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Dusan Zrnic and Michael Istok

Abstract

Doppler spectra of a tornado were collected with a radar having a large unambiguous velocity range, ±91 m s−1. Thus for the first time a presentation of nonaliased spectra was possible, showing direct measurement of radial velocities. By fitting the tornado model spectrum to data, the radius of maximum winds and tornado center location are deduced. Tornado spectral signature is defined as a double peak, symmetric with respect to the mean wind spectrum. Histograms of maximum measured wind speeds (from spectrum skirts) for two tornadic storms are obtained, and the histograms of velocity difference (between the left and right spectrum skirt) suggest that smaller scale turbulence (<500 m) is principally responsible for spectrum broadness.

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Michael J. Istok and Richard J. Doviak

Abstract

A technique to separate ordered flow in a tornadic thunderstorm from the random velocities associated with turbulence is described. The relative importance of the ordered flow shear and turbulence in the broadening of the Doppler velocity spectrum is evaluated by least-squares fitting an assumed linear model of radial velocities to measured ones over an angular analysis domain (about 3° in azimuth and 3° in elevation). Fields and cumulative probabilities of Doppler spectral widths associated with turbulence and velocity shear, of root-mean-square (rms) velocity residuals, and of the turbulent kinetic energy dissipation rates ε are presented. In order to estimate ε from measurements of Doppler spectral width, the outer scale of the inertial subrange of turbulence must be at least three times larger than the size of the radar's resolution volume. Wind fields synthesized from the Doppler data of two radars are related to the turbulent kinetic energy dissipation rate and rms velocity residual fields. The observed relationship between wind gradients, rms residuals, and dissipation rates suggests the expected cascade of turbulent kinetic energy as it moves from the largest length scales down to the smallest.

Within this storm, turbulence contributed much more to spectral broadening than ordered flow shear. However, in a very small portion of the storm, shear and nonturbulent eddies are responsible for nearly all spectral broadening. Fifty percent of the volume of this tornadic storm had ε larger than 0.1 m2 s−3.

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Steven V. Vasiloff, Richard J. Doviak, and Michael T. Istok

Abstract

The Next Generation Weather Radar (NEXRAD) may use a 5-min volume scan to monitor thunderstorms and provide hazard warnings. Short-lifetime, low-altitude wind shear near airports is a hazard to safe flights that deserves special attention. An interlaced scanning strategy is examined for its effects on the accuracy and reliability of some NEXRAD storm analysis and tracking algorithms that require noninterlaced data. By increasing the elevation step to twice its normal value and starting every other scan at the second step of the corresponding 5-min sequence, a pair of 2½-min sequences is achieved. These can be recombined for use in the NEXRAD algorithms while providing a shorter period between observations of rapidly developing phenomena such as low-altitude wind shear. It is found that differences between storm cell attributes derived from successive non-interlaced scans are about the same as differences between values obtained from interlaced and noninterlaced volume scans for the same time period. Thus, interlaced scanning may halve the wind shear warning time to be provided by the proposed NEXRAD noninterlaced scan strategy without significantly compromising the evaluation of storm attributes. Growth rates of reflectivity and updraft speed for several cells during the growth stage of a severe thunderstorm have been assessed in relation to the need for 2½-min updates to resolve severe thunderstorm phenomena. Results indicate that the growth rates are not so rapid as to require interlaced scanning for this purpose.

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Valery M. Melnikov, Michael J. Istok, and John K. Westbrook

Abstract

Radar echoes from insects, birds, and bats in the atmosphere exhibit both symmetry and asymmetry in polarimetric patterns. Symmetry refers to similar magnitudes of polarimetric variables at opposite azimuths, and asymmetry relegates to differences in these magnitudes. Asymmetry can be due to different species observed at different azimuths. It is shown in this study that when both polarized waves are transmitted simultaneously, asymmetric patterns can also be caused by insects of the same species that are oriented in the same direction. A model for scattering of simultaneously transmitted horizontally and vertically polarized radar waves by insects is developed. The model reproduces the main features of asymmetric patterns in differential reflectivity: the copolar correlation coefficient and the differential phase. The radar differential phase on transmit between horizontally and vertically polarized waves plays a critical role in the formations of the asymmetric patterns. The width-to-length ratios of insects’ bodies and their orientation angles are retrieved from matching the model output with radar data.

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Mark Weber, Kurt Hondl, Nusrat Yussouf, Youngsun Jung, Derek Stratman, Bryan Putnam, Xuguang Wang, Terry Schuur, Charles Kuster, Yixin Wen, Juanzhen Sun, Jeff Keeler, Zhuming Ying, John Cho, James Kurdzo, Sebastian Torres, Chris Curtis, David Schvartzman, Jami Boettcher, Feng Nai, Henry Thomas, Dusan Zrnić, Igor Ivić, Djordje Mirković, Caleb Fulton, Jorge Salazar, Guifu Zhang, Robert Palmer, Mark Yeary, Kevin Cooley, Michael Istok, and Mark Vincent

Abstract

This article summarizes research and risk reduction that will inform acquisition decisions regarding NOAA’s future national operational weather radar network. A key alternative being evaluated is polarimetric phased-array radar (PAR). Research indicates PAR can plausibly achieve fast, adaptive volumetric scanning, with associated benefits for severe-weather warning performance. We assess these benefits using storm observations and analyses, observing system simulation experiments, and real radar-data assimilation studies. Changes in the number and/or locations of radars in the future network could improve coverage at low altitude. Analysis of benefits that might be so realized indicates the possibility for additional improvement in severe-weather and flash-flood warning performance, with associated reduction in casualties. Simulations are used to evaluate techniques for rapid volumetric scanning and assess data quality characteristics of PAR. Finally, we describe progress in developing methods to compensate for polarimetric variable estimate biases introduced by electronic beam-steering. A research-to-operations (R2O) strategy for the PAR alternative for the WSR-88D replacement network is presented.

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