Hook echo

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. 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. Due to the extreme unpredictability 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 . Radar was first widely used for meteorological purposes during World War II; and 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, IL 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 echos 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 echos, 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 -strong thunderstorms with the potential to produce tornadoes; found in supercell thunderstorms - . 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 airmass. 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. 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. Those two currents have a rotation, due to the vertical windshear, and interact to form a mesocyclone. Tightening of the rotation due to the interaction of those two air currents near the surface will create the 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. Also, especially preceding the advent of the more advanced Doppler weather radars, by the time a hook echo could be detected a tornado had often already formed—especially when the storm was far away from the location of the radar.