Published: 2008-04-20 13:39:27
Updated: 2008-04-20 13:39:27
Posted April 20, 2008
MIKE MOSS SAYS: Steve, A hook echo is fairly loosely defined but frequently involves a radar echo taken at a low elevation angle that has an elongated aspect ratio in the direction of storm movement, a high reflectivity value at the core, a strong reflectivity gradient along one side of the storm and a broader gradient along the other, and a "hook" or curved appendage extending from the rear flank of the storm, usually the right rear in the northern hemisphere. Between the hook and the main body of the storm is a "weak echo region" with little or no reflectivity, and this region is often covered by an overhang of suspended precipitation that can be seen at higher radar beam elevations. An example of a hook echo is shown, and is a fairly typical case for a supecell thunderstorm in the United States, likely moving toward the northeast, with a high reflectivity gradient adjacent to a weak echo region along the right side (relative to it's motion) of the storm, and with a hook along the right rear flank of the storm. The hook portion of the echo is produced by precipitation wrapping around a circulation called the mesocyclone, a few hundred yards to a few miles in diameter, and if a tornado is present, it is often located near or within the hook area. Unless the storm is extremely close to the radar site, the circulations associated with the actual tornado are too small to resolve by radar, and the mesocyclone rotation stands in as a proxy to indicate the possible presence or future development of a tornado. On rare occasion when the storm is very close to a radar site, a high reflectivity spot can be seen along or near the hook area, and is associated with debris being swept above the ground by the tornado.
So, in answer to your last question, we and other meteorologists do indeed still look for hook echoes as a flag for possible severe weather. Up to date radar technology means that there are many other pieces of infomation that get added to the diagnostic mix, though, and these can be very helpful since from time to time the reflectivity pattern associated with a storm may mimic a hook when a true mesocyclone is not present, or in other cases a mesocyclone may be there but may be masked by a lack of precipitation or by an adjacent cell. For that reason, the doppler capabilities of radar are used to look for strong areas of horizontal wind shear associated with wind blowing strongly toward the radar very nearly adjacent to winds blowing strongly away. Such a "couplet" of sufficient magnitude can be a good indicator of mesocyclone presence, with the next step being to look for some vertical consistency to the rotation. If it extends over some vertical depth and maintains some continuity over time, then the chances a tornado may be produced increase considerably. When these characteristics are present in addition to a classic hook echo, there's definitely reason for concern, and while a tornado isn't guaranteed even then, a warning would almost always be issued. Even if a tornado is not present, such a storm would have a high potential to produce damaging straight-line wind gusts or hail meeting severe criteria.
One other tidbit to watch for in radar animations is that the most well-formed supecell storms have a propagation component to the their movement that makes them appear to move in a direction that is somewhat to the right of other showers and non-supercell storms in the area. When the radar is peppered with a number of cells and lines, this can occasionally make the stronger supecells stand out, in additon to their doppler velocity signatures and hook echoes.