Undular bore
Encyclopedia
Meteorology
Meteorology is the interdisciplinary scientific study of the atmosphere. Studies in the field stretch back millennia, though significant progress in meteorology did not occur until the 18th century. The 19th century saw breakthroughs occur after observing networks developed across several countries...
, an undular bore is a wave
Wave
In physics, a wave is a disturbance that travels through space and time, accompanied by the transfer of energy.Waves travel and the wave motion transfers energy from one point to another, often with no permanent displacement of the particles of the medium—that is, with little or no associated mass...
disturbance in the Earth's atmosphere
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...
and can be seen through unique cloud formations. They normally occur within an area of the atmosphere which is stable in the low levels after an outflow boundary
Outflow boundary
An outflow boundary, also known as a gust front, is a storm-scale or mesoscale boundary separating thunderstorm-cooled air from the surrounding air; similar in effect to a cold front, with passage marked by a wind shift and usually a drop in temperature and a related pressure jump...
or a cold front
Cold front
A cold front is defined as the leading edge of a cooler mass of air, replacing a warmer mass of air.-Development of cold front:The cooler and denser air wedges under the less-dense warmer air, lifting it...
moves through.
In hydraulics
Hydraulics
Hydraulics is a topic in applied science and engineering dealing with the mechanical properties of liquids. Fluid mechanics provides the theoretical foundation for hydraulics, which focuses on the engineering uses of fluid properties. In fluid power, hydraulics is used for the generation, control,...
, an undular bore is a (weak) bore
Bore
Bore may refer to:* Bore , the diameter of a cylinder in a piston engine* Bore , the interior chamber of a wind instrument* Bore , a district of Ethiopia that includes the town of Bore* Boré, Mali...
with a standing wave
Standing wave
In physics, a standing wave – also known as a stationary wave – is a wave that remains in a constant position.This phenomenon can occur because the medium is moving in the opposite direction to the wave, or it can arise in a stationary medium as a result of interference between two waves traveling...
pattern at the downstream (subcritical flow|subcritical]] side.
Overview
Undular bores are usually formed when two air masses of different temperatures collide. When a low level boundary such as a cold front or outflow boundary approaches a layer of cold, stable air, it creates a disturbance in the atmosphere producing a wave-like motion, known as a gravity waveGravity wave
In fluid dynamics, gravity waves are waves generated in a fluid medium or at the interface between two media which has the restoring force of gravity or buoyancy....
. Although the undular bore waves appear as bands of clouds across the sky, they are transverse waves, and are propelled by the transfer of energy from an oncoming storm and are shaped by gravity. The ripple like appearance of this wave is described as the disturbance in the water when a pebble is dropped into a pond or when a moving boat creates waves in the surrounding water. The object displaces the water or medium
Transmission medium
A transmission medium is a material substance that can propagate energy waves...
the wave is travelling through and the medium moves in an upward motion. However, because of gravity, the water or medium is pulled back down and the repetition of this cycle creates the transverse wave motion.
The undular bore's period can measure 5 miles (8 km) peak to peak and can travel 16 kilometres per hour (9.9 mph) to 95 kilometres per hour (59 mph). The medium it travels through is the atmosphere
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...
. There are several varying types of ‘‘bores’’ in different layers of the atmosphere, such as the mesospheric bore which occurs in the mesosphere
Mesosphere
The mesosphere is the layer of the Earth's atmosphere that is directly above the stratosphere and directly below the thermosphere. In the mesosphere temperature decreases with increasing height. The upper boundary of the mesosphere is the mesopause, which can be the coldest naturally occurring...
.
Undular bores are believed to be catalysts for thunderstorms. Although a thunderstorm helps create an undular bore, an undular bore can in turn intensify a thunderstorm because it further disturbs the atmosphere. “Undular bores can have an effect on severe weather conditions,” said Tim Coleman, Atmospheric Scientist, National Space Science and Technology Center
National Space Science and Technology Center
The National Space Science and Technology Center is a joint research venture between NASA and the seven research universities of the state of Alabama, represented by the Space Science and Technology Alliance. The aim of the NSSTC is to foster collaboration in research between government,...
(NSSTC). “A tornado
Tornado
A tornado is a violent, dangerous, rotating column of air that is in contact with both the surface of the earth and a cumulonimbus cloud or, in rare cases, the base of a cumulus cloud. They are often referred to as a twister or a cyclone, although the word cyclone is used in meteorology in a wider...
in Birmingham, Alabama
Birmingham, Alabama
Birmingham is the largest city in Alabama. The city is the county seat of Jefferson County. According to the 2010 United States Census, Birmingham had a population of 212,237. The Birmingham-Hoover Metropolitan Area, in estimate by the U.S...
in April 1998 that came in contact with an undular bore increased in size and intensity.”
