Constant absolute vorticity trajectories (CAVT)

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CONSTANT ABSOLUTE VORTICITY TRAJECTORIES (CAVT)

It has been proved mathematically (under some rigid assumptions) that air parcels moving in the atmosphere conserve their absolute vorticity; that is, the absolute vorticity values do not change. The most important of the assump-tions is that no convergence or divergence occurs. Actual observations under conditions of no convergence or divergence bear out this theory. Therefore, as a parcel moves from latitude to latitude in the Northern Hemisphere it describes a definite pattern called a constant absolute vorticity trajectory (CAVT). Figure 8-5-4 is an example of a CAVT. Constant absolute vorticity trajectory is defined as the latitudinal path of an air parcel whose absolute vorticity is unchanging. Constant absolute vorticity trajectories are directly related to the movement of long waves.

Figure 8-5-4.—Samp1e CAV trajectory.

The amplitude and wavelength of a long wave trough varies depending on the initial speed, direction, and latitude of the air parcel at the long wave’s inflection points. Inflection points are simply those points on the long wave where wave curvature changes from cyclonic to anticyclonic and vice versa. These are shown in figure 8-5-5 as points A, B, C, and D.

The wind at the inflection points is the controlling factor in determining the future amplitude and wavelength of the long wave using CAVT tables. Both amplitude and wavelength increase with an increase in wind speed. Wind direction, however, may increase or decrease amplitude and wavelength, depending on the angle created by the wind direction at the inflection point (angle formed by a wind direction arrow and the latitude line). When the inflection angle is small, because of a basic westerly wind component, waves tend to be flat and elongated.

On the other hand, a northerly or southerly wind component creates a large inflection angle, and waves exhibit great amplitude and wavelength. Constant absolute vorticity trajectory tables were developed to provide a means of forecasting the future position of long waves. Because CAVT tables are used in forecasting, I will not elaborate

Figure 8-5-5.—Sinusoidal vorticity path.

any further at this time. These tables will be discussed in more detail in the AG 1 rate training manual.

References

Aerographer’s Mate 1 & C, NAVEDTRA 10362-B1, Naval Education and Training Program Development Center, Pensacola, Fla., 1974.

Convergence and Divergence, Parts I & II, CNATT-L123 PAT, Naval Air Technical Training Command, Memphis, Term.

Forecasting for the Mid-Latitudes, Vol. II, NAVEDTRA 40502, Naval Education and Training Support Center, Pacific, 1978.

Meteorological Wind-Scales, Naval Weather Research Facility 34-0961-048, Norfolk, Va., 1961.

Practical Methods of Weather Analysis and Prognosis, Naval Operations, Washington, D.C., 1952.

Rotational Motion, Part I—Terminology and Related Concepts, Training Command, Memphis, Term.

Rotational Motion, Part II—CA VT and the Vorticity Theorem, Training Command, Memphis, Term.

Smithsonian Institute, Smithsonian Meteoro-logical Tables, Washington, D.C., 1951.

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