An experiment was carried out at the Arecibo Observatory in Puerto Rico in August 1985 to study Doppler velocities in a thunderstorm environment with a beam pointed 2.5 degrees off-vertical. Researchers detected two types of echoes associated with lightning. The first was associated with scattering from the lightning channel itself and had characteristics similar to those observed previously with meteorological radars. The second appeared to be due to scattering from the turbulence organized by phase fronts of an acoustic wave generated by lightning. The observations were consistent with a wave traveling at a velocity near the speed of sound and having a vertical phase velocity component of 40 m/s

Holden, D. N.

Larsen, M. F.

English

NASA Technical Reports Server

ABSTRACT
N87-10444
3.2.4 OBSERVATIONS OF THUNDER WITH THE ARECIBO VHF _%DAR
D. N. Holden and M. F. La;sen "_--_
Department of Physics and Astronomy
C1 em son University
Clemson, SC 29634-1911
An experiment was carried out at the Arecibo Observatory in Puerto Rico in
August 1985 to study Doppler velocities in a thunderstorm environment with a
beam pointed 2.5 ° off-vertical. We have detected two types of echoes
associated with lightnin 8. The first is associated with scattering from the
llghtnlng channel itself and has characteristics similar to those observed
previously with meteorological radars. The second appears to be due to
scattering from the turbulence organized by phase fronts of an acoustic wave
generated by lightnin 8. The observation described here isconslstent with a
wave traveling at a velocity near the speed of sound and having a vertical
phase velocity component of 40 m/s.
INTRODU CT ION
A number of investigators, including LIGDA (1950), ATLAS (1958), HOLP_S
et al. (1980) and MAZUR et al. (1984), have observed transient echoes
associated with lightning at shorter wavelengths typical of meteorological
radars (e.g., S band). These transient echoes are generally attributed to
scattering from the plasma in the lightning channel itself, as described by
DAWSON (1972). The plasma is expected to move with the surrounding air, and,
indeed, the transient echoes at S band (MAZUR st al., 1984) and our
observations at UHF have mean Doppler shifts typical of the air motions inside
a cloud and the spectral widths are in agreement with the broadening expected
due to the effects of atmospheric turbulence.
ROTTGER (1981), GAGE et al. (1978), and FUKAO et al. (1985) have already
shown that both the precipitation echoes and the "clear air" echoes due to
scattering by turbulent variations in the refractive index can be detected at
wavelengths near 6 m. The relative contributions of the two scattering
mechanisms depend on the radar wavelength, the intensity of the turbulence, and
the intensity of the precipitation, but usually it is not difficult to separate
the effects since the precipitation and air motions will be different, except
for the smallest droplets.
There have been very few observations of Doppler velocities in a
thunderstorm environment at wavelengths longer than a few tens of centimeters.
Exceptions include the experiments of LARSEN et al. (1982) at UHF and those of
ROTTGER (1981), GAGE et al. (1978), and FUKAO et 81. (1985) at VHF. In most of
the experiments, the coherent integration has been sufficiently long to exclude
the observation of effects on a time scale comparable to the scale of lightning
or acoustic waves. In August 1985, we carried out an experiment with the new
VHF radar located at the Areciho Observatory in Puerto Rico in which the raw
pulse-to-pulse data were recorded for later analysis. The observations show
what we believe is the first detection of acoustic waves generated by
lightning.
DESCRIPTION OF THE DATA
The VHF radar with a frequency of 46.8 MHz and UHF radar with a frequency
of 430 _z were operated on five separate afternoons in August 1985. We will
focus on the VHF radar data for the afternoons of August 4th and August 7th.
https://ntrs.nasa.gov/search.jsp?R=19870001011 2020-03-23T14:36:11+00:00Z
148 _
