Vertical Velocity Charts

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Vertical Velocity Charts

Vertical velocity charts are currently being transmitted over the facsimile network and are computed by numerical weather prediction methods.

The charts have plus signs indicating upward motion and minus signs indicating downward motion. The figures indicate vertical velocity in centimeters per second (cm/sec). With the larger values of upward motions (plus values) the likelihood of clouds and precipitation increases. However, an evaluation of the moisture and vertical velocity should be made to get optimum results. Obviously, upward motion in dry air is not as likely to produce precipitation as upward motion in moist air.

Studies have shown that surface cyclones and anticyclones are not independent of developments in the upper atmosphere, rather, they work in tandem with one another. The relationship of the cyclone to the large-scale flow patterns aloft must therefore be a part of the daily forecast routine.

Many forecasters have a tendency to shy away from the subject of vorticity, as they consider it too complex a subject to be mastered. By not considering vorticity and its effects, the forecaster is neglecting an important forecasting tool. The principles of vorticity are no more complicated than most of the principles of physics, and can be understood just as readily. In the following section we will discuss the definition of vorticity, its evaluation, and its relationships to other meteorological parameters.

VORTICITY

LEARNING OBJECTIVES: Recognize the two components of relative vorticity. Define the term absolute vorticity. Determine vorticity impacts on weather processes.

Vorticity measures the rotation of very small air parcels. A parcel has vorticity when it spins on its axis as it moves along its path. A parcel that does not spin on its axis is said to have zero vorticity. The axis of spinning or rotation can extend in any direction, but for our purposes, we are mainly concerned with the rotational motion about an axis that is perpendicular to the surface of the Earth. For example, we could drop a chip of wood into a creek and watch its progress. The chip will move downstream with the flow of water, but it may or may not spin as it moves downstream. If it does spin, the chip has vorticity. When we try to isolate the cause of the spin, we find that two properties of the flow of water cause the chip to spin: (1) If the flow of water is moving faster on one side of the chip than the other, this is shear of the current; (2) if the creek bed curves, the path has curvature. Vorticity always applies to extremely small air parcels; thus, a point on one of our upper air charts may represent such a parcel. We can examine this point and say that the parcel dots or does not have vorticity. However, for this discussion, larger parcels will have to be used to more easily visualize the effects. Actually, a parcel in the atmosphere has three rotational motions at the same time: (1) rotation of the parcel about its own axis (shear), (2) rotation of the parcel about the axis of a pressure system (curvature), and (3) rotation of the parcel due to the atmospheric rotation. The sum of the first two components is known as relative vorticity, and the sum total of all three is known as absolute vorticity.

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