P r = P t G 2 λ 2 ( 4 π ) 3 R 4 η Δ V = P t G 2 π 2 64 λ 2 ︸ c r Δ V | K | 2 R 4 Z = C r Δ V | K | 2 R 4 Z. Snow generally is not as serious as rain in reducing radar performance. Unless doppler processing is used, S band and higher frequency radars are seriously degraded (clobbered) in rain. Rain clutter can seriously affect the performance of radars at L band and higher. “Sea spikes” are the dominant clutter mechanism at these frequencies. On the other hand, Bragg scatter does not apply at the higher microwave frequencies. The theory of radar sea clutter at HF and VHF is readily described by Bragg scatter. Theoretical models of clutter echo are poor and remote sensing results over land might be misleading. With the possible exception of vertical incidence, the description of land clutter at higher grazing angles is not as good. The available information on land clutter at low grazing angles is quite good. Land clutter is a more serious limitation than sea clutter. It is far more difficult to characterize clutter than to characterize receiver noise. Ĭlutter can be more harmful than receiver noise in limiting the ability of a radar to detect targets.The following brief statements review the general nature of radar clutter: The chief characteristic of clutter is its variability. Measured amplitude distributions “almost never pass rigorous statistical hypothesis tests for belonging to Weibull, lognormal, or other theoretical distributions that have been tried.” There is little difference in the clutter echo between vertical and horizontal polarization. The variation of the average clutter echo with weather, season, and from day-to-day is small. The effect of vertical discrete objects on the overall clutter strength is large even when these objects are relatively sparse. Most of the significant clutter echoes come from spatially localized or discrete vertical features such as trees, fences, buildings, and high regions of terrain. Ground clutter of rural terrain at low gazing angles as a function of frequency, showing mean values of clutter strength (open circles), standard deviation (vertical bars), and the extreme measured values (horizontal bars). Since hail can cause the rainfall estimates to be higher than what is actually occurring, steps are taken to prevent these high dBZ values from being converted to rainfall.Fig. Hail is a good reflector of energy and will return very high dBZ values. These values are estimates of the rainfall per hour, updated each volume scan, with rainfall accumulated over time. Depending on the type of weather occurring and the area of the U.S., forecasters use a set of rainrates which are associated to the dBZ values. The higher the dBZ, the stronger the rainrate. Typically, light rain is occurring when the dBZ value reaches 20. The scale of dBZ values is also related to the intensity of rainfall. The value of the dBZ depends upon the mode the radar is in at the time the image was created. Notice the color on each scale remains the same in both operational modes, only the values change. The other scale (near left) represents dBZ values when the radar is in precipitation mode (dBZ values from 5 to 75). One scale (far left) represents dBZ values when the radar is in clear air mode (dBZ values from -28 to +28). Each reflectivity image you see includes one of two color scales. The dBZ values increase as the strength of the signal returned to the radar increases. So, a more convenient number for calculations and comparison, a decibel (or logarithmic) scale (dBZ), is used. Reflectivity (designated by the letter Z) covers a wide range of signals (from very weak to very strong). "Reflectivity" is the amount of transmitted power returned to the radar receiver. The colors are the different echo intensities (reflectivity) measured in dBZ (decibels of Z) during each elevation scan.
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