The VHF transmitter operates at 50 kW peak power. The system uses the 300-m
diameter dish, of which the Yagi feed at the focus illuminates 200 m for an
effective beamwidth of less than 2 ° . The beam was pointed at 2.5 ° zenith angle
in this experiment and operated in one of two data-taking modes. The first
used a l-_sec pulse length and coherent integration online to give an effec-
tive sampling time of 92 msec. The second mode used a 2-_sec pulse length and
an interpulse period of 750 usec. The raw data were then recorded on magnetic
tape in this mode, so that the IPP and sampling interval were the same, or we
could coherently integrate off line to produce a smaller Nyquist frequency if
desired.
An electric field change meter and two tipping-bucket rain gauges were
also installed at the site and operated in conjunction with the radar
measurements. The field change meter was used to determine the time of
occurrence of lightning, as well as a qualitative estimate of the distance of
the discharge from the radar.
DOPPLER SPECTRA
Figure 1 shows an example of Doppler spectra over a range of heights
obtained with Mode I when a thunderstorm was overhead. The heights in range
gates 48 through 81 show a contribution primarily from the "clear air" scatter.
These spectra are wider than the spectra observed in a nonconvective
environment. The upper heights, e.g., gates 82 to 95, show spectra that are
broadened to such an extent that it is difficult to discern a peak. The power
profile at the right of Figure I shows that these heights are characterized by
high rather than low power levels.
The maximum unambiguous velocity using Mode I was +8.7 m/s. The
lightning-associated spectra have widths which are of t_e order of the spectral
window since the power is essentially constant across the window. A possible
explanation is that the mean Doppler velocity is much greater than the maximum
resolvable velocity. We expect that the spectral width would scale in same way
with the mean velocity and would account for these observations. Although a
large velocity would cause aliasing, the aliasing itself would not account for
the increase in the width of the spectra. We infer that lightning was present
in the beam when only same of the range gates show the broad spectra, as in
Figure 1.
An example of the spectra obtained using Mode II is shown in Figure 2.
The increased time resolution made it possible to attain much larger
unambiguous velocity determinations. We found no evidence of lightning in the
beam on the days when data were taken with Mode II. However, the electric
field change data did show evidence of more distant lightning discharges.
Figure 2 shows such spectra recorded at the same time as a more distant dis-
charge. Of particular interest are the features with velocities near +_30 m/s
in gates 19 to 23 which appear only for the time required to produce the
spectra (less than 4 sec). The change in sign of the vertical Doppler shift
between gates 20 and 21 could be attributable to vertical phase variation in
the wave or to the geometrical relation between the observing angle of the
radar and the location of the source, as we will show in the next section.
INTERPRETATION OF MEASURED VELOCITIES
The signals measured at VHF will have a component due to the scattering
from the lightning channel itself, a component due to the backscatter from
refractive index variations, and a component due to the scatter from liquid or
frozen droplets. The first does not appear to affect the observations on
August 4th, based both on the characteristics of the spectra and the electric
field change records which did not indicate any lightning nearby. It is
149
THE LEFT RND RIGHT RXES ON THE PROFILE RRE GATE NUMBER RND SLRNT RRNGE, RESPECTIVELY
THE DATA DATE IS 85219 HEIGHT RESOL'N = 150 METERS
POINTS PER SPECTRA = 32 FILE NUMBER = 1
THE TRPE NUMBER IS R03408 BEGINNING RECORD = 130
HT OF LOW GATE = 4.7 KM MSL ENDING RECORD = 1691
NO. OF INCOH INTEG. = 3 NYQUIST FREQUENCY = 5. 4 HZ
9,
6.
w
L
80. IOO,
POWER (OB)
Figure 1. VHF Doppler spectra integrated for nine seconds, with spectra plotted
