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J. I. Metcalf
and
J. D. Echard

Abstract

A coherent polarization-diversity (dual channel) radar can be used to obtain spectral functions analogous to the four parameters derived from a noncoherent polarization-diversity radar. Formulations for the two power spectra and the cross spectrum are developed theoretically, and their interpretation is illustrated by model calculations. Analytical results include the derivation of raindrop size distributions, separation of Doppler velocity components due to air velocity and raindrop fallspeed, and improved identification of hail in rain. Effects of turbulence are illustrated and propagation effects are discussed briefly. The coherent polarization-diversity radar has potential value for a wide variety of research areas, including weather modification, severe storms, precipitation microphysics and electromagnetic propagation.

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E. F. Greneker
,
C. S. Wilson
, and
J. I. Metcalf

Abstract

Joint observations by radar and high-frequency sferics detectors at Georgia Institute of Technology provided unique data on the Atlanta tornado of 24 March 1975. The classic hook echo was detected by radar at a range of about 26 km, 15 min before the tornado touched down. While the tornado was on the ground the sferics burst rate was very low, despite very high values recorded immediately before and after this interval. This observation, together with visual reports of a strong cloud-to-ground discharge at the time of tornado touchdown, suggests an interaction of the tornado with the electric field of the storm.

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D. Atlas
,
J. I. Metcalf
,
J. H. Richter
, and
E. E. Gossard

Abstract

No abstract available.

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D. Atlas
,
J. I. Metcalf
,
J. H. Richter
, and
E. E. Gossard

Abstract

Ultra-high resolution (2m) radar observations show the amplification of unstable Kelvin-Helmholtz (KH) waves, the development of roll vortices, their breaking and the resulting turbulence, and appear to represent our first view of the life cycle of clear air turbulence. The KH waves are initiated at the base of an inversion at which the Richardson number, Ri, is slightly positive just prior to wave action, and above which Ri≫0. Accordingly, only a small enhancement of the wind shear at the interface will reduce Ri to the critical value (0–0.25) required to trigger KH waves. The KH waves also trigger stable waves in the dynamically stable stratum immediately above. Quantitative measurements indicate reflectivities typically 10 times greater, and occasionally 300 times greater, than the previously recorded maximum, but in strata of only a few meters vertical extent. Large-volume averaging by the prior low-resolution radars accounts largely for the discrepancy. The thinness of some of the scatter layers and the smoothness of the reflectivity contours precludes turbulent eddies exceeding a few meters, but the high reflectivities require major centimetric scale perturbations in refractivity. Direct measurements of microscale perturbations of the required magnitude by Lane, though rare, support the deductions. The origin of this microscale turbulence, especially in layers of large dynamic stability, is a mystery deserving attention. The intermittency of the KH wave activity and the undulations of the layer of large refractivity variance explain the previously reported patchiness of turbulence in and near stable strata, but raise serious questions as to the validity of long-path (duration) measurements of turbulence spectra. Both the form and intensity of the turbulence spectrum are also strongly dependent on height and the “age” of CAT.

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James I. Metcalf
,
Alexander W. Bishop
,
Richard C. Chanley
,
Timothy C. Hiett
, and
Pio J. Petrocchi

Abstract

The Geophysics Directorate of the U.S. Air Force developed a unique 11-cm (S-band) coherent polarimetric radar. The radar can transmit signals of alternating orthogonal polarizations, with either circular or linear basis, and receive signals of polarizations identical and orthogonal to that of the transmitted signal. The received signals, comprising logarithmic power and in-phase and quadrature components of both polarizations, are sampled in 50 selectable gates and recorded for off-line analysis. The radar antenna and the polarimetric data acquisition are controlled by interactive computer programs. Measurements of meteorological backscatter have been used to characterize the performance of the radar, to describe microphysical attributes of clouds and precipitation, and to identify the effects of changing electric fields on the orientations of hydrormeteors. Examples of these measurements are shown. Related facilities are described briefly.

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