Occurrences
Rare but not unknown in a great many locations, the waves appear with some predictability and regularity in the Gulf of Carpentaria during Spring. They have been seen as frequently as six days in a row according to reports by the two pilots who have most experience withsoaring these sometimes enormous examples of the undular bore, known in Australia as the Morning Glory cloud
Morning glory cloud
The Morning Glory cloud is a rare meteorological phenomenon occasionally observed in different locations around the world. The southern part of Northern Australia's Gulf of Carpentaria is the only known location where it can be predicted and observed on a more or less regular basis. The settlement...
.
In hydraulics
The term “bore” is also used to describe positive surges advancing in shallow waters. When the surge's Froude numberFroude number
The Froude number is a dimensionless number defined as the ratio of a characteristic velocity to a gravitational wave velocity. It may equivalently be defined as the ratio of a body's inertia to gravitational forces. In fluid mechanics, the Froude number is used to determine the resistance of an...
is less than 1.4 to 1.7 (i.e. between unity and 1.4 to 1.7), the advancing front is followed by a train of well-defined free-surface undulations. The surge is then called an undular surge or undular bore.
The undulations form a standing wave
Standing wave
In physics, a standing wave – also known as a stationary wave – is a wave that remains in a constant position.This phenomenon can occur because the medium is moving in the opposite direction to the wave, or it can arise in a stationary medium as a result of interference between two waves traveling...
pattern, relative to the undular bore front. So, the phase velocity
Phase velocity
The phase velocity of a wave is the rate at which the phase of the wave propagates in space. This is the speed at which the phase of any one frequency component of the wave travels. For such a component, any given phase of the wave will appear to travel at the phase velocity...
(propagation velocity relative to still water) of the undulations is just high enough to keep the undulations stationary relative to the bore front. Now, in water waves, the group velocity
Group velocity
The group velocity of a wave is the velocity with which the overall shape of the wave's amplitudes — known as the modulation or envelope of the wave — propagates through space....
(which is also the energy-transport velocity) is less than the phase velocity
Phase velocity
The phase velocity of a wave is the rate at which the phase of the wave propagates in space. This is the speed at which the phase of any one frequency component of the wave travels. For such a component, any given phase of the wave will appear to travel at the phase velocity...
. Therefore, on average, wave energy of the undulations is transported away from the front, and contributing to the energy loss in the region of the front.
A related occurrence of positive surges is the tidal bore
Tidal bore
A tidal bore is a tidal phenomenon in which the leading edge of the incoming tide forms a wave of water that travel up a river or narrow bay against the direction of the river or bay's current...
in estuaries
Estuary
An estuary is a partly enclosed coastal body of water with one or more rivers or streams flowing into it, and with a free connection to the open sea....
.
See also
- Altostratus undulatus cloudAltostratus undulatus cloudThe altostratus undulatus is a type of low altocumulus cloud with signature undulations within it. These undulations may be visible , but frequently they are indiscernible to the naked eye...
- DerechoDerechoA derecho is a widespread and long-lived, violent convectively induced straight-line windstorm that is associated with a fast-moving band of severe thunderstorms in the form of a squall line usually taking the form of a bow echo...
- Gravity waveGravity waveIn fluid dynamics, gravity waves are waves generated in a fluid medium or at the interface between two media which has the restoring force of gravity or buoyancy....
- Hydraulic jumpHydraulic jumpA hydraulic jump is a phenomenon in the science of hydraulics which is frequently observed in open channel flow such as rivers and spillways. When liquid at high velocity discharges into a zone of lower velocity, a rather abrupt rise occurs in the liquid surface...
- Morning Glory cloudMorning glory cloudThe Morning Glory cloud is a rare meteorological phenomenon occasionally observed in different locations around the world. The southern part of Northern Australia's Gulf of Carpentaria is the only known location where it can be predicted and observed on a more or less regular basis. The settlement...
- Tidal boreTidal boreA tidal bore is a tidal phenomenon in which the leading edge of the incoming tide forms a wave of water that travel up a river or narrow bay against the direction of the river or bay's current...
External links
- LiveScience article
- Satellite image of undular bore over Gulf of Mexico
- Possible undular bore over Arabian Sea
- One NASA site, of undular bore wave over Iowa
- Another NASA site, of undular bore wave over Arabian Sea
- Strange Storm Shaped by Gravity Wave Live Science, 15 October 2007
- Hubert Chanson (2007). Hydraulic Jumps: Bubbles and Bores. In: 16th Australasian Fluid Mechanics Conference (AFMC), Gold Coast, Queensland, Australia, 3-7 Dec. 2007, Plenary lecture, pp. 39-53
- Gravity wave over Iowa - Youtube video
- Morning Glory Waves of the Gulf of Carpentaria - Articles and Images, 1989 on
