Outflow Boundary - Origin

Origin

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 4 kilometres (2.5 mi) in diameter or less, distinguishing them as a type of downburst and apart from common wind shear which can encompass greater areas. They are normally associated with individual thunderstorms. Microburst soundings show the presence of mid-level dry air, which enhances evaporative cooling.

Organized areas of thunderstorm activity reinforce pre-existing frontal zones, and can outrun cold fronts. This outrunning occurs within the westerlies in a pattern where the upper level jet splits into two streams. The resultant mesoscale convective system (MCS) forms at the point of the upper level split in the wind pattern in the area of best low level inflow. The convection then moves east and toward the equator into the warm sector, parallel to low-level thickness lines. When the convection is strong and linear or curved, the MCS is called a squall line, with the feature placed at the leading edge of the significant wind shift and pressure rise which is normally just ahead of its radar signature. This feature is commonly depicted in the warm season across the United States on surface analyses, as they lie within sharp surface troughs.

A macroburst, normally associated with squall lines, is a strong downburst larger than 4 kilometres (2.5 mi). A wet microburst consists of precipitation and an atmosphere saturated in the low-levels. A dry microburst emanates from high-based thunderstorms with virga falling from their base. All types are formed by precipitation-cooled air rushing to the surface. Downbursts can occur over large areas. In the extreme case, a derecho can cover a huge area more than 200 miles (320 km) wide and over 1,000 miles (1,600 km) long, lasting up to 12 hours or more, and is associated with some of the most intense straight-line winds, but the generative process is somewhat different from that of most downbursts.