for gates 44 through 91 and a height profile of mean velocity, total power,
and noise level for 256 gates. Positive Doppler velocity (toward the radar)
is to the right of center, each spectra is scaled to its own peak.
Note the large variance in the upper gates (above 90) as compared with the
lower ones.
unreasonable to expect that either solid or liquid precipitation will travel at
velocities close to 40 m/s since terminal velocities, even for hail, are less
than 20 m/s (PRUPPACHER and KLETT, 1980) and would be only downward directed;
further, no precipitation was recorded by the rain gauges.
Vertical motions of 40 m/s inside the cloud cannot be excluded absolutely
but appear unlikely. It would be easier to explain the observed velocities as
being due to the vertical projection of a near-horizontal acoustic phase
velocity. However, it cannot be that we are scattering from 3-m acoustic
waves, because in that case only waves propagating parallel to the radar be_,
contribute to the backscatter. Also, a wavelength of 3 m is not consistent
with the wave properties derived below. A possible explanation is that 3-m
irregularities are acting as a tracer of the motion of the acoustic front.
The observations can be interpreted in terms of the dispersion relation
for acoustic-gravity waves (YEH and LIU, 1974).
2 (1 _a2/_ 2)kh2 (I - _b2/_) + kz2 = k ° - •
The behavior will be complicated by the background temperature gradients and
150
:., i_i
GAtE l GATE 9
i i • i i
i ; 1 i q
^ A
A
i _ i . A i i
A
i /I _ I i i
GATE 17 GATE 25
THE LEFT AND RIGHT AXES ON THE PROFILE RRE GATE NUMBER AND SLRNT RRNGE. RESPECTIVELY
DATE: YERR DAY NUMBER = 216 HEIGHT RESOL'N = 300 METERS
POINTS PER SPECTRR = 64 NUMBER OF COH. RDD'NS = 16
THE TAPE NUMBER IS RO993 BEGINNING RECORD = 6
PLOT NUMBER 1 OF THIS RECORD TIME HHMMSSIOTHS = 1512544
NO. OF INCOH INTEG. = 5 NYOUIST FREOUENCY = 41. Z HZ
66
|.
40. 60. 80. 100. 120.
POWER (DB)
Figure 2. VHF Doppler spectra taken in mode II (see text). Note the large
Doppler shift in gates 19 through 23. The spectra are integrated for 3.8
seconds, the anomalous echoes did not appear in the previous spectra nor in
the next. The time of these spectra is coincident with a lightning
discharge as indicated by an electric field change meter.
other effects not accounted for in this simple form of the dispersion relation.
Here, k =_/c , _ = Co/2H is the acoustic cut-off frequency, and_0 .O a
is the Brunt-Vazsala frequency. The local speed of sound is c , the scale
O
height is H, and k and k are the horizontal and vertical wave number
• Z
components, respectively. The background parameters were calculated from the
San Juan radiosonde data closest to the time of the observations.
We can only look at the behavior of the wave solutions in certain limits
since we do not know the horizontal wave number. The first limit corresponds
to a horizontal wave number that is much smaller than the vertical wave number
which we calculate from the power profile to be k = 2w/2400 m. The solution
for the wave period then gives a value of T = 7.9 s z which is on the acoustic
branch of the dispersion relation. The total phase velocity is close to the
speed of sound as determined from the sounding and has a large horizontal
componen t.
If we assume that the horizontal and vertical wave numbers are comparable
in magnitude, one solution corresponds to a period of 7.3 rain which is in the
gravity regime, the other solution corresponds to a period of 5.6 s which is
also on the acoustic branch. However, the phase velocity of the gravity wave
is much less than the observed velocity, and the observation could only be
151
explained if the mean air motion over the pulse volume was of the order of
30-40 m/s.
A point source at some distance laterally will produce an acoustic wave
that is propagating more or less horizontally above the vertically pointing
radar. A wave traveling at the speed of sound (%300 m/s) can produce a
vertical component of the phase velocity of _30-40 m/s if it enters the pulse
