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UNIT 8—LESSON 5

ROTATIONAL MOTION AS IT AFFECTS THE ATMOSPHERE

OVERVIEW 

Define vorticity and explain the two types. 

State how CAVT tables are used.

OUTLINE

Vorticity

Constant absolute vorticity trajectories (CAVT)

ROTATIONAL MOTION

An air-parcel pushing through Earth’s atmosphere has three spinning motions: (1) It rotates about its own axis; (2) it rotates about the axis of its pressure system; and (3) it rotates about Earth. All of these motions occur simultaneously. Measuring the spin imparted to an air parcel is known as vorticity measurement. In meteorology, vorticity measurements are applied only to those spinning motions having axes perpendicular to Earth’s surface. Air spinning about a high- or low-pressure system is an example of such motion, as these systems extend vertically upward into the atmosphere (a perpendicular axis). Vorticity measurement gained prominence in the 1950’s and 1960’s, and over the last 30 years it has become an important forecasting tool, especially in the field of numerical forecasting. This discussion will be brief, as I will simply explain the three spinning motions and the two types of vorticity. A more in-depth discussion of this subject and its forecasting application can be found in the AG 1 rate training manual.

Learning Objective: Define vorticity and explain the two types.

VORTICITY

A parcel has vorticity when it spins as it moves along its path. Depending on the direction of the spin, the vorticity is either positive or negative. In the Northern Hemisphere, if a parcel spins in a counterclockwise direction (cyclonically), it has positive vorticity. If a parcel spins clockwise (anticyclonically), it has negative vorticity. If a parcel does not spin, it is said to have zero vorticity.

For an example of how spin is imparted to air parcels, we could drop a chip of wood into a stream and watch its progress. The chip will move downstream with the flow of water, but it may or may not spin. If it spins, it has vorticity. Two properties of the stream cause the chip to spin: (1) Water moving faster on one side of the chip than on the other (a shear effect) and (2) curves in the stream bed (a curvature effect). When we measure the spin created by these two components (shear and curvature), we can determine relative vorticity.

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