Bow echo - AbsoluteAstronomy.com

A bow echo is a term describing the characteristic radar return from a mesoscale convective system

Mesoscale Convective System

A mesoscale convective system is a complex of thunderstorms that becomes organized on a scale larger than the individual thunderstorms but smaller than extratropical cyclones, and normally persists for several hours or more...

that is shaped like an archer’s bow

Bow (weapon)

The bow and arrow is a projectile weapon system that predates recorded history and is common to most cultures.-Description:A bow is a flexible arc that shoots aerodynamic projectiles by means of elastic energy. Essentially, the bow is a form of spring powered by a string or cord...

. These systems can produce severe straight-line winds and occasionally tornado

Tornado

A tornado is a violent, dangerous, rotating column of air that is in contact with both the surface of the earth and a cumulonimbus cloud or, in rare cases, the base of a cumulus cloud. They are often referred to as a twister or a cyclone, although the word cyclone is used in meteorology in a wider...

es, causing major damage.

Research

The term bow echo was first used by Dr. Theodore Fujita

Ted Fujita

was a prominent severe storms researcher. His research at the University of Chicago on severe thunderstorms, tornadoes, hurricanes and typhoons revolutionized knowledge of each.- Biography :Fujita was born in Kitakyūshū, Fukuoka Prefecture, Japan...

of the University of Chicago in his 1978 paper “Manual of Downburst Identification for Project NIMROD.” In 2004, research was done to better anticipate the formation of bow echoes. They studied the formation of bow echoes from weakly organized squall lines, and supercell

Supercell

A supercell is a thunderstorm that is characterized by the presence of a mesocyclone: a deep, continuously-rotating updraft. For this reason, these storms are sometimes referred to as rotating thunderstorms...

s. They determined that bow echoes were most likely to occur in weakly organized cells. In 2007, from February 28 to March 1, there was a Midwest Bow Echo Workshop, where meteorologists gathered to share their research and findings with each other to better understand bow echoes.

Formation

A bow echo is associated with squall line

Squall line

A squall line is a line of severe thunderstorms that can form along or ahead of a cold front. In the early 20th century, the term was used as a synonym for cold front. It contains heavy precipitation, hail, frequent lightning, strong straight-line winds, and possibly tornadoes and waterspouts....

s 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

Wind shear

Wind shear, sometimes referred to as windshear or wind gradient, is a difference in wind speed and direction over a relatively short distance in the atmosphere...

exists in the lower 2 to 3 km of the atmosphere

Atmosphere

An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...

. 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 derecho

Derecho

A derecho is a widespread and long-lived, violent convectively induced straight-line windstorm that is associated with a fast-moving band of severe thunderstorms in the form of a squall line usually taking the form of a bow echo...

Rear inflow jet

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.

Book end vortices

After the rear inflow jet has bowed the storm system, book end or line end vortices develop on either side of the jet. These vortices are similar in strength. Due to the small size of the bow echo, the vortices help enhance the mid-level flow between them. This strengthens the rear inflow jet. The surface winds increase from the descending jet. As the life of the storm increases, the Coriolis force acts to intensify the cyclonic vortex and weaken the anticyclonic vortex. The system then develops an asymmetric comma-shaped echo. Some embedded tornado

Tornado

es or gustnado

Gustnado

A gustnado is a specific type of short-lived, low-level rotating cloud that can form in a severe thunderstorm. The name is a portmanteau of "gust front tornado", as gustnadoes form due to non-tornadic cyclonic features in the downdraft from the gust front of a strong thunderstorm, especially one...

es develop within these vortices.

Strongest Winds

Damaging straight-line winds often occur near the center of a bow echo. Damage from all severe thunderstorm winds account for half of all severe reports in the lower 48 states of the US and is more common than damage from tornadoes. Wind speeds can reach up to 100 mph (160 km/h) and can produce a damage path extending for hundreds of miles. Bow echoes are capable of producing straight-line winds that are just as strong as many tornadoes. A strong bow echo will produce more widespread and intense damage than the majority of tornadoes. Also, bow echoes create a favorable environment for tornadoes to form.

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