JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 116, D03111, doi:10.1029/2010JD015233, 2011

Comparison of positive and negative compact
intracloud discharges
Ting Wu,1 Wansheng Dong,1 Yijun Zhang,1,2 and Tao Wang1
Received 22 October 2010; revised 19 November 2010; accepted 21 December 2010; published 15 February 2011.

[1] Prior studies have found that positive and negative compact intracloud discharges(+CIDs and −CIDs, according to the physics sign convection) occur at distinctly different
altitudes, which correspond to different regions in a thunderstorm. On the basis of a large
number of CIDs of both polarities recorded by our VLF/LF lightning location network,
characteristic differences between +CIDs and −CIDs are discussed in this study. The
results reveal that −CID is a more special type of discharge. Compared with +CIDs, −CIDs
produce larger electric field changes on average, and they are more isolated from otherdischarge processes. A locating method based on ionospheric reflection pairs of CIDs isdeveloped, which confirms that −CIDs do occur at higher altitudes. The relationshipbetween CIDs and convective strength is also analyzed. Out of nine storms analyzed in thisstudy, eight produce fewer −CIDs than +CIDs. The percentage of −CIDs seems to increase
with the convective strength. Although −CIDs are relatively rare, their occurrences are
more temporally compact; that is, a large portion of −CIDs are produced in a very shortperiod. +CIDs also have this characteristic, but it is not as pronounced as that of −CIDs.
Citation: Wu, T., W. Dong, Y. Zhang, and T. Wang (2011), Comparison of positive and negative compact intraclouddischarges, J. Geophys. Res., 116, D03111, doi:10.1029/2010JD015233.

1. Introduction[2] CIDs are a distinct class of intracloud discharge processes. They are accompanied by powerful radiation in the
HF and VHF radio bands [Le Vine, 1980; Smith et al.,1999a; Jacobson, 2003], and they also produce large electric field changes that are comparable to that produced byreturn strokes [Le Vine, 1980; Cooray and Lundquist, 1985;
Willett et al., 1989; Smith et al., 1999a]. CIDs are usuallyfound isolated with other discharge processes at a time scaleof several milliseconds [Willett et al., 1989; Smith et al., 1999a,2002]. However, Rison et al. [1999] observed 13 CIDs
which were all followed by other intracloud discharges
within 10 ms. Further research indicated that at least a
portion of CIDs are probably the initial events of lightning
flashes [Betz et al., 2008]. CIDs can be conveniently
observed from the satellite [Holden et al., 1995; Light andJacobson, 2002] and thus may serve as a proxy of global
convective activity for satellite‐based remote sensing, so
there has been a growing interest in the association between
CIDs and severe tropospheric convection in recent years. It
is generally agreed that CID rates are statistically correlatedwith convective strength, although there are still many
uncertainties [Suszcynsky and Heavner, 2003; Jacobson andHeavner, 2005; Wiens et al., 2008]. CIDs are also called
1
Laboratory of Lightning Physics and Protection Engineering, Chinese
Academy of Meteorological Sciences, Beijing, China.
2State Key Laboratory of Severe Weather, Chinese Academy of
Meteorological Sciences, Beijing, China.

Copyright 2011 by the American Geophysical Union.
0148‐0227/11/2010JD015233“narrow bipolar events” or NBEs because of their narrow andbipolar radiation field pulses [e.g., Willett et al., 1989].
[3] Smith et al. [2004] calculated heights of over
100,000 CIDs and they found that +CIDs occurred between
7 and 15 km and −CIDs occurred between 15 and 20 km.
The result indicates that −CID is probably a type of discharge that occurs between upper positive charge region andthe screening charge layer where normal intracloud discharges rarely occur. Therefore, it can be speculated that
+CIDs and −CIDs are produced in largely different meteorological context and many of their characteristics may also
be different from each other. Unfortunately, as far as weknow, there are few studies concerning the differences
between +CIDs and −CIDs. In this paper, we present
observations of both polarities of CIDs by multiple stationsof fast electric field change meters and discuss differences in
multiple characteristics of +CIDs and −CIDs comprising thelarge peak electric field changes, temporal isolation, heightof occurrence and their relationship with convective
strength.

