A bow echo is the characteristic radar return from a

mesoscale convective system A mesoscale convective system (MCS) 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. A mesoscale conv ...

that is shaped like an archer's bow. These systems can produce severe straight-line winds and occasionally

tornado A tornado is a violently rotating column of air that is in contact with the surface of Earth and a cumulonimbus cloud or, in rare cases, the base of a cumulus cloud. It is often referred to as a twister, whirlwind or cyclone, although the ...

es, causing major damage. They can also become derechos or form Line echo wave pattern (LEWP).

Research

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

supercell A supercell is a thunderstorm characterized by the presence of a mesocyclone, a deep, persistently rotating updraft. Due to this, these storms are sometimes referred to as rotating thunderstorms. Of the four classifications of thunderstorms (su ...

s. 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.

Formation

A bow echo is associated with

squall line A squall line, or quasi-linear convective system (QLCS), is a line of thunderstorms, often forming along or ahead of a cold front. In the early 20th century, the term was used as a synonym for cold front (which often are accompanied by abrupt a ...

s or lines of convective

thunderstorms A thunderstorm, also known as an electrical storm or a lightning storm, is a storm characterized by the presence of lightning and its acoustic effect on the Earth's atmosphere, known as thunder. Relatively weak thunderstorms are som ...

. 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 (; also written windshear), sometimes referred to as wind gradient, is a difference in wind speed and/or direction over a relatively short distance in the atmosphere. Atmospheric wind shear is normally described as either vertical ...

exists in the lower 2 to 3 km of the

atmosphere An atmosphere () is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosph ...

. 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.MetEd Registration Page
/ref> Especially strong bow echoes that cause devastating damage all along the width of the storm are often called derechos.

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.AMS Glossary
Due to the small size of the bow echo, the vortices help enhance the mid-level flow between them, which strengthens the rear inflow jet. The surface winds increase from the descending jet. As the life of the storm increases, the

Coriolis force In physics, the Coriolis force is a pseudo force that acts on objects in motion within a frame of reference that rotates with respect to an inertial frame. In a reference frame with clockwise rotation, the force acts to the left of the motio ...

acts to intensify the cyclonic vortex and weaken the

anticyclonic A high-pressure area, high, or anticyclone, is an area near the surface of a planet where the atmospheric pressure is greater than the pressure in the surrounding regions. Highs are middle-scale meteorological features that result from interpl ...

vortex. The system then develops an asymmetric comma-shaped echo. Some embedded

es or

gustnado A gustnado is a brief, shallow surface-based vortex which forms within the downburst emanating from a thunderstorm. The name is a portmanteau by elision of "gust front tornado", as gustnadoes form due to non-tornadic straight-line wind features ...

es develop within these vortices. Bow echo diagram

Strongest winds

Damaging straight-line winds often occur near the center of a bow echo. Damage from all severe thunderstorm winds accounts for half of all severe reports in the lower 48 states of the US, and is more common than damage from tornadoes. In a type of long-lived and powerful bow echo known as a derecho, wind speeds can reach up to or exceed 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. Indeed, a strong bow echo will produce more widespread and intense damage than the majority of tornadoes. Also, bow echoes in the form of a line echo wave pattern create a favorable environment for tornadoes to form. The semiarid climate and rugged terrain in the interior west of the United States do not favour the development of bow echoes. However, on 21 April 2011, a bow echo associated with a fast-moving mid-tropospheric perturbation formed across the Great Salt Lake (GSL) in Utah, producing damaging winds along its path.

Notable bow echo storms

In 1674, the city of

Utrecht Utrecht ( ; ; ) is the List of cities in the Netherlands by province, fourth-largest city of the Netherlands, as well as the capital and the most populous city of the Provinces of the Netherlands, province of Utrecht (province), Utrecht. The ...

in the

Netherlands , Terminology of the Low Countries, informally Holland, is a country in Northwestern Europe, with Caribbean Netherlands, overseas territories in the Caribbean. It is the largest of the four constituent countries of the Kingdom of the Nether ...

was devastated by a storm now thought to have been a bow echo storm. Some of the damage to the city is still visible, and severe storm activity was recorded across other areas of

Europe Europe is a continent located entirely in the Northern Hemisphere and mostly in the Eastern Hemisphere. It is bordered by the Arctic Ocean to the north, the Atlantic Ocean to the west, the Mediterranean Sea to the south, and Asia to the east ...

. In the 2014 Pentecost weekend storms in Europe, a violent bow echo moved across the

Rhine-Ruhr The Rhine-Ruhr metropolitan region () is the Metropolitan regions in Germany, largest metropolitan region in Germany, with over ten million inhabitants. A wikt:polycentric, polycentric conurbation with several major urban concentrations, the reg ...

metropolitan region, causing 6 fatalities and an estimated 650 million euros in damages. Hailstorm Piracicaba 2024-10-11

On the evening of October 11, 2024, a series of bow echoes swept across the

state of São Paulo State most commonly refers to: * State (polity), a centralized political organization that regulates law and society within a territory **Sovereign state, a sovereign polity in international law, commonly referred to as a country **Nation state, a ...

Brazil Brazil, officially the Federative Republic of Brazil, is the largest country in South America. It is the world's List of countries and dependencies by area, fifth-largest country by area and the List of countries and dependencies by population ...

. The first storm hit the city of São Paulo, bringing widespread damage with wind gusts of up to 75 mph (120 km/h) and resulting in two deaths, one directly and one indirectly caused. Damages were estimated at 1.65 billion BRL. Another storm moved toward

Piracicaba Piracicaba ( ) is a Municipalities of Brazil, Brazilian municipality located in the Interior of São Paulo, interior of São Paulo (state), São Paulo state, in the Southeast Region, Brazil, Southeast Region of Brazil. It serves as the main city ...

and its

metropolitan area A metropolitan area or metro is a region consisting of a densely populated urban area, urban agglomeration and its surrounding territories which share Industry (economics), industries, commercial areas, Transport infrastructure, transport network ...

, producing wind gusts of up to 52 mph (85 km/h) and small hail. Both storms brought down trees and light poles. Six additional fatalities were reported elsewhere in the state due to the severe weather linked to the bow echoes.

References

External links

Manual of downburst identification for Project NIMROD
(NASA Technical Reports Server) (May 1978)
The Structure and Evolution of Squall Line and Bow Echo Convective Systems
(NWS)

(SPC)
What Creates a Bow Echo?

Glossary of Meteorology

Midwest Bow Echo Workshop

The Bow Echo and MCV Experiment: Observations and Opportunities
(''BAMS'')
The Bow Echo: Observations, Numerical Simulations, and Severe Weather Detection Methods
(''WAF'')

{{DEFAULTSORT:Bow Echo Radar meteorology Storm Severe weather and convection Weather hazards ja:気象レーダー#ボウエコー