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David Atlas

Abstract

Mutually conflicting observational requirements such as high resolution, large-scan swath width, high-scan speed, and adequate signal dwell time have restricted the development of spaceborne meteorological radar from the measurement of precipitation. The approach suggested here, which involves adaptive sampling by the radar only in regions likely to be occupied by precipitation as seen by a forward-looking passive imager, appears to overcome these long-standing obstacles.

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David Atlas

Abstract

The use of a dual-arm non-coherent bistatic tropo-scatter link is proposed as a means of measuring the cross-path motion of the wind. The arms are skewed symmetrically to either side of the great circle path so that one is pointing upwind and the other downwind. The signals received on each arm are then associated with Doppler frequency shifts of opposite sign. When summed at the receiver, their beating produces a signal fluctuation spectrum having a secondary peak equal to the difference in the mean Doppler frequencies on the two arms. This is directly proportional to the crosswind. Another approach involves the measurement of the mean-square signal fluctuation rate or spectrum variance on each arm of the link separately and then with both arms combined. The latter is the power-weighted sum of the variances on the individual arms plus a term which is proportional to the difference of the mean Doppler frequencies between the two arms of the link, and thus to the speed of the crosswind.

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David Atlas

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David Atlas

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David Atlas

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David Atlas

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David Atlas

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David Atlas

Abstract

Although the radar-reflectivity factor Z = ∑Nd 6 is a function of the liquid-water content W, median volume diameter d 0 (or other characteristic diameter), and breadth of the drop-size distribution, correlations of Z vs. W, and Z vs. d 0, provide reasonable estimates of W and d 0 from Z alone. If two of the latter three are known, considerably more reliable estimates of the third parameter are possible. Estimates may also be improved somewhat by an observation of cloud type. A theoretically feasible, though instrumentally complex method for measuring cloud water-content is also described. The possibility of using reflectivity measurements for warnings of icing severity is discussed. The reflectivity statistics indicate that natural clouds have a preference for drop-size spectra between the n = 2 (B) and 3 (C) distributions of the “multi-cylinder” family.

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David Atlas

Abstract

The theory of the back-scatter from invisible atmospheric targets which are considered to be likely sources of “angel” echoes is reviewed. Special attention is given to planes and bubbles and vapor sheaths of large radius of curvature with cross sections which cannot be obtained from the conventional far-zone radar theory. The cross sections of such surfaces approximate reported “angel” cross sections if the surface reflection coefficient and corresponding refractive index gradient is large but not inconceivable. The segments of these surfaces need only be approximately as large as the first Fresnel zone in order to behave essentially like the entire surface. In the microwave band, the required sizes are of the order of a few meters at short ranges. Partial focusing by surfaces concave toward the radar may enhance the cross sections further. However, surface roughness may reduce the cross sections significantly.

While it may be difficult to visualize essentially smooth atmospheric surfaces as large as 5 to 10 m, the alternative solution, which assumes bubbles or eddies of small radius of curvature, requires extremely large and questionable refractive index gradients. Although the question is left open for future validation, the author believes, on the basis of limited observations, that the first alternative is the more likely one. This is indicated in several important cases of “angel” activity.

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David Atlas

Abstract

Radar and meteorological observations of four sea-breeze cases are presented. A clear-cut association between the echoes and the meteorological events is demonstrated. Where direct measurements are not available, the echo patterns are consistent with a rational physical picture of the sea breeze. Birds, insects, and other particulate matter are precluded by a combination of direct observation of the small pulse volume, by echo magnitude considerations, and by echo pattern. Scattering theory can account for the magnitude of the coherent layer echoes on the vertical beam, at the sea-breeze inversion with large but reasonable refractive-index gradients of 4 to 9 N units per cm or with vapor gradients of about 1 to 2 mb per cm. Incoherent echoes on the horizontal beam appear to come from vapor sheaths of large radius of curvature, with index gradients of similar magnitude, which are oriented parallel to the sea-breeze front. While this unusual result is supported by the streamer-form echo patterns and the steady sea-breeze winds, further verification is required.

The horizontal beam echoes disappear with the onset of the usually deeper mesoscale southwest sea breeze because this air, with a long overwater trajectory, has a vertically homogeneous moisture distribution which prevents sharp vapor fluctuations. The crucial condition necessary for the occurrence of persistent sea-breeze echoes on a horizontal beam is the sharp moisture lapse in the low levels.

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