Anomalous Propagation

Anomalous Propagation (AP) is a false reflectivity echo on radar; an echo that is not precipitation. Radar return from AP is unpredictable, often contaminates precipitation measurements and can cause the generation of erroneous rainfall estimates used in hydrology products.

Typical ground clutterThere are occasions when it will be very difficult to determine AP from real echoes. Always get a second opinion for what is displayed by the radar. That second opinion is gained by looking at adjacent radars to determine if they are "seeing" the same thing.

There are two main types of AP and false echo seen on the RIDGE radar; Ground Clutter and Superrefraction. Ground Clutter is the most common false echo and is usually seen in every radar image. When atmospheric conditions are such where there are low-level inversions (air temperature increasing with height instead of the typical decreasing with height) ground clutter can be very pronounced.

Ground clutter is the easiest false echo to recognize since it does not move in any organized fashion, it has no kind of structure to it that is similar to real precipitation, and it is usually close to the radar. The ground clutter in clear air mode is often more prevalent than precipitation mode. This is because the radar is in its most sensitive operation thereby "seeing" smaller objects such as dirt, dust and bugs.

The image (above right) shows false radar echos from the radar beam reflecting off of the ground. The sky was clear at the time this image was recorded. 

Superrefraction is where the radar beam is bent greater than normal back toward the earth. If the bending is severe it can actually intersect with the earth's surface. With superrefraction, not only can the radar not see the more distant "real" precipitation, it can also appear the most real. Superrefraction is more likely when a strong low-level inversion is in place, especially when the low-level air is moist and the air above the inversion is very dry.

In the left image (below) from the Fort Worth Doppler radar, there are two main areas where thunderstorms are indicated; one to the north of the radar and one to the southwest. Mouseover the image to see the area which is AP. This is confirmed by looking at that same region with the Norman, OK radar (below right). Mouseover that image to see the lack of thunderstorms in the same region.

Mouseover to see AP from the Fort Worth radar
AP from the Fort Worth radar
Mouseover to see same region from the Norma, OK radar
Norma, OK radar without AP

The AP seen on the Fort Worth radar developed over about a 40 minute period after some showers dissipated. View a loop of the development of the AP.

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