Супержұлынды дауылдардағы құйымшақ тәрізді радиолокациялық белгілер

Weather radar signature indicating tornadic circulation in a supercell thunderstorm

A hook echo is a pendant or hook shaped weather radar signature as part of some supercell thunderstorms. 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 even debris being wrapped around the supercell. It is one of the classic hallmarks of tornado producing supercells. 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.

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. 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 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. 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 data from 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 produce tornadoes.

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.