Editing Hook echo

{{short description|Weather radar signature indicating tornadic circulation in a supercell thunderstorm}}
[[Image:Tornadic classic supercell radar.gif|thumb|250px|right|Classic-style hook echo of the F5 [[1999 Bridge Creek-Moore tornado]].]]
A '''hook echo''' is a pendant or hook-shaped [[weather radar]] signature as part of some [[supercell]] [[thunderstorm]]s. It is found in the lower portions of a storm as air and precipitation flow into a [[mesocyclone]], resulting in a curved feature of [[reflectivity]]. The echo is produced by rain, hail, or debris being wrapped around the supercell.<ref>{{cite book |editor-last = Glickman |editor-first = Todd S.  |title = Glossary of Meteorology |publisher = American Meteorological Society |edition = 2nd |year = 2000 |url = http://glossary.ametsoc.org/wiki/Hook_echo |section=Hook Echo |isbn = 978-1-878220-34-9 }}</ref> It is one of the classic hallmarks of [[tornado]]-producing supercells.<ref name="review">{{cite journal |last = Murkowski |first = Paul M. |title = Hook Echoes and Rear-Flank Downdrafts: A Review |journal = Mon. Wea. Rev. |volume = 130 |issue = 4 |pages = 852–76 |year = 2002 |doi = 10.1175/1520-0493(2002)130<0852:HEARFD>2.0.CO;2 |bibcode = 2002MWRv..130..852M |s2cid = 54785955 |doi-access = free }}</ref> The [[National Weather Service]] may consider the presence of a hook echo coinciding with a [[tornado vortex signature]] as sufficient to justify issuing a [[tornado warning]].<ref>{{cite web |last = Angel |first = Jim |title = ISWS is Pioneer in Tracking Tornadoes by Radar |publisher = Illinois State Water Survey |date = Apr 9, 2013 |url = http://www.isws.illinois.edu/hilites/press/130409radar.asp |access-date = 2013-05-22 |archive-date = 2013-06-01 |archive-url = https://web.archive.org/web/20130601140628/http://www.isws.illinois.edu/hilites/press/130409radar.asp |url-status = dead }}</ref><ref>{{Cite web | url = http://www.wdtb.noaa.gov/modules/twg02/TWG2002.pdf | title = Tornado Warning Guidance | date = Spring 2002 | publisher = National Weather Service | access-date = June 16, 2013 | url-status = dead | archive-url = https://web.archive.org/web/20130306015738/http://www.wdtb.noaa.gov/modules/twg02/TWG2002.pdf | archive-date = March 6, 2013 }}</ref>

==History==
[[File:NEXRAD radar of an EF2 tornado in Kansas on March 13, 2024.png|thumb|250px|right|Classic hook echo can be seen for this [[Tornado_outbreak_of_March_13–15,_2024|Kansas EF2 tornado]] in 2024]]
Because of the unpredictable and potentially catastrophic nature of tornadoes, the possibility of detecting tornadoes via radar was discussed in the meteorological community in the earliest days of meteorological radar.<ref name="isws.illinois.edu">Huff, F.A., H.W. Hiser, and S.G. Bigler, 1954: [http://www.isws.illinois.edu/pubdoc/RI/ISWSRI-22.pdf Study of an Illinois tornado using radar, synoptic weather and field survey data]. Report of Investigation 22, Champaign, IL, pp. 73</ref> The first association between tornadoes and the hook echo was discovered by E.M. Brooks in 1949.<ref name=Brook>{{Cite journal | last= Brooks | first= E. M. |year = 1949 | title= The tornado cyclone | journal = [[Weatherwise]] | volume = 2 | issue = 2 | pages = 32–33 | doi= 10.1080/00431672.1949.9930047 | bibcode= 1949Weawi...2b..32B }}</ref> Brooks noted circulations with radii of approximately 8–16&nbsp;km on radar. These circulations were associated with supercell thunderstorms and were dubbed “tornado cyclones” by Brooks.

