Quantcast Analysis Procedures

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Analysis Procedures

As with any weather analysis, you must look to the past to better understand the present. Make sure past history is transferred onto the current streamline chart. In streamline analysis the past locations of cyclones, anticyclones, waves, neutral points, and asymptotes of convergence and divergence are marked using a yellow pencil. Your next step is to evaluate the plotted data for obvious errors. The last preliminary step is dependent on your access to real-time satellite pictures. Pictures coinciding with your chart time should be examined.

Before drawing your first streamline, you should tentatively locate and mark (in pencil) the center positions of anticyclonic and cyclonic centers and their associated neutral points. Also, mark the center positions of tropical depressions, storms, and hurricanes/typhoons according to information taken from appropriate advisories or warnings, and label the centers with standard symbology. Note that each of the above features is a singular point. Draw the streamlines in the same manner as any other cyclonic circulation. However, do not forget that on upper-level charts (300- or 200-mb), the wind flow changes from cyclonic to anticyclonic. As hurricanes track northward and enter midlatitudes, the stream-line analysis is dropped and a contour (pres-sure) analysis is begun. It is up to the command to determine at what point the switch is made.


Figure 9-2-11.—Neutral points in the streamlines.





Figure 9-2-12.—Low-level streamline-surface contour analysis combination.

Figure 9-2-12 illustrates the low-level streamlines associated with a hurricane and the switch from streamline to contour analysis.

A recommended area in which to start drawing streamlines is at the base of the subtropical ridge line. The wind flow in this area is fairly straight (undisturbed). These winds are the trades, and they are the dominant feature of most tropical streamline analyses. Draw a few streamlines through this area first. Then, analyze the major


Figure 9-2-13.—Streamline-isotach analysis over a large ocean area.

features in the subtropical ridge—the anti-cyclones, and neutral points. Upon completing this, locate and draw streamlines within any other extensive area of undisturbed wind flow. Your next step is to look for cyclones and their associated neutral points. Asymptotes are then drawn to these singular points and also to areas of confluent and difluent winds. After all major features and asymptotes are drawn, sketch additional streamlines until a smooth and con-tinuous pattern of wind direction is established. You should remember that some interpolation is almost always required in drawing streamlines. Streamlines must parallel the wind arrows, although some leeway is given in areas of wind speeds that are 5 knots or less.

The analysis of the wind field is incomplete without the isotachs. They are dashed green lines which, like streamlines, are first sketched in pencil. They represent areas of equal wind speed. Isotachs are drawn at 5-knot intervals up to 20 knots, then every 10 knots thereafter. Figure 9-2-13 is an example of an isotach pattern typically found on a streamline analysis. The major axes of isotach patterns tend to parallel streamlines. The higher wind-speed areas show elongated patterns, while areas of low wind speed show a much broader isotach pattern. Color coding is used to denote certain wind speed areas. Areas with speeds 30 knots or greater are shaded in purple, while areas with wind speeds of 10 knots or less are shaded yellow. In addition to this shading, the terms MAX and MIN are placed inside the highest and lowest isotachs respectively.

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