A hook echo is a pendant or hook-shaped
weather radar signature as part of some
supercell thunderstorm
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 some ...
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 reflectance of the surface of a material is its effectiveness in reflecting radiant energy. It is the fraction of incident electromagnetic power that is reflected at the boundary. Reflectance is a component of the response of the electronic ...
. The echo is produced by rain, hail, or even debris being wrapped around the supercell. It is one of the classic hallmarks of
tornado
A tornado is a violently 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. It is often referred to as a twister, whirlwind or cyclone, alt ...
-producing supercells.
The
National Weather Service
The National Weather Service (NWS) is an agency of the United States federal government that is tasked with providing weather forecasts, warnings of hazardous weather, and other weather-related products to organizations and the public for the ...
may consider the presence of a hook echo coinciding with a
tornado vortex signature as sufficient to justify issuing a
tornado warning.
History
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.
[Huff, F.A., H.W. Hiser, and S.G. Bigler, 1954]
Study of an Illinois tornado using radar, synoptic weather and field survey data
Report of Investigation 22, Champaign, IL, pp. 73 The first association between tornadoes and the hook echo was discovered by E.M. Brooks in 1949.
Brooks noted circulations with radii of approximately 8–16 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
Urbana-Champaign, Illinois on 9 April 1953.
This event was unintentionally discovered by Illinois State Water Survey
electrical engineer
Electrical engineering is an engineering discipline concerned with the study, design, and application of equipment, devices, and systems which use electricity, electronics, and electromagnetism. It emerged as an identifiable occupation in the l ...
Donald Staggs.
Staggs was repairing and testing an experimental
precipitation
In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. ...
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
was a Japanese-American meteorologist whose research primarily focused on severe weather. His research at the University of Chicago on severe thunderstorms, tornadoes, hurricanes, and typhoons revolutionized the knowledge of each. Although ...
also documented hook echoes with various supercell thunderstorms which occurred on 9 April 1953 - the same day as the Huff et al. discovery.
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. Fulks analyzed
wind velocity
In meteorology, wind speed, or wind flow speed, is a fundamental atmospheric quantity caused by air moving from high to low pressure, usually due to changes in temperature. Wind speed is now commonly measured with an anemometer.
Wind spee ...
data from
Doppler weather radar units which were installed in
Central Oklahoma
Central Oklahoma is the geographical name for the central region of the U.S. state of Oklahoma. It is also known by the Oklahoma Department of Tourism designation, Frontier Country, defined as the twelve-county region including Canadian, Grady, ...
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
produce tornadoes.
Interpretation
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
The bounded weak echo region, also known as a BWER or a vault, is a radar signature within a thunderstorm characterized by a local minimum in radar reflectivity at low levels which extends upward into, and is surrounded by, higher reflectivities a ...
or BWER.
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 less 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.
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.
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
hydrometeors 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.
Observational limitation
Hook echoes are not always obvious. Particularly in the
Southern United States
The Southern United States (sometimes Dixie, also referred to as the Southern States, the American South, the Southland, or simply the South) is a geographic and cultural region of the United States of America. It is between the Atlantic Ocean ...
, 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.
See also
*
Bow echo
*
Bounded weak echo region
The bounded weak echo region, also known as a BWER or a vault, is a radar signature within a thunderstorm characterized by a local minimum in radar reflectivity at low levels which extends upward into, and is surrounded by, higher reflectivities a ...
*
Lemon technique The Lemon technique is a method used by meteorologists using weather radar to determine the relative strength of thunderstorm cells in a vertically sheared environment. It is named for Leslie R. Lemon, the co-creator of the current conceptual mode ...
*
Rear flank downdraft
*
Convective storm detection
References
{{reflist, 2
External links
Formation and Steering Mechanisms of Tornado Cyclones and Associated Hook EchoesMesoanalysis of the Illinois Tornadoes of 9 April 1953July 10th, 2000 Severe Weather (NWS)- includes discussion of tornadic radar signatures
NWS - Analysis of May 3, 1999 Oklahoma City Tornado Signature
Radar meteorology
Severe weather and convection
Tornado