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 as sufficient to justify issuing a tornado warning.

The hook echo has been recognized as a sign of tornado development early in weather radar's history. In 1949, E. M Brooks had referred to circulations having radii of approximately 8–16 km with supercells thunderstorms and named them tornado cyclones. The first documented tracking of a hook echo associated with a tornado was on April 9, 1953 by the Illinois State Water Survey near Urbana-Champaign, during preparations for an early test of radar's ability to measure rainfall rates by Donald Staggs electrical engineer of Dr. Huff research team.

Ted Fujita documented hook echoes associated with other supercells on the same day as Huff team made their observations. He inferred the concept of thunderstorm rotation from viewing the evolution of the hook echoes, which he studied in detail. In 1962, J. R. Fulks was the first to hypothesize on the formation of hook echoes. Wind velocity data obtained following the installation of weather radars having Doppler effect capabilities in central Oklahoma in the late 1960s confirmed an association between hook echoes and strong horizontal shear zones associated with storm rotation and tornadoes.

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.

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