A bow echo is a term describing the characteristic radar return from a
mesoscale convective systemthat is shaped like an archer’s bow. These systems can produce severe straight-line winds and occasionally tornadoes, causing major damage.
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 shearexists in the lower 2 to 3 km of the atmosphere. While similar to squall lines, bow echoes are smaller in scale; this creates their extreme intensity. The "bow shaped" echo is a result of focusing of the strong flow at the rear of the system. [ [http://www.meted.ucar.edu/mesoprim/severe2/ MetEd Registration Page ] ] 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. [ [http://www.meted.ucar.edu/mesoprim/severe2/ MetEd Registration Page ] ]
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. [ [http://amsglossary.allenpress.com/glossary/search?id=bow-echo1 AMS Glossary ] ] 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. [ [http://www.meted.ucar.edu/mesoprim/severe2/ MetEd Registration Page ] ] As the life of the storm increases, the
Coriolis forceacts to intensify the cyclonicvortex and weaken the anticyclonicvortex. The system then develops an asymmetric comma-shaped echo. [ [http://www.meted.ucar.edu/mesoprim/severe2/ MetEd Registration Page ] ] [ [http://amsglossary.allenpress.com/glossary/search?id=bow-echo1 AMS Glossary ] ] Some embedded tornadoes or gustnadoes develop within these vortices.
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 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. [ [http://www.nssl.noaa.gov/primer/wind/wind_basics.html# Questions and Answers about Damaging Winds: Basics ] ] 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.
Mesoscale convective system
Rear inflow jet
Convective storm detection
* [http://www.crh.noaa.gov/lmk/soo/docu/bowecho.php The Structure and Evolution of Squall Line and Bow Echo Convective Systems] (NWS)
* [http://www.spc.noaa.gov/misc/AbtDerechos/bowechoprot.htm Bow Echo Prototype] (SPC)
* [http://www.theweatherprediction.com/habyhints2/386/ What Creates a Bow Echo?]
* [http://amsglossary.allenpress.com/glossary/search?id=bow-echo1 Glossary of Meteorology]
* [http://www.crh.noaa.gov/lmk/?n=bowechoworkshop Midwest Bow Echo Workshop]
* [http://ams.allenpress.com/perlserv/?request=get-document&doi=10.1175%2FBAMS-85-8-1075 The Bow Echo and MCV Experiment: Observations and Opportunities] ("BAMS")
* [http://ams.allenpress.com/perlserv/?request=get-abstract&doi=10.1175%2F1520-0434(1995)010%3C0203%3ATBEONS%3E2.0.CO%3B2 The Bow Echo: Observations, Numerical Simulations, and Severe Weather Detection Methods] ("WAF")
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