The first documented association between a hook echo and a confirmed tornado occurred near [[Champaign–Urbana metropolitan area|Champaign–Urbana, Illinois]], on 9 April 1953.<ref name="isws.illinois.edu"/><ref>{{cite web |last = Angel |first = Jim |title = 60th Anniversary of the First Tornado Detected by Radar |work = Illinois State Climatology |publisher = Illinois State Water Survey |date = Apr 9, 2013 |url = https://climateillinois.wordpress.com/2013/04/09/60th-anniversary-of-the-first-tornado-detected-by-radar/ |access-date = May 22, 2013 }}</ref> This event was unintentionally discovered by Illinois State Water Survey [[electrical engineer]] Donald Staggs.
Staggs was repairing and testing an experimental [[precipitation]] measurement radar unit when he noticed an unusual radar echo which was associated with a nearby thunderstorm. The unusual echo appeared to be an area of precipitation in the shape of the number six - hence the modern term “hook echo”. Staggs chose to record the echo for further analysis by [[meteorologists]]. Upon review of the unusual echo data, meteorologists F.A. Huff, H.W. Heiser, and S.G. Bigler determined that a destructive tornado had occurred in the geographical location which corresponded with the "six-shaped" echo seen on radar.

Prominent [[severe storm]] researcher [[Ted Fujita]] also documented hook echoes with various supercell thunderstorms which occurred on 9 April 1953 - the same day as the Huff et al. discovery.<ref name=Fujita>{{Cite journal | last=Fujita | first= T. T. | year = 1958 | title= Mesoanalysis of the Illinois tornadoes of 9 April 1953 | journal =  Journal of Meteorology| volume= 15 | issue= 3 | pages = 288–296 | doi=10.1175/1520-0469(1958)015<0288:MOTITO>2.0.CO;2|bibcode = 1958JAtS...15..288F | doi-access= free }}</ref> After detailed study of the evolution of hook echoes, Fujita hypothesized that certain strong thunderstorms may be capable of rotation.

J.R. Fulks developed the first hypothesis on the formation of hook echoes in 1962.<ref>{{Cite book | last= Fulks | first = J. R. | year = 1962 | title = On the Mechanics of the Tornado | work = National Severe Storms Project Rep. No. 4 | publisher = U. S. Weather Bureau }}</ref> Fulks analyzed [[wind speed|wind velocity]] data from [[Weather radar#Velocity|Doppler weather radar]] units which were installed in [[Central Oklahoma]] in 1960. Doppler data on wind velocity during thunderstorms demonstrated an association between strong horizontal [[wind shear]] and mesocyclones, which were identified as having the potential to [[tornadogenesis|produce tornadoes]].<ref name="review" />

== Interpretation ==
[[File:Supercell-above.svg|thumb|left|Diagram of air current in a supercell]]
Hook echoes are a reflection of the movement of air inside and around a supercell thunderstorm. Ahead of the base of the storm, the inflow from the environment is sucked in by the instability of the air mass. As it moves upward, it cools slower than the cloud environment, because it mixes very little with it, creating an echo free tube which ends at higher levels to form a [[bounded weak echo region]] or BWER.<ref name="review"/> At the same time, a mid-level flow of cool and drier air enters the thunderstorm cloud. Because it is drier than the environment, it is more dense and sinks down behind the cloud and forms the [[rear flank downdraft]], drying the mid-level portion of the back of the cloud. The two currents form a vertical windshear, which then develops rotation and can further interact to form a mesocyclone. Tightening of the rotation near the surface may create a tornado.<ref name="review" />

[[File:05june-dow7-wide.gif|thumb|upright=1.5|A [[Doppler on Wheels]] image of a tornadic thunderstorm near [[La Grange, Wyoming]] (USA) captured during the [[VORTEX2]] project. In the velocity image on the left, Blues/green represent winds moving towards the radar, and reds/yellows indicate winds moving away from the radar. In the reflectivity image on the right, the main body of the storm can be seen, with the appendage on the bottom of the storm being a hook echo.]]
Near the interaction zone at the surface, there will be a dry slot caused by the updraft on one side and the cloudy area below the rear flank downdraft on the other side. This is the source of the hook echo seen on radar near the surface. Hook echoes are thus a relatively reliable indicator of tornadic activity; however, they merely indicate the presence of a larger mesocyclone structure in the tornadic storm rather than directly detecting a tornado.<ref name="review"/> During some destructive tornadoes, debris lofted from the surface may be detected as a "[[debris ball]]" on the end of the hook structure. Not all thunderstorms exhibiting hook echoes produce tornadoes, and not all tornado-producing supercells contain hook echoes.

