Transient changes in Doppler spectra of precipitation associated with ...

JOURNAL

OF GEOPHYSICAL

RESEARCH, VOL. 92, NO. D6, PAGES 6699-6704, JUNE 20, 1987

Transient Changes in Doppler Spectra of Precipitation Associated With Lightning VLADISLAV MAZUR, DUSAN S. ZRNIC', AND W. DAVID RUST National SevereStormsLaboratory, National Oceanicand AtmosphericAdministration,Norman, Oklahoma

From radar observationsobtained at vertical incidence,a transient increase(5-10 dB on the average) of noise power in the Doppler spectra associatedwith lightning is analyzed. To explain this effect, three hypothesizedcausesare tested: (1) a sudden increase in echo power caused by lightning within the Fourier analysis window, (2) refractive index discontinuity, and (3) drop oscillation; refractive index discontinuityand drop oscillationare both producedby a lightning shockwave propagatingthrough the sampling volume. The data analysis strongly suggeststhat the refractive index discontinuity is mainly responsible for the transient effect.

1.

INTRODUCTION

Recently, scientists have begun to use Doppler radar to study lightning and its effects on precipitation [Zrnic' et al., 1982; Lhermitte, 1982; Williams and Lhermitte, 1983]. A distinct separation of the Doppler velocity spectra of lightning from the spectra of precipitation has allowed determination of several properties of intracloud lightning flashes,such as the presence of continuous current and the acceleration of the lightning channel by the earth's magnetic field and buoyancy [Mazur et al., 1985]. In the majority of Doppler spectra containing lightning contributions, Mazur et al. [1984] found a short-term increasein the power of spectral skirts, lasting tens of milliseconds. It is such transient changes that are studied here.

2.

The

facilities

DATA COLLECTION AND PROCESSING

for

data

collection

at the

National

Severe

Storms Laboratory in Norman, Oklahoma, included an S-band Doppler radar with a vertically pointed, shrouded antenna, a VHF lightning-mapping system with a 60ø vertical observational cone, and instruments for measuring electric field changesassociatedwith lightning. The radar character-

spectra is described by Mazur et al. [1985]; 128 data points (about 100 ms) weighted with the yon Hann (raised cosine) data window are Fourier transformed to calculate the Doppler spectra. To examine fast time variations of spectral parameters, we used sequencesof overlapped (by 75 ms) spectra, which produced a time-sampling resolution of 25 ms for the analysis. In order to evaluate the changes in spectral power due to lightning, we also calculated the differences between each current spectra Pt(f) and the average spectrum Pay(f) over a 1-s interval precedingthe lightning. If 10 log Pr(f) -- 10 log Pav(f) were used as a differencespectrum,its plot would show fluctuations about 0 dB, where lightning is not affecting the P•(J). Becausesuch a plot indicates a ratio of the compared spectra, the small changes in spectral skirts would be overemphasized,while much larger changesin spectral peaks would be deemphasized.In order to represent equally absolute values in power changes for all spectral coefficients,we have

chosenlog [P•c(f)- P•(f)] as a "difference spectrum." Note that P•- P,• can become negative, in which case the logarithm does not exist. However, this rarely occurred in the spectra with lightning that we observed. 3.

istics follow:

Wavelength One-way 3-dB beamwidth Peak transmitter power Pulse repetition time Pulse width Antenna one-way gain

10.52 cm 3ø 20 kW 768 #s 0.25 #s 35 dB

Effective area of the

3 m2

antenna aperture Receiver bandwidth

10 MHz

The antenna shroud, made of a radio-absorbing material, significantly decreasesthe sidelobes.The first side lobe is at least 50 dB below the main lobe of the two-way antenna pattern. The data collectedin 16 disk-shapedresolution volumes (45 m deep, 300 m apart) consistedof 4096 digital complex samples lasting 3.14 s per record, with 73-ms intervals between individual records. A multistep procedure to identify lightning

Copyright 1987 by the American GeophysicalUnion. Paper number 7D0262. 0148-0227/87/007D-0262505.00 6699

OBSERVATIONS

The spectra from resolution volumes where lightning occurred exhibited a transient increase of power in the form of a peak and, in about 90% of the cases,also exhibited a burst of power in the spectral skirts. The average spectrum of precipitation over a 1-s interval before lightning occurrenceis shown in Figure 1, and the observed changesin power are illustrated with a series of difference spectra (Figure 2). The mean noise level is obtained from the average spectrum and used as a reference level for the series of difference spectra. The mean velocity of precipitation in the resolution volume is downward

and equal to about 3.3 m s- x The differencespectra allow us to extract transient changes in spectral power density. The first of 17 overlapped difference spectra (Figure 2) indicates no change in spectral skirts; note that the difference is far below the noise level of the average spectrum. A lightning peak appears in the region of updraft velocities in spectra 2 through 12. The difference in spectral skirts from the averagepower density (Figure 1) increaseswith the growth of the lightning peak, reaching about 7 dB above the referencelevel of noise in spectrum 4, and decreasingin spectra 5 and 6. Our general impression(from 46 cases)is that the power in spectral skirts increasessimultaneouslywith the

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MAZURETAL.' DOPPLER SPECTRA OFPRECIPITATION

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