Concept
A bow echo is the characteristic radar return from a mesoscale convective system that is shaped like an archer's bow. These systems can produce severe straight-line winds and occasionally tornadoes, causing major damage. They can also become derechos or form Line echo wave pattern (LEWP). The term "bow echo" was first used by Theodore Fujita in his May 1978 paper "Manual of Downburst Identification for Project NIMROD." In 2004, research was done to better anticipate the formation of bow echoes, specifically the formation of bow echoes from weakly organized squall lines and supercells. Researchers determined that bow echoes were most likely to occur in weakly organized cells. A Midwest Bow Echo Workshop was held in 2007, at which meteorologists gathered to share their research to better understand bow echoes. A bow echo is associated with squall lines or lines of convective thunderstorms. These echoes can range in size from 20 to 200 km, and have a life span of 3 to 6 hours. Bow echoes tend to develop when moderate to strong wind shear exists in the lower 2 to 3 km of the atmosphere. While similar to squall lines, bow echoes are smaller in scale and are moved by the wind inside them. They tend to push outward and after time die out. A bow echo also lowers the chance of a tornado being formed in the storm itself. The "bow shaped" echo is a result of focusing of the strong flow at the rear of the system. Especially strong bow echoes that cause devastating damage all along the width of the storm are often called derechos. The formation of a bow echo requires a strong elevated rear inflow jet at mid-levels. The strength of the cold pool and mesohigh at the surface as well as warmer temperatures aloft due to convection works to create a mesolow at mid-levels which strengthens the jet. Upon reaching the edge of the convection the jet descends and spreads along the surface, generating straight-line winds. After the rear inflow jet has bowed the storm system, book end or line end vortices develop on either side of the jet.
