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D. Sirmans
and
R. J. Doviak

Abstract

First results derived from an NSSL effort to develop a real-time Plan Position Indicator (PPI) display of Doppler isotachs are described. The mean Doppler velocity is estimated for multiple range locations by measuring the phase change of the complex echo envelope over time intervals equal to the pulse repetition period. This technique, phase change per pulse pair, provides velocity estimates directly from samples of the Doppler time record to circumvent spectrum computation.

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R. J. Doviak
and
D. Sirmans

Abstract

In this note we propose a technique to uncouple the tie between unambiguous Doppler velocity and range. Transmission of orthogonally polarized waves in pairs of pulses increases a weather Doppler radar's capability to unambiguously resolve velocity spectrum mean and variance at simultaneous range locations sampled in real time.

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R. J. Doviak
,
D. Sirmans
,
D. Zrnic
, and
G. B. Walker

Abstract

Data show that Doppler shift of storm echoes and the range extent of severe storm systems encompass a span exceeding the unambiguous resolving capability of centimetric radars. Echo coherency which is related to Doppler spectrum width and radar pulse repetition frequency (PRF) places limits on pulse-Doppler radar's unambiguous range and velocity measurements. Statistics on severe storm Doppler velocities and spectrum widths are given to evaluate limits for radars operating at different wavelengths.

The maximum range to which coherent Doppler measurements can be obtained is related to shear, turbulence and radar parameters. Data suggest that either eddy dissipation is high (ε > 1 m2 s−3) in large (20%) portions of severe storms or that many of the measured Doppler spectrum widths arise from vortices of scales small compared to the pulse volume but outside the equilibrium range of turbulence scales. Rangescrambled echoes are most annoying because they obscure observation, whereas velocity ambiguities can usually be resolved. The extent to which overlaid echoes obscure observation of mesocyclones is estimated for scattered and squall line thunderstorms. Staggered PRF techniques are shown to increase the unambiguous range and velocity limits of pulse-Doppler radars.

This study concludes that, aside from attenuation considerations, the biggest obstacle to shorter wavelength Doppler radar observation of severe storms is the larger occurrence of overlaid echoes resulting from the smaller unambiguous range necessary to meet echo coherency requirements.

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Peter S. Ray
,
R. J. Doviak
,
G. B. Walker
,
D. Sirmans
,
J. Carter
, and
B. Bumgarner

Abstract

On 20 April 1974 a tornadic storm passed between the two NSSL Doppler radars spaced about 42 km apart. Both radars simultaneously collected Doppler data throughout the storm. Air motions synthesized from these data provide the first three-dimensional display of Doppler-derived wind fields in a tornadic storm. Cyclonic circulation, associated with the tornado, and regions of intense up- and down-drafts are clearly evident.

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