Anomalous Propagation is abbreviated as AP. AP and false echoes are one of the biggest enemies of the radar meteorologist. Those without experience in radar meteorology are often "burned" by AP and false echoes. AP is a false reflectivity echo on radar (a reflectivity echo that is NOT precipitation, especially echoes produced by unusual rates of refraction in the atmosphere). With experience you will be able to recognize which echoes are AP / false echoes and which are precipitation. If the index of refraction is at a standard level, computer algorithms can remove some of the AP and false echoes. In cases where the index of refraction is unusual, AP is much more likely to show on radar. In extreme cases, the air near the ground may be so cold and dense that a radar beam that starts out moving upward is bent all the way down to the ground. This produces strong echoes at large distances from the radar. Generally, the Doppler velocity of AP echoes is zero, but the echoes themselves can move around because propagation characteristics of the atmosphere keep changing. There are 4 common AP and false echo signatures. Each of these will be discussed below: a. GROUND CLUTTER- This is the most common false echo. It results from backscattering from the radar beam sidelobes. Ground clutter is most common when low elevation beam angles are used and in cases where a low-level inversion is in place. Ground clutter is the easiest false echo for the radar operation to recognize. It is easy to recognize since ground clutter 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. b. HIGH SUPERREFRACTION- Superrefraction is also termed ducting if the radar beam bend down toward the earth's surface enough to actually intersect the earth's surface. When radar beams bounce off the earth's surface some of that energy will backscatter to the radar to show AP. Ducting is more likely when a strong lower tropospheric inversion is in place, especially when the low-level air is moist and the air above the inversion is very dry. Ducting is more likely in the morning since inversions tend to be stronger in the morning (colder air under warmer air). When ducting occurs, ground clutter echoes are also much larger in spatial coverage since side lobes are more easily bounced off the earth's surface. c. ECHOES BEYOND MAXIMUM UNAMBIGUOUS RANGE- Radars send pulses out at the rate a several hundred per second. This can be done since radar beams move at the speed of light. After radar sends a single beam out, it listening for returning energy. The time it listens for return echoes from that single pulse is limited by the Pulse Repetition Frequency that the radar is set to. If energy from a previous radar beam is returned after radar has already sent out another pulse, this tardy backscattered energy will show at an incorrect range from the radar. The radar thinks all returns are from the pulse it just sent out. These can be some of the most difficult false echoes to recognize since the returns have a precipitation structure to them but they can be easily recognized if multiple radars are used and higher tilt angles are used. Often these misplaced echoes can be recognized since they have a grainy structure to them since these were echoes detected at a long range from the radar set. d. BIOLIFE, MOUNTAINS, ETC.- Mountains are an AP problem in some locations. The amount of reflectivity from the mountains will depend on the elevation angle of the radar beam and the current index of refraction in the atmosphere. There are all kinds of other objects that can produce echoes that are not precipitation including aircraft, birds, dust, and strong density gradients. The last thing a weather person would want to do is track AP or false echoes on radar thinking it is real precipitation. It is important to acquaint yourself with the situations in which AP and false echoes are likely to show on the radar sites within your forecast region. If you have data from more than one radar, AP and false echoes are easy to identify because the neighboring radars generally won't see the same thing. If you have just one radar but can look at different angles, you will find that AP and false echoes tends to be confined to the lowest elevation angles. When the radar is pointed higher in the sky, it's unlikely that the beam can be bent backward all the way to the ground. If you're stuck with just one radar and one elevation angle, you have to get more imaginative. The structure of AP echo tends to be "unusual", with sudden, sharp changes in intensity. Also, its motion tends to be erratic, although this can also be misleading. The best points of comparison are surface observations and satellite data. |