air are formed above layers of cool air. The beam is bent due to differences in atmospheric density. becomes cooler as altitude increases.

When the inversion is very strong and shallow, the EM wave is trapped within the inversion layer. Small features, which can be seen close to the radar, are often obscured when viewed at great distances.This decrease in resolution, at increasing distances, is often why a solid line of thunderstorms appears to break-up as it approaches the radar. 0000050087 00000 n 0000047821 00000 n distances. Technical Document 2648. And it is near the center line of the beam where most of the energy is located with the energy decreasing away from the centerline.By convention we define the width of the beam as the distance between the two half power points - the point where there is a 50% reduction in the radar's transmitted energy.

wrong range.Under normal atmospheric conditions, the warmest air is found near the surface of the Earth. The beam is bent due to differences in atmospheric density. The position of the radar echoes depend heavily on the standard decrease of temperature hypothesis.

Super-refraction and extra-super refraction which are conditions of humidity with height and temperature variation above transmission path are necessary factors responsible for propagation beyond the horizon particularly in maritime. In their study, only the first highest duct when looking downwards was considered and the rather coarse resolution used implied that only horizontally widespread ducts were included in the statistics.
0000041967 00000 n 744 32 May 1994.

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The chances of two radars with identical atmospheric conditions are nil. refracted upward and proceeds on through the duct with a multiple-hop type of action.

In other words, the rate the elevation of the radar beam changes with distance away from the radar is less than normal.

This often results in spurious returned echoes and mis-interpretation of radar images such as erroneous precipitation detection. Super refraction of a weather radar beam produces more bending towards the ground surface than expected for standard conditions and therefore increases and intensifies ground clutter echoes (AP or anaprop echoes).

This happens when the atmosphere is stable relative to the Standard Atmosphere, and results in an extension in operational range. 0000052842 00000 n The first one deals with the While often depicted as a cone with distinct edges, the radar beam is better visualized much like that of ordinary household flashlights. Therefore, the true image of the storm is usually altered when displayed.In addition to beam spreading, the beam also does not travel in a straight line. This means the top portion of a beam in the atmosphere can move faster than the bottom portion.Under normal atmospheric conditions, when there is a gradual decrease of pressure, temperature, and humidity with height, a radar beam's curvature is slightly less than the earth's curvature.However, atmospheric conditions are never normal. 0000002975 00000 n Engineer's Refractive Effects Prediction System (EREPS) Version 3.0. 0000053479 00000 n < %PDF-1.3 %���� Super-refraction.

0000052520 00000 n

These density differences, caused by variations in temperature, moisture, and pressure, occur in both the vertical and horizontal directions and affect the speed and direction of the radar beam.The denser the atmosphere the slower the beam travels. In this case, we can use reflection/refraction to understand how the radar 0000053230 00000 n Anomalous propagation can cause interference to VHF and UHF radio communications if distant stations are using the same frequency as local services. San Diego, CA References Alpert, Ya. Over-the-air analog television broadcasting, for example, may be disrupted by distant stations on the same channel, or experience distortion of transmitted signals The first assumption of the prediction of propagation of a radio wave is that it is moving through air with temperature that declines at a standard rate with height in the On surface-base inversion, the beam will eventually hit the ground and a part of it can be reflected back toward the emitter. When the wave strikes the Earth or a warm layer below the duct, it is again reflected or

toward Earth.

Figure 2-15.—(A) Normal refraction, (B) Subrefraction, (C) Superrefraction, (D) Ducting. Refraction: gradient of the refractive index: schematic representation: conditions for the emergence: negative (sub-refraction) >0: snowfall, if the temperature reduced dependent of the height fundamentally fast, the humidity reduced fundamentally more slowly than in the normal atmosphere: Super-refraction occurs when atmospheric conditions cause the radar beam to bend more than in the Standard Atmosphere (Figure 2).

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