Naked Science Forum
Life Sciences => The Environment => Topic started by: Titanscape on 10/04/2009 07:45:17
-
Why if up high there are jet streams, at 300-400km/h, are there none down at ground level?
How is it that sometimes they are incurred to hit the ground in storms?
-
There are low level jet streams, they are typically found at heights of 2 to 3 km, although they can be as low as 300 foot. Some of their given names are the nocturnal jet,the Great Plains low level jet, extratropical cyclone low level jet, African jet , east african low level jet and the Somali jet.
An easy to understand piece can be found in the USAToday:
How nighttime, low-level jet streams form
A fast-moving river of air known as a "jet" sometiems forms at about 1,000 feet above the ground after sunset.
On a clear evening the atmosphere cools down. And if conditions are calm, a stable temperature "inversion" sets up where cool air aloft, which is heavier than warm air, sinks to the ground and any leftover warm air sits on top of it. The stable air in an inversion acts like a nearly solid object and allows the air above it to flow rapidly past the inversion like wind blowing over water.
Differences in air pressure on either side of the developing low-level jet help to concentrate the flow of air into a corridor or stream less than several hundred miles wide. Winds in the stream can flow at speeds of 60 mph or more. Mountain ranges can further enhance low-level jet stream winds.
Nighttime, low-level jet streams are marked by a rapid change in wind speed with height. The sudden shift in wind can catch late-night fliers by surprise and be hazardous to landing aircraft. And low-level jet streams can form other hazards for pilots.
When strong winds blow across a body water waves form. The atmosphere reacts in a similar manner forming waves of air on the edges of the inversion layer called turbulence. Pilots try to avoid flying into air turbulence because at the very least it causes a bumpy ride. Turbulence can become quite severe and some aircraft have been damaged by turbulence.
On the central Plains of the USA, the nighttime, low-level jet stream also feeds developing thunderstorms with warm, moist air. Large, nocturnal thunderstorm clusters called mesoscale convective complexes, or MCCs, can span areas as large as whole states, blocking flight paths with torrential downpours, deadly lightning, damaging winds and even tornadoes.
The light of day kills the low-level jet. Once the sun begins to heat the land, the lower atmosphere begins to mix as the warm air rises, breaking the inversion. As this happens,the jet rises in some places and sinks in others like a giant roller coaster. Without a smooth surface to glide over, the jet encounters friction and slows down. But the same conditions the next night could allow the low-level jet to reform with equal strength and similar consequences.
Nighttime, low-level jets aren't confined to the Plains states. In 1991, Doppler radar in Sterling, Va., thoroughly documented a low-level jet for the first time east of the Appalachians. As daylight faded, forecasters noticed winds increasing rapidly with height to the 1,000 foot level while surface winds remained at a tranquil 6 knots. Pilots reported low-level wind shear with no thunderstorms in the area.
The nighttime, low-level jet stream isn't unique to the United States either. It's been detected in Canada, China, Australia, Argentina and western Europe.
If you want something meatier, then try these links:
Diurnal Variation and Horizontal Extent of the Low-Level Jet over the Northern Gulf of California (http://ams.allenpress.com/perlserv/?request=get-document&doi=10.1175%2F1520-0493(1998)126%3C2017%3ADVAHEO%3E2.0.CO%3B2)
WORLD METEOROLOGICAL ORGANIZATION AVIATION HAZARDS EDUCATION AND TRAINING PROGRAMME (http://www.caem.wmo.int/moodle/file.php?file=/1/AVIATION_HAZARDS_FINAL.pdf.)
Weather and climate modification problems and prospects (http://books.google.co.uk/books?id=lz4rAAAAYAAJ&printsec=titlepage#PPP1,M1)