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Outflow boundary
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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.
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Encyclopedia
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 as downbursts can be generated in environments of vertical wind shear and mid-level dry air. Microbursts have a diameter of influence less than while macrobursts occur over a diameter greater than . Wet microbursts occur in atmospheres where the low levels are saturated, while dry microbursts occur in drier atmospheres from high-based thunderstorms.
Definition
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. Outflow boundaries can persist for over 24 hours and travel hundreds of miles/kilometers from their area of origin. 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".
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. 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.
Special types
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. The term was defined as affecting an area in diameter or less, 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. A macroburst is a strong downburst larger than . 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. All types are formed by precipitation-cooled air rushing to the surface.
Effects
Gust fronts create low-level wind shear which can be hazardous to planes when they takeoff or land.
See also
External links
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