FORMATION AND STEERING MECHANISMS OF TORNADO CYCLONES AND ASSOCIATED HOOK ECHOES

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  • 1 The University of Chicago, Chicago, Ill.
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Abstract

An attempt is made to explain the mechanism of a hook-echo formation on the southwestern edge of an eastward-moving cumulonimbus cell. The conditions necessary for originating a hook-echo circulation through this proposed mechanism are: significant absolute vorticity within the subcloud layer, intense updraft to bring the low-level moist air into the cloud, and a vertical wind shear which steers the cumulonimbus cell with a velocity which is considerably different from that of the low-level winds. The Magnus effect involving the steering current and the spiraling up-draft is considered to be the force which directs the hook-echo circulation at low levels toward the southwestern edge of the major thunderstorm cell. A kinematic diagram with the absolute tangential speed and the radius of the cyclone as coordinates is also presented, as well as some speculation on the conservation of absolute circulation and absolute vorticity.

Abstract

An attempt is made to explain the mechanism of a hook-echo formation on the southwestern edge of an eastward-moving cumulonimbus cell. The conditions necessary for originating a hook-echo circulation through this proposed mechanism are: significant absolute vorticity within the subcloud layer, intense updraft to bring the low-level moist air into the cloud, and a vertical wind shear which steers the cumulonimbus cell with a velocity which is considerably different from that of the low-level winds. The Magnus effect involving the steering current and the spiraling up-draft is considered to be the force which directs the hook-echo circulation at low levels toward the southwestern edge of the major thunderstorm cell. A kinematic diagram with the absolute tangential speed and the radius of the cyclone as coordinates is also presented, as well as some speculation on the conservation of absolute circulation and absolute vorticity.

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