Quantcast Identification of weather echos on RHI scopes

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IDENTIFICATION OF WEATHER ECHOES ON RHI SCOPES

Range height indicators provide very valuable information that is not available in any other way. When used, the antenna is fixed with respect to azimuth and scans in the vertical only. The presen-tation on the RHI is a vertical cross section of the atmosphere along the preset azimuth.

Cloud Types

In general, it is rather simple to identify the various cloud types that produce echoes on the RHI scope. However, instead of word descriptions, refer to the reproduced RHI pictures in figure 10-4-4, views A, B, C, D, and E. In view A, the broken pattern shows convective-type showers. The fact that the echo does not extend above 11,000 feet indicates that the showers are weak. The shower cells shown in the lower levels of view B appear to originate from a thick cloud layer extending from 12,000 to 26,000 feet.

In view C, a definite series of stratified layers is shown between 8,000 and 18,000 feet. The relative intensity of the layers can be determined by progressively decreasing the gain. The most intense layers will continue to show up on the scope as the gain is decreased. A thick layer directly above the station is shown in view D. A short distance away, precipitation reaches the ground. The advance of the precipitation with time can be followed by watching the blank area near the station. The streaky nature of the echo in the upper levels indicates shower activity. In view E, three layers are pictured with precipitation falling through them. Again, a cellular structure seems indicated.

Bright Band

Observers using an RHI scope frequently speak of a bright band echo pattern appearing on their scope. This bright band is caused by a concentrated layer of water-covered ice particles, created when snow falls into a layer above freezing temperatures. 

The snow particles become water-coated as they melt, and by the time the melting process is complete, two things happen: (1) there’s an increase is the concentration of particles in the melting layer, and (2) the water-covered particles produce higher reflectivity values than the pure snow/ice particles above the freez-ing level. A well-defined thin horizontal bright band is indicative of very stable air, while a bright band that is ill defined or nonexistent is associated with extremely unstable air.

The significance of the bright band is its rela-tionship to the freezing level. When the bright band is detected on the RHI scope, it is safe to say the freezing level is located at the level corresponding to the top of the band or perhaps 1,000 feet (300 meters) higher.

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