2. Data[4] In the summer of 2007, a VLF/LF lightning location
network consisting of 7 stations of fast electric field change
meters were established in Guangzhou, China. All theelectric field change meters were identical to each other andwere similar in principle to that described by Kitagawa and
Brook [1960]. The electric field change system had a decay
time constant of 1 ms. The output was digitized at 10 MHz
with a resolution of 12 bits and each record had duration of
1 ms. The software‐based triggering system was similar to

D03111

1 of 9

D03111WU ET AL.: COMPARISON OF POSITIVE AND NEGATIVE CIDS

D03111location systems such as LASA [Shao et al., 2006], which
has baselines of larger than 100 km. The compact configuration of our network can provide thorough and specified
observations of those storms passing over. There is also an
obvious limitation, though, that the location accuracy isrelatively low for the discharges far away. Figure 1 shows
two images of composite radar reflectivity with locations of
CIDs within 6 min of the radar time. As in Figure 1a, thestorm is close to the stations and all the CIDs are located at
the high‐reflectivity core. In Figure 1b, the storm is morethan 100 km away from the stations, and some of the CIDs
are located out of the high‐reflectivity core. However, theresult is still generally reasonable.
[6] Classification criteria for CIDs are similar to those
employed by Smith et al. [2002] and Hamlin et al. [2007].
For each discharge event, if the mean pulse width of waveforms recorded by all the stations is ≤7 ms, and if the
maximum of posttrigger SNR value is ≥23 dB, then theevent is determined as a CID.
[7] In this paper, records of 19 days (from 6 August to
24 August) are used. There are 14 storm days in thisperiod. A total of 11,876 CIDs was recorded comprising7882 +CIDs and 3994 −CIDs. Physics sign convention forelectric field change polarity is used throughout this paper,thus a negative cloud‐to‐ground return stroke produces anegative polarity field change signal. Typical waveforms of+CIDs, −CIDs and negative return strokes (−RSs) are shownin Figure 2. Differences between CIDs and −RSs are obvious,and ionospheric reflection pairs of CIDs are easily discerned. Note that the scale of electric field changes was notcalibrated so the digital unit (DU) is used throughout thispaper.

3. Results

Figure 1. Composite radar reflectivity with locations of
CIDs within 6 min of the radar time. Red dots represent
observation stations. Red plus sign represents the radar station. Black triangles represent CIDs. The red circle represents an equidistance of about 150 km from the central
station. Black circles are to illustrate the cell creation procedure discussed in section 3.4.

that of the upgraded LASA (Los Alamos Sferic Array) [Shaoet al., 2006], using floating trigger thresholds to prevent disturbance of fluctuating background noise. Continual datatransmission from the digitizer to PC memory ensured thatevery electric field change signal exceeding the thresholdvalue could be recorded without any dead time. Each
recorded event was time stamped with GPS receivers, andthree‐dimensional position of the sources can be determined
employing differential time‐of‐arrival (DTOA) technique.
[5] Five peripheral stations were located about 10 km to
the central station, and a remote station was located about
32 km away from the central station. These seven stations
are represented by red dots in Figure 1. Such a compact
configuration is largely different from that of other lightning

3.1. Peak Amplitude of Electric Field Changes[8] Peak amplitudes of electric field changes produced by+CIDs and −CIDs are compared with that of 138,148 −RSsobserved during the same period similar to previous studies[Le Vine, 1980; Willett et al., 1989; Smith et al., 1999a]. In
order to use the simple 1/R distance relation to normalize
electric field change amplitude, only those discharge events
located more than 50 km away from the central station areused to calculate the geometric mean value of electric fieldchanges. The geometric mean values of electric field changes
normalized to 100 km are 353, 506 and 187 DU for +CIDs,−CIDs and −RSs, respectively.
[9] In order to analyze the distribution of electric fieldchanges, the value of 0–3000 DU is divided into 60 segments with 50 DU interval, and the value of larger than3000 DU is set as another segment, thus totally 61 segments.
Then the percentage of events that fall in each segment iscalculated for +CIDs, −CIDs and −RSs. The result is shownin Figure 3. The distribution of −RSs is largely different
from that of +CIDs and −CIDs, showing a predominantportion of −RSs having values below 500 DU. The normalized geometric mean value of −RSs is 187 DU, which is
0.53 of that of +CIDs and 0.37 of that of −CIDs. It seems
that CIDs generally produce larger electric field changes
than −RSs do. This result is in accordance with a recent
study on +CIDs [Nag et al., 2010].

