THUNDERSTORMS. Consider, for the moment, the typical physical appearance and behavior of a thunderstorm. Usually, the onset of rain is quite sharp and the precipitation is heavy. As quickly as the thunderstorm passes, the rain stops. It is not surprising, therefore, that the echo that is returned from a thunderstorm is almost always bright (good raindrop targets) and reasonably sharp edged. The brightness and sharpness of a thunderstorm echo distinguishes it from almost any other type of echo. 

You can follow the development of a thunderstorm quite clearly on the PPI. The horizontal extent can be determined by watching how many degrees of azimuth are covered by the echo. If there is only one thunderstorm in the area, sector scan is desirable. Sector scanning permits searching between specific azimuth coor-dinates instead of the entire 360-degree circle.

The location of the active cells within a thunderstorm are found by using the radars gain control. The higher the gain setting, the higher the power of the set. The highest setting shows up everything that the radar is capable of seeing. As the gain is reduced, the less intense part of the echo fades out. Since the cells are the most active part of the thunderstorm, they are the last to disappear as the gain is reduced.

LIGHTNING. It has been observed that lightning strokes show up on the PPI. Usually, the strokes cannot be identified by the naked eye. Their persistence is too short for spotting and identification. Motion pictures of the PPI with active thunderstorms do show the lightning strokes. When seen, there is no mistaking their identity. On the scope, they resemble their exact appearance in the sky.

TORNADOES. One of the most controver-sial issues in radar meteorology is the identifica-tion of tornadoes on a PPI. In general, the contention is that the tight circulation associated with a tornado shows a hook, V, or eye on an intense thunderstorm echo. There are many instances in which a PPI picture was taken of such a manifestation while a tornado was in progress. The location of the hook or V coincided exactly with the known location of the tornado. Unfor-tunately, hooks, Vs, and eyes are also seen on intense thunderstorms when tornadoes are not present. In a still picture, those associated with the destructive storms are no different from those that are not. Only when the manner of hook development is watched over a period of time can an experienced observer distinguish between tornado hooks and meaningless hooks, and even then there may be doubt.

COLD FRONTS AND SQUALL LINES. In considering the appearance of cold fronts and squall lines on a PPI scope, start with a concept of how these phenomena appear in nature. An inactive cold front contains no precipitation and does not show up on radar, while a well-defined active cold front and/or a squall line contain convective clouds and are marked by a band of active precipitation. On the PPI, all areas of precipitation show up. The typical, unmistakable appearance of a front or squall line is a narrow band of discrete echoes oriented in a line, moving across the scope as a unit. See figure 10-4-2.

WARM FRONTS.The classic picture of a warm frontal region is a wide cloud shield con-taining layered, precipitating clouds with perhaps a few thunderstorms penetrating the layers. On the PPI, the warm frontal picture is substantially the same. A large part of the scope is covered by soft echoes (indicating continuous rather than showery precipitation). Usually, at full gain setting, thunderstorms, even if present, do not show up. They are hidden in the continuum of the warm-front echoes. With reduced gain setting,

Figure 104-2.Line echo wave pattern (LEWP) in a squall line as it appears on a PPI scope.

however, the overall echo tends to disappear and the thunderstorm cells remain.

RAIN OR SNOW.Since water droplets scatter about five times as much energy as cor-responding snow crystals, the return from snow tends to be weaker, and the differences of intensity within a snowstorm are generally much less than in a rainstorm. Both texture and behavior help in distinguishing rain echoes from those of snow. A typical PPI presentation of snow is a uniformly hazy or coarse echo with very diffused edges. The texture of snow echoes is often described as soft in contrast to the sharp or hard echoes produced by rain. 

Another clue as to what kind of echo is in-volved may be found by varying the gain setting. There seems to be much more variation in the intensity of rain than of snow. A rain echo, as the gain is reduced, does not disappear as a unit. Certain portions fade out much more quickly than others. With snow, there is much greater likelihood that the entire echo will disappear at once. This characteristic may be used to good advantage in identifying the nature of the echo-producing substance.

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