A Demonstration of Antenna Beam Errors in Radar Reflectivity Patterns

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  • 1 Air Force Cambridge Research Laboratories, Sudbury, Mass
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Abstract

Measurement errors caused by antenna main-lobe and side-lobe distortion are computed for seven model thunderstorm reflectivity distributions observed over various ranges by model antenna beams with half-power widths of one and two degrees. Errors in echo top height measured by the two-degree beam, somewhat larger than the one-degree beam errors, rarely exceed ±5000 ft at ranges up to 25 n mi. In very intense storms, however, and at ranges of 50 mi or more, the measured echo top may be displayed at least 20,000 ft above the actual storm top. In extreme situations the displayed echo top may be double the true storm top. Measured maximum reflectivity may decrease with range by a much as 8 db at 50 mi. However, the reflectivity measured at 30,000 ft, roughly midway between maximum reflectivity and storm top, is largely independent of range out to 150 mi. The detectability of a hole surrounded by intense precipitation may be dependent on reduced radar sensitivity. A fairly accurate estimate of hole size is not possible unless its edges are sharp and its actual width exceeds twice the half-power antenna beam width.

Several methods are considered for correcting antenna beam distortion of echo tops. None of them are versatile enough to be used with confidence in all cases. The most promising one for sharp-edged reflectivity structures is a nomogram technique suggested by Probert-Jones, wherein side-lobe errors are eliminated by suitable reduction in radar sensitivity.

Abstract

Measurement errors caused by antenna main-lobe and side-lobe distortion are computed for seven model thunderstorm reflectivity distributions observed over various ranges by model antenna beams with half-power widths of one and two degrees. Errors in echo top height measured by the two-degree beam, somewhat larger than the one-degree beam errors, rarely exceed ±5000 ft at ranges up to 25 n mi. In very intense storms, however, and at ranges of 50 mi or more, the measured echo top may be displayed at least 20,000 ft above the actual storm top. In extreme situations the displayed echo top may be double the true storm top. Measured maximum reflectivity may decrease with range by a much as 8 db at 50 mi. However, the reflectivity measured at 30,000 ft, roughly midway between maximum reflectivity and storm top, is largely independent of range out to 150 mi. The detectability of a hole surrounded by intense precipitation may be dependent on reduced radar sensitivity. A fairly accurate estimate of hole size is not possible unless its edges are sharp and its actual width exceeds twice the half-power antenna beam width.

Several methods are considered for correcting antenna beam distortion of echo tops. None of them are versatile enough to be used with confidence in all cases. The most promising one for sharp-edged reflectivity structures is a nomogram technique suggested by Probert-Jones, wherein side-lobe errors are eliminated by suitable reduction in radar sensitivity.

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