Range-folded data can impact products and algorithms that use reflectivity data. When range-folded data are present, corrupted data are displayed with valid data. Products using multiple reflectivity scans may be affected as well. The impact of doppler velocity and spectrum width range folding is significant, both in the radars unambiguous range limits and in areas of significant velocity data lost due to ambiguous returns. In addition, range ambiguous values are treated as missing by the velocity-based algorithms and can, therefore, seriously impact those algorithms.

Assessing Impacts of Range-Folded Data on Velocity Products

The presence of range-folded (overlaid) data on Mean Radial Velocity products is inevitable. The inability to determine velocity estimates for the sample volumes results from the inability of the range unfolding algorithm to distinguish between power returns from two or more sample volumes at the same relative location within different trips. Therefore, valid velocity estimates can be derived for only one corresponding sample volume along each radial, The ring of range-folded (overlaid) data at the beginning of the second and subsequent trips is caused by the first trip ground clutter and is a common result of this range unfolding limitation.

NON-METEOROLOGICAL RADAR ECHOES 

This section will briefly describe methods of identifying and assessing the impacts of ground clutter, anomalous propagation, sidelobes, and solar effects.

Prior to calculation of reflectivity, velocity, or spectrum width, return signals from ranges within the radars normal ground clutter pattern are processed to remove most of the signal returned from targets that are stationary (part B of the FMH-11). The portion of signal not removed, called residual clutter, will remain as part of many of the products.

RECOGNITION OF RESIDUAL GROUND CLUTTER. A low-elevation Reflectivity product will show ground clutter close to the radar or distant mountainous terrain, It will normally appear as a cluster of points (having a speckled nature) or as a large area of contiguous returns. Errors are recognizable as wedges having sharp radial discontinuities from adjacent regions and whose predominant colors differ markedly. Errors may also appear as radial spikes several volume samples in length, whose velocities are shifted toward high positive or negative values. A time lapse of Reflectivity products will show no movement of these returns. With increasing antenna elevations, ground clutter returns will disappear. Generally, mean velocities will be near zero and spectrum widths will be very narrow.

ASSESSING IMPACTS OF RESIDUAL GROUND CLUTTER. Residual ground clutter near the radar may be recognized by its speckled appearance. When it is imbedded in a meteorological signal over the radar, the velocity dealiasing algorithm may introduce errors in the velocity field due to radial and azimuthal gate-to-gate shears greater than the Nyquist velocity. This problem is most likely to occur in the Clear Air mode using VCP (volume coverage pattern) 31, where the Nyquist velocity is about 21 kts. If this problem occurs in the Precipitation mode, the induced shears may be picked up by the mesocyclone algorithm and carried as a feature.

Ground Clutter Returns from Anomalous Propagation

Anomalous propagation of the radar beam is caused by nonstandard atmospheric temperature or moisture gradients (part B of the FMH-11). Super refraction, which is frequently caused by temperature inversions, bends the beam toward the earth and can cause the radar to detect ground returns from distances far exceeding the normal ground clutter area.

RECOGNITION OF GROUND CLUTTER RETURNS FROM ANOMALOUS PROPAGATION. With increasing antenna elevation, these returns will usually disappear. A time lapse of Reflectivity products may show apparent motion or changing patterns. There can be a 20 dBZe (a decibel of the equivalent radar reflectivity factor) or more difference between adjacent returns in the absence of precipitation, mean velocities maybe near zero, and spectrum widths may be narrow. Ground returns from anomalous propagation mixed with precipitation may result in large spectrum width values and low velocities. Erratic movement of ground returns from anomalous propagation, in comparison with the motion of precipitation echoes, may also be seen.

ASSESSING IMPACTS OF GROUND CLUTTER RETURNS FROM ANOMALOUS PROPAGATION. Ground return from anomalous propagation mainly affects interpretation of reflectivity echoes in the affected areas. It can cause erroneous output and increase edit time of the radar coded message in these areas. It may also affect algorithmic output; for example, if reflectivity is greater than 30 dlil~, erroneous identification of a storm may occur and precipitation accumulation values may be degraded. Super refraction of the radar beam frequently occurs behind the cold air outflow regions of thunderstorms. In these instances, the precipitation processing algorithms may erroneously interpret the ground returns as precipitation echoes and significantly overestimate the precipitation.

REMOVAL OF GROUND CLUTTER RETURNS F R OM ANOMALOUS PROPAGATION. Anomalous propagation can be removed, to a large extent, by application of the clutter filter to the elevation angles affected. This is accomplished by overriding the clutter map resident in the RDA through editing of the Clutter Suppression Regions menu at the Unit Control Position. Up to 15 clutter suppression regions can be edited in which three levels of suppression can be defined for the reflectivity and doppler channels. Unit Radar Committee guidance on the use of the clutter map editor must be obtained. If weather is not a factor, that is, when operating in the Clear Air mode or when ground clutter or anomalous propagation is in a precipitation-free sector in the Precipitation mode, it is reasonable to apply the clutter filter. If anomalous propagation is mixed with precipitation, the filter should not be applied.

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