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RADAR DETECTION OF THE SEA BREEZE

David AtlasGeophysics Research Directorate, Air Force Cambridge Research Center

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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.

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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