Pulse duration, t, is the length of time in microseconds (sec or 10-6 s) that a pulse is transmitted. The pulse duration of the WSR-88D in short pulse mode (VCP 11, 12, 21, 121 or 32) is 1.57 sec. Multiplying pulse duration by the speed of light (c » 3 x 108 m/s) produces a pulse volume whose pulse length, H, is ~ 500 m. In long pulse mode (VCP 31), the pulse duration is 4.7 sec producing a pulse length, H, of ~ 1500 m (4,625 ft). In long pulse mode, the total power received from a given target would correspondingly be three times greater than in short pulse mode. As a result, the WSR-88D is more sensitive when operating in long pulse mode than in short pulse mode. The WSR-88D operates in alternating long/short pulse mode.
|Figure 1. Example of pulse duration, t, and listening period, . Time is along the x axis and, when multiplied by the speed of light, c, yields the actual physical length of the pulse, H.|
The radar sends a pulse of energy out, and then waits for a return echo. The length of time that the radar "waits" is based upon the "range" of the radar to detect useful echoes. This is known as Pulse Repetition Time. The number of pulses in a unit time is known as the Pulse Repetition Frequency (PRF).
The range of the current WSR-88D (NEXRAD) radars is about 230 km. The radars can "see" beyond that range, but the echo resolution is very poor beyond that radius.
So the radar "waits" for the length of time it would take for the return pulse to return (all at about the speed of light) before sending out another pulse. The trouble is, the first pulse will have traveled out beyond 230 km and could have encountered another scatterer (although poorly). That scatterer will return a pulse of energy to the radar that will arrive slightly after the radar has sent out a second pulse. The radar will interpret this return energy as an echo from a source at a very nearby range. This is a false echo and the process by which false echoes appear in this manner is called "range folding".
This is the way it would work. Suppose the maximum unambiguous range is 230 kilometers. If a storm is located at 345 kilometers from the radar, the radar will detect the storm as being 115 kilometers from the radar instead. Any energy returned to radar beyond 230 kilometers would be range folded. This occurs because the radar energy bounced off the distant storm is returning to the radar after the radar has already sent out another pulse and is "listening" for return echoes.
To some extent, range folded data can be "unfolded" by computer software (for example, comparing returns the radar receives by briefly using different pulse repetition frequencies and artificially changing the maximum unambigous range). In areas in which this cannot be done, the color designation will indicate that range folding is an issue (RF) and the echoes in that area should not be accepted.
The term "gate" is used to denote the radar's "pixel" or resolution in which a unit percentage of the radar's energy is concentrated. Since the radar beam spreads out, the same unit percentage of energy takes up a larger area. Thus, the "gate" are larger, the further one gets out from the radar.
Because of this, the resolution of radar data decreases with distance from the radar. These range gates become larger with increasing distance from the radar site since the beam is expanding as it moves away from the radar site. Large range gates will result in less resolution. As beam spreading increases (diameter of cone increases), rainfall intensity is increasingly underestimated, rainfall areal coverage is increasingly overestimated and precipitation and/or heavy precipitation in only part of the range gate increasingly becomes averaged over a larger area.
The image below illustrates the issues associated with beam spreading. The radar site is KUEX and is at the extreme left of the image. Notice as one examines the dispaly further outward on radials, the range gates become larger. The portion of the echoes at the edge of the radar look more grainy and the reflectivity areas more "smeared" than those close to the radar.
So....keep in mind that the storms near the edge of the radar are in actuality smaller than shown by radar (because of beam filling issues). Also, the storms near the edge of the radar are also being sampled at a higher elevation than the storms close to radar.