Quantcast Tropical storms (hurricanes/typhoons)

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TROPICAL STORMS (HURRICANES/ TYPHOONS).— here is no standard description that can be applied to the appearance of tropical storms on radar screens. Nonetheless, a generalized description can be given with the understanding that any specific storm may be an oddity. Assume that a hurricane/typhoon approaches your station, passes close by, and then moves off. Until the rain shield associated with the storm is within approximately 250 miles of your radar set, the PPI, regardless of the power of the set, will not show any evidence of the storm. This is the result of the Earth’s curvature and radar beams traveling on a horizontal path. The farther the beams travel, the greater the separation between the beams and the surface of Earth. At a range of approximately 250 miles, the beams exceed the height of the hurricane clouds, and the storm goes undetected. As the storm continues to approach your station, echoes will begin to appear on the PPI scope. The echoes are almost identical to those produced by warm fronts, and if you are not aware of a hurricane’s presence from other sources, the echoes could easily be mistaken as being warm frontal echoes. As a hurricane approaches closer, the echoes begin to develop distinctive patterns. They acquire structure and appear as a series of concentric bands. See figure 10-4-3.

The position of the eye of the storm may be approximated by finding the center of curvature of these bands. Actually, the bands seem to spiral about the eye, and as more and more of the hur-ricane is seen on the PPI, the spiral pattern becomes more evident. The eye of the storm appears as a blank area on the scope, because there is no precipitation within the eye to reflect radar beams. Radar shows that the eye shifts, forms and reforms, or develops asymmetrically with respect to the nearest precipitation bands. As the storm moves away from your station, the radar picture usually becomes less definitive, because the trailing half of a tropical storm usually contains less precipitation than the leading half.

Plotting the Movement of Weather Echoes

Weather echoes are plotted in several ways. A suggested method is to place an acetate overlay over the PPI scope and mark the four directional reference points (N, E, S, and W) and a center reference point. When severe weather is ap-proaching your station, use a grease pencil to outline (mark) the weather echo(s). At designated times, depending on the speed of movement of the weather, mark the new location(s) of the echo(s).

Figure 10-4-3.—Hurricane showing echo-free eye on PPI scope.

The movement can be extrapolated to determine which storms, if any, will affect your station. Obviously, you should focus your attention on potentially critical upstream areas. When extrapolating the movement, allow for modifications to the weather by significant local terrain features and for changes in the size and intensity of the echo(s) as indicated by successive radar sweeps.

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