The use of Doppler weather radar systems, such as [[NEXRAD]], allows for the detection of strong, low-level mesocyclones that produce tornadoes even when the hook echo is not present and also grant greater certainty when a hook echo is present. By detecting [[hydrometeor]]s moving toward and away from the radar location, the relative velocities of air flowing within different parts of a storm are revealed. These areas of tight rotation known as "velocity couplets" are now the primary trigger for the issuance of a tornado warning. The [[tornado vortex signature]] is an algorithm-based detection of this.<ref>{{cite web| last = Paul Schlatter, Warning Decision Training Branch| title = WSR-88D Distance Learning Operations Course; Topic 5, Lesson 19| access-date = June 16, 2013| date = September 2009| url = http://www.wdtb.noaa.gov/courses/dloc/topic5/lesson19/player.html| archive-url = https://web.archive.org/web/20130227212912/http://www.wdtb.noaa.gov/courses/dloc/topic5/lesson19/player.html| archive-date = February 27, 2013| url-status = dead}}</ref>

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== Observational limitation ==

Hook echoes are not always obvious. Particularly in the [[Southern United States]], thunderstorms tend to take on a structure of more precipitation surrounding a mesocyclone, which leads to the high precipitation (HP) variation supercell that obscures the hook shape. HP supercells instead often have a high reflectivity pendant or front flank notch (FFN), appearing like a  "kidney bean" shape. Another limiting factor is radar resolution. Prior to 2008, NEXRAD had a range resolution of 1,000 meters, while the processes which lead to a hook echo happen on a smaller scale.<ref>{{cite web|title=NWS Louisville: Supercell Structure and Dynamics|url=http://www.crh.noaa.gov/lmk/soo/docu/supercell.php|access-date=1 June 2013}}</ref>

== See also ==
* [[Bow echo]]
* [[Bounded weak echo region]]
* [[Lemon technique]]
* [[Rear flank downdraft]]
* [[Convective storm detection]]

== References ==
{{reflist|2}}

== Further reading ==
* {{cite journal |last1=Fujita |first1=Tetsuya |title=Formation and Steering Mechanisms of Tornado Cyclones and Associated Hook Echoes |journal=Monthly Weather Review |date=1965-02-01 |volume=93 |issue=2 |pages=67–78 |doi=10.1175/1520-0493(1965)093<0067:FASMOT>2.3.CO;2 |bibcode=1965MWRv...93...67F |url=https://journals.ametsoc.org/view/journals/mwre/93/2/1520-0493_1965_093_0067_fasmot_2_3_co_2.xml |language=EN |issn=1520-0493|url-access=subscription }}
* {{cite journal |last1=Fujita |first1=Tetsuya |title=Mesoanalysis of the Illinois Tornadoes of 9 April 1953 |journal=Journal of the Atmospheric Sciences |date=1958-06-01 |volume=15 |issue=3 |pages=288–296 |doi=10.1175/1520-0469(1958)015<0288:MOTITO>2.0.CO;2 |bibcode=1958JAtS...15..288F |language=EN |issn=1520-0469|doi-access=free }}
* {{cite web |last1=Wade |first1=Patrick |title=Tornadoes' 'hook echo' discovered here 60 years ago |url=https://www.news-gazette.com/news/tornadoes-hook-echo-discovered-here-60-years-ago/article_cc5a6a2e-a968-5e0c-be7d-ada419641841.html |website=[[The News-Gazette]] |accessdate=2023-07-03 |place=Champaign |language=en |date=2013-04-07}}
* {{cite journal |last1=Burgess |first1=Donald W. |last2=Magsig |first2=Michael A. |last3=Wurman |first3=Joshua |last4=Dowell |first4=David C. |last5=Richardson |first5=Yvette |title=Radar Observations of the 3 May 1999 Oklahoma City Tornado |journal=Weather and Forecasting |date=2002-06-01 |volume=17 |issue=3 |pages=456–471 |doi=10.1175/1520-0434(2002)017<0456:ROOTMO>2.0.CO;2 |language=EN |issn=1520-0434|doi-access=free |bibcode=2002WtFor..17..456B }}

[[Category:Radar meteorology]]
[[Category:Severe weather and convection]]
[[Category:Tornado]]