volume at a large zenith angle, say 85 ° . Also, there would be upward and down-
ward velocity components above and below the height corresponding to the height
of the source. Figure 3 shows the geometry of a single source, displaced
horizontally 6 km and downward 0.2 kin. The downward displacement accounts for
the larger upward velocities observed in the upper range gates. The
temperature profile to the right indicates an inversion at an altitude of
around ii kin. The inversion may have provided some ducting of the acoustic
wave which would explain the appearance of the anomalous echoes in only a few
ga tes.
OONCLUS ION
Experiments carried out with the 46.8-MHz Arecibo radar during August
1985, have shown that there is a component in backscattered signals produced in
connection with lightning events that are not observed at shorter wavelengths,
e.g., S band. There are many unknowns in the analysis of these new echoes, as
detailed above, but is plausible that the echoes are due to scattering from
refractive index variations associated with acoustic waves generated by
IRADAR BEAM
WAVE FRONTS
VERTICAL VELOCITY
COMPONENTS
13 km(=I)
12
DU _TI_
ISOURCE
iI
I0
\
t • | |
-69 ° _40 ° _20 °
Temperature - Celsius
Figure 3. Schematic representation of lightning produced acoustic wave
as measured by vertically pointing VHF Doppler radar. The distance
from the source is estimated to be 6 kilometers, horizontally, where
the angles have been enlarged for the sake of clarity. The tempera-
ture profile on the right is taken from the San Juan radiosonde.
Note the inversion that occurs at approximately 11 km msl.
152
lishtnin 8. The data taken when lightning was some distance from the radar
shows a pattern of positive and nesative velo¢itles with amplitudes on the
order of 20-40 m/s. The obse_vatlons can be explained if the velocity is the
line-of-sight component of the acoustic phase velocity traced by 3-m
irregularitles and the source was some 6 fun _ay. The data taken when
lishtnin 8 is in the beam are more difficult to interpret, partly because in
that particular data-takin 8 mode, a smaller value for the maximum unambisuous
velocity resulted in aliaslng of the Doppler spectra. We cannot rule out that
acoustic waves were responsible for these echoes, as well.
REFER_qCES
Atlas, D. (1958), Radar lishtnin 8 echoes and atmospherics in vertical cross-
section, in Recent Advances in Atmospheric Electricity, edited by L. G.
Smith, 441-459, Pergamon, New York.
Dawson, G. A. (1972), Radar as a diasnostic tool for lishtnins, J. Geophys.
Res., 77, 4518-4527.
Fukao, S., K. Wakasugi. T. Ssto, S. Morimoto, T. Tsuda, I. Hirota, I. Kimura,
and S. Kato (1985), Direct measurement of air and precipitation particle
motion by VHF Doppler radar, June, 1985 issue of Nature.
Gase, K. S., J. L. Green, and T. E. VanZandt (1978), Application of the VHF
pulsed Doppler radar to cloud physics research, Preprint volume, Conf.
on Cloud Physics and Atmospheric ElectricitT, July 31 - Ausust 4, 1978,
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Holmes, C. R., E. W. Szymanski, S. J. Szyumanski, and C. B. Moore (1980),
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Larsen, M. F., W. E. Swartz, and R. F. Woodman (1982), Gravity-wave generation
by thunderstorms observed with a vertically-pointing 430 MHz radar,
Geophys. Res. Lett., 9, 571-574.
Ligda, M. G. H. (1950), _ightning detection by radar, Bull. Am. Meteorol.
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Mazur, V., D. S. Zrnlc, and W. D. Rust (1984), Lightning and precipitation
velocity determined with a vertically looking radar, Preprlnts, 22nd Conf.
on Radar Meteorology, Sept. 10-13, Zurich Switzerland.
Pruppacher, H. R., and J. D. Klett (1980), Microphysics of Clouds and Precipi-
tation, D. Riedel Publishing Co.
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598.
Yeh, K. C., and C. H. Liu (1974), Acoustic-gravity waves in the upper atmo-
sphere, Rev. Geophys. Space Phys., 12, 193-216.


Observations of thunder with the Arecibo VHF radar

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