Outflow, inmeteorology, is air that flows outwards from a storm system. It is associated with ridging, oranticyclonic flow. In the low levels of thetroposphere, outflow radiates from thunderstorms in the form of a wedge of rain-cooled air, which is visible as a thin rope-like cloud onweather satellite imagery or a fine line onweather radar imagery. For observers on the ground, a thunderstorm outflow boundary often approaches in otherwise clear skies as a low, thick cloud that brings with it agust front.
Low-level outflow boundaries can disrupt the center of smalltropical cyclones. However, outflow aloft is essential for the strengthening of a tropical cyclone. If this outflow is restricted or undercut, the tropical cyclone weakens. If two tropical cyclones are close, the upper-level outflow from the upwind system can limit the development of the other system.
For thunderstorms, outflow tends to indicate the development of a system. Large quantities of outflow at the upper levels of athunderstorm indicate its development. Too much outflow in the lower levels of a thunderstorm, however, can choke off thelow-level inflow which fuels it.[1]Squall lines typically bow out the most, or bend the most convex outward, at the leading edge of low level outflow due to the formation of a mesoscalehigh-pressure area which forms within the stratiform rain area behind the initial line. Thishigh-pressure area is formed due to strong descending motion behind the squall line, and could come in the form of adownburst.[2]
The "edge" of theoutflow boundary can often be detected byDoppler radar (especially in clear air mode). Convergence occurs along the leading edge of the downdraft. Convergence of dust, aerosols, and bugs at the leading edge will lead to a higher clear air signature.Insects andarthropods are swept along by the prevailing winds, making them good indicators of the presence of outflow boundaries.[3] The signature of the leading edge is also influenced by thedensity change between the cooler air from the downdraft and the warmer environmental air. This density boundary will increase the number of echo returns from the leading edge. Clouds and new thunderstorms also develop along the outflow's leading edge. This makes it possible to locate the outflow boundary when using precipitation mode on a weather radar. Also, it makes outflow boundaries findable within visible satellite imagery as a thin line of cumuliform clouds which is known as anarcus, or arc, cloud. The image to the right depicts a particularly strong outflow boundary ahead of a line of storms. Often, the outflow boundary will bow in the direction it is moving the quickest.[4]
The development of a significantmesoscale convective complex can send out a large enough outflow boundary to weaken thecyclone as the tropical cyclone center moves into the more stableair mass behind the leading edge of thunderstorm outflow, or outflow boundary.[5] Moderate verticalwind shear can lead to the initial development of the convective complex and surface low similar to the mid-latitudes, but it must relax to allowtropical cyclogenesis to continue.[6]
While the most obvious motion of clouds is toward the center, tropical cyclones also develop an upper-level (high-altitude) outward flow of clouds. These originate from air that has released its moisture and is expelled at high altitude through the "chimney" of the storm engine.[7] This outflow produces high, thincirrus clouds that spiral away from the center. The clouds are thin enough for the sun to be visible through them. These high cirrus clouds may be the first signs of an approaching tropical cyclone.[8] As air parcels are lifted within the eye of the storm thevorticity is reduced, causing the outflow from a tropical cyclone to haveanticyclonic motion. If two tropical cyclones are in proximity to one another, the outflow from the system downstream (normally to the west) can hinder the development of the system upstream (normally to the east).[9]
Low-level outflow boundaries from thunderstorms are cooler and more moist than theair mass the thunderstorm originally formed within due to itswet bulbing byrain,[10] forming a wedge of denser air which spreads out from the base of the parent thunderstorm. Ifwind speeds are high enough, such as duringmicroburst events, dust and sand can be carried into thetroposphere, reducing visibility.[11] This type of weather event is known as ahaboob, and is most common in the late spring withinSudan.[12] Upper-level outflow can consist of thickcirrus clouds which would then obscure the sun and reduce solarinsolation around the outermost edge of tropical cyclones.
