day could fly. Some of the planes were virtually stationary in the sky as they flew head on into the jet streams.

Today we know that jet streams are indeed real, and they are an important part of the motion of the atmosphere. There are two main jet streams in each hemisphere, called the polar jet and the subtropical jet. They rotate in a general west to east direction high above Earth, but they frequently twist and turn, producing meanderings of north and south winds on occasion. There are times where the polar jet and subtropical jet can intersect, producing highly complex effects in the upper troposphere.

The subtropical jet is found at latitudes around 25 to 35 degrees north and south of the equator, and the polar jet between the 50 and 60 degree latitudes. All these jet streams occur at the boundaries of cold and warm air in the upper atmosphere and are typically 1.5–4.5 km across.

A jet stream made visible by cloud as it crosses eastern Canada in May 1991. Jet streams can make a significant difference in flying times across Australia. If favoured by a jet stream, a flight from Perth to Sydney can take half an hour less than the reverse trip, Sydney to Perth. (NASA Space Shuttle image - click to enlarge)

They are of considerable importance in the development and steering of low-pressure systems, and meteorologists pay careful attention to their location and general movement. The aviation industry is also very interested in jet streams because of their effect on flight times. For example, when travelling from Perth to Sydney, ‘hitching a ride’ on the high-level subtropical jet stream can save significant time and fuel. Conversely, flying into the jet stream will slow the ground speed of the aircraft considerably, and in many cases the captain will attempt to fly above or below it to save time.

Another problem with the jet stream is its association with high-level clear air turbulence—or CAT—which can be uncomfortable or downright dangerous for passenger aircraft. Because jet streams are zones of high-speed winds running within the broad-scale atmospheric flow, there are surrounding regions where the wind speeds change quickly with both height and horizontal distance. This is called wind shear and a high-speed aircraft flying through it will experience ‘bumpiness’, something like a car driving over a series of potholes. This is called turbulence.

In most cases turbulence is only slight, but sometimes severe buffeting can occur, sufficient to throw objects around the cabin and injure passengers.

Tropical cloud bands

Jet streams play an important role in the generation of tropical cloud bands that periodically develop near the equator and then move to the mid latitudes—from the southwest in the northern hemisphere and the northwest in the southern hemisphere.

These cloud bands are the result of complex interaction between warm sea surface temperatures and upper wind patterns, notably the subtropical jet streams. They are important sources of rain in some mid-latitude areas, including Australia, where they are known as northwest cloud bands.

A northwest cloudband streams across Australia on 30th July 2008.
(Bureau of Meteorology image - click to enlarge)

In a large part of western and central Australia, more than 60 per cent of the April–October rainfall is contributed by northwest cloud bands that form to the south of Indonesia before streaming in from the northwest to form a cloud ‘conveyor belt’ that can be between 3000 and 8000 km long.

When a northwest cloud band interacts with a cold frontal system over southern areas of Australia, significant inland rain can result.

Reference: The Complete Book of Australian Weather by Richard Whitaker
Allen and Unwin 2010, ISBN978 1 74175 734 7

1 comment:

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