Gust front
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Gust_front".

For information on the book Gust Front by author John Ringo, see Legacy of the Aldenata.
Outflow boundary on radar with radial velocity and frontal boundary drawn in.
Outflow boundary on radar with radial velocity and frontal boundary drawn in.

An outflow boundary is a storm-scale or mesoscale boundary separating thunderstorm-cooled air (outflow) 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. Outflow boundaries may persist for 24 hours or more after the thunderstorms that generated them dissipate, and may travel hundreds of miles from their area of origin. New thunderstorms often develop along outflow boundaries, especially near the point of intersection with another boundary (cold front, dry line, another outflow boundary, etc.)

Outflow boundaries create low-level wind shear which can be hazardous during aircraft takeoffs and landings. Strong versions of these features known downbursts can be generated in environments of vertical wind shear and mid-level dry air. Microbursts have a diameter of influence less than 4 kilometres (2.5 mi) while macrobursts occur over a diameter greater than 4 kilometres (2.5 mi). Wet microbursts occur in atmospheres where the low levels are saturated, while dry microbursts occur in drier atmospheres from high-based thunderstorms.

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Definition

Thunderstorm with lead gust front
Thunderstorm with lead gust front

An outflow boundary, also known as a gust front or arc cloud, is an outflow boundary that is the leading edge of gusty, cooler surface winds from thunderstorm downdrafts; sometimes associated with a shelf cloud or roll cloud. A pressure jump is associated with its passage.[1] Outflow boundaries can persist for over 24 hours and travel hundreds of miles/kilometers from their area of origin.[2] A wrapping gust front is a front that wraps around the mesocyclone, cutting off the inflow of warm moist air and resulting in occlusion. This is sometimes the case during the event of a collapsing storm, in which the wind literally "rips it apart".[3]

Appearance

Sometimes a gust front can be seen on weather radar, showing as a thin arc or line of weak radar echos pushing out from a collapsing storm. The thin line of weak radar echoes is known as a fine line.[4] In most cases, these echoes represent insects that have been caught in the colliding winds at the front. Occasionally, winds caused by the gust front are so high in velocity, that they may also show up on radar. This cool outdraft can then energize other storms which it hits by assisting in updrafts. Gust fronts colliding from two storms can even create new storms. However, there is usually no rain accompanying the shifting winds.

At the surface, clouds of dust can be raised by outflow boundaries. An expansion of the rain shaft near ground level, in the general shape of a foot, is a telltale sign of a downburst. Gustnadoes, short-lived vertical circulations near ground level, can be spawned by outflow boundaries.[5]

Special types

Illustration of a microburst. Note the downward motion of the air until it hits ground level. It then spreads outward in all directions. The wind regime in a microburst is opposite to that of a tornado.
Illustration of a microburst. Note the downward motion of the air until it hits ground level. It then spreads outward in all directions. The wind regime in a microburst is opposite to that of a tornado.
Main article: Microburst

A microburst is a very localized column of sinking air known as a downburst, producing damaging divergent and straight-line winds at the surface that are similar to but distinguishable from tornadoes which generally have convergent damage.[6] The term was defined as affecting an area 4 kilometres (2.5 mi) in diameter or less,[7] distinguishing them as a type of downburst and apart from common wind shear which can encompass greater areas. Microburst soundings show the presence of mid-level dry air, which enhances evaporative cooling.[5] A macroburst is a strong downburst larger than 4 kilometres (2.5 mi).[8] A wet microburst consists of precipitation and an atmosphere saturated in the low-levels. A dry microburst emminates from high-based thunderstorms with virga falling from their base.[5] All types are formed by precipitation-cooled air rushing to the surface.

Effects

Effect of wind shear on aircraft trajectory. Note how merely correcting for the initial gust front can have dire consequences.
Effect of wind shear on aircraft trajectory. Note how merely correcting for the initial gust front can have dire consequences.
See also: Wind shear

Gust fronts create low-level wind shear which can be hazardous to planes when they takeoff or land.[9]

See also

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References

  1. ^ Glossary of Meteorology. Gust Front. Retrieved on 2008-07-09.
  2. ^ National Weather Service. Outflow Boundary. Retrieved on 2008-07-09.
  3. ^ National Weather Service. Wrapping Gust Front. Retrieved on 2008-07-09.
  4. ^ Glossary of Meteorology. Fine Line. Retrieved on 2008-07-09.
  5. ^ a b c Fernando Caracena, Ronald L. Holle, and Charles A. Doswell III. Microbursts: A Handbook for Visual Identification. Retrieved on 2008-07-09.
  6. ^ Lyndon State College Meteorology. How to distinguish between tornado and microburst (straight-line) wind damage. Retrieved on 2008-07-09.
  7. ^ National Weather Association. Welcome to Lesson 5. Retieved on 2008-07-09.
  8. ^ Dr. Ali Tokay. CHAPTER #13: Thunderstorms. Retrieved on 2008-07-09.
  9. ^ Diana L. Klingle, David R. Smith, and Marilyn M. Wolfson. Gust Front Characteristics as Detected by Doppler Radar. Retrieved on 2008-07-09.

External links
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