2 of 9

D03111WU ET AL.: COMPARISON OF POSITIVE AND NEGATIVE CIDS

D03111Figure 2. Typical waveforms of (a) +CIDs, (b) −CIDs, and (c) −RSs.

[10] Ratio of normalized mean value of +CIDs to −CIDs
is 0.70. Their distributions of peak values have the same
trend (Figure 3); the difference is in that a considerably
higher percentage of −CIDs than +CIDs have large peakvalues of electric field changes. For example, there are 35%
of −CIDs having peak values larger than 800 DU, comparedwith a value of 12% for +CIDs. Besides, there are 14 −CIDswith peak values larger than 3000 DU, while the maximum
peak value of +CID is only 2900 DU. Therefore, −CIDs
show a much higher possibility to produce very large
electric field changes.
3.2. Temporal Isolation[11] The characteristic of temporal isolation of CIDs haslong been noticed [e.g., Willett et al., 1989]. More recent

observations indicate that at least a significant portion of
CIDs serve as the initiation process of normal lightning[Rison et al., 1999; Betz et al., 2008]. However, it is still
unclear to what extent are CIDs isolated with other discharge processes and whether there is any difference
between +CIDs and −CIDs. In this study, on the basis oflarge samples of both +CIDs and −CIDs, we try to present
some statistical results concerning the characteristic oftemporal isolation of +CIDs and −CIDs.
[12] Since each waveform record in our study is 1 ms
long, and other discharge pulses are rarely observed in thesame record with a CID, we simply define the time difference between two discharge processes as the difference oftrigger times of the two records. Considering the possibility
that two or more independent discharge processes occur

3 of 9

D03111

D03111WU ET AL.: COMPARISON OF POSITIVE AND NEGATIVE CIDS

Figure 3. Distributions of normalized peak value of electric field changes of −RSs, +CIDs, and −CIDs. Each percentage is calculated by dividing the total number of
events of each type by the number of events of this type thatfall in each 50 DU long segment.

simultaneously at different places, we only consider records
that are located within 20 km from each other to computethe time differences. Besides, the detection efficiency isrelatively low for discharges far away and it is possible todetect CIDs while miss other discharges. In order to limit
such detection bias, only those discharges located less than
100 km from the central station are considered. There are atotal of 3205 +CIDs and 1969 −CIDs in this area.
[13] 11.7% of +CIDs are found to be followed by otherdischarge processes within 10 ms (defined as preceding‐discharge percentage), and 1.6% of +CIDs occur within
10 ms after other discharge processes (defined as following‐discharge percentage). The corresponding percentages for−CIDs are 4.4% and 1.7%. It is difficult to tell the distinguishable features merely from these values, but whencomparing with the corresponding values of normal discharge processes, the significance of the results is obvious.
There are a total of 244,989 triggering events in the analysis
area, and normal discharge processes are picked out simply
by excluding CID records from these triggering events. It isfound that the preceding‐discharge percentage of normal
discharge processes is 13.0%, and the following‐discharge
percentage is 13.1%. Table 1 summarizes these results.
[14] The above results have twofold meaning. First, it isnatural that the preceding‐ and following‐discharge percentages of normal discharge processes are almost the same.
However, by comparison, the preceding‐discharge percentages of both polarities of CIDs are much larger than theirfollowing‐discharge percentages. It indicates that bothpolarities of CIDs have much higher chance to serve as theinitiation processes than to be initiated by other discharge
processes. Second, the preceding‐discharge percentage of+CIDs, which is close to that of normal discharge processes,
indicates that a significant portion of +CIDs are indeed
followed by other discharge processes. Such example is notdifficult to find in our data set. Figure 4 shows an example
of a +CID followed by typical intracloud discharges. The
preceding‐discharge percentage of −CIDs is much smaller
than that of +CIDs, indicating that −CIDs are more likely tobe temporally isolated from other discharge processes. This
is in accordance with the observation by Rison et al. [1999]
that all CIDs initiating normal intracloud discharges werepositive polarity.
[15] It is interesting to note that of all the CIDs found to be
associated with other discharge processes, 2 +CIDs precede−RSs, another 2 follow −RSs, 1 −CID precedes −RS, and3 −CIDs follow −RSs. Nag et al. [2010] observed three+CIDs preceding CGs by 72–233 ms and 4 +CIDs during or
after CGs based on 500 ms long electric field records, anddistances between +CIDs and CGs were mostly less than
15 km. They also found 6 +CIDs occurring in pairs. It seems
that CIDs can be associated with various kinds of discharge
processes, although CIDs associated with normal intraclouddischarges are significantly more common.
3.3. Ionospheric Reflection Pairs and Heightof Occurrence
3.3.1. Method of 3‐D Locating CIDs With Their
Ionospheric Reflection Pairs[16] The method of employing ionospheric reflection pairs
to calculate source height of CIDs proposed by Smith et al.
[1999a] is very useful for a lightning location system that is
only capable of determining 2‐D source locations like theone used in their work. It is also useful for a lightning
location system that has very short baselines such as the one
in our study. This method virtually adds some stations withvery long baselines, so it can significantly increase thedetection accuracy for discharges far away from the network.
[17] Smith et al. [1999a] employed ionospheric reflection
pairs to calculate source height of CIDs and ionosphericvirtual height after 2‐D locations of CIDs had been
determined. This method can be developed to calculate 3‐D
locations of CIDs and ionospheric virtual height simultaneously based on multiple‐station observations. As in Smithet al. [1999a], we have the following relations according to
Figure 5:ctia ¼

qﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ qﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
ð2H  h  zi Þ2 þri2  ðh  zi Þ2 þri2

ð1Þ

ctib ¼

qﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ qﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
ð2H þ h  zi Þ2 þri2  ðh  zi Þ2 þri2

ð2Þ

where tia is the arrival time difference of signal A and B, andtib is the arrival time difference of signal A and C. Both of tia

Table 1. Characteristics of Temporal Isolation

+CID
−CID
Normal discharge process

4 of 9Preceding‐Discharge
Percentage

Following‐
Discharge
Percentage

11.7%
4.4%
13.0%

1.6%
1.7%
13.1%

D03111WU ET AL.: COMPARISON OF POSITIVE AND NEGATIVE CIDS

D03111Figure 4. A CID followed by typical intracloud discharges. The distance between two successive discharge events calculated from their locations provided by the lightning location network is no larger than
3.5 km, which indicates that these discharges probably belong to the same process.
and tib can be measured directly from the waveforms. ri is
given byri ¼ R  arccos½sinðx0 Þ sinðxi Þ þ cosðx0 Þ cosðxi Þ cosðy0  yi Þ ð3Þ

where R is the radius of the Earth, x0 and y0 are the latitude andlongitude of the source location, and xi and yi are the latitude
and longitude of the ith station.
[18] In equations (1) and (2) the Earth is treated as a flat
plane. In this study, CIDs with ionospheric reflection pairs
are mostly observed between 100 km and 300 km from the
network. The ones nearer cannot produce clear reflection
pairs and the ones farther cannot be effectively detected. In
this range, the error caused by the flat Earth model is around
100 m, which is trivial and can be neglected. Equation (3)is based on spherical coordinates rather than Cartesian
coordinates because it turns out that such consideration does
not significantly increase the complexity of calculation andit makes the result closer to reality.
[19] Substituting equation (3) into equations (1) and (2),the two equations have four unknowns, so records of two
stations can provide a set of solutions. Since a discharge
event is usually observed by 5 or more stations simulta-

neously, the optimization method is used to minimize thefollowing function:f ð H; h; x0 ; y0 Þ
qﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ qﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ2n Xctia  ð2H  h  zi Þ2 þri2 þ ðh  zi Þ2 þri2¼
i¼1

qﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ qﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ2
þ ctib  ð2H þ h  zi Þ2 þri2 þ ðh  zi Þ2 þri2

ð4Þ

where n is the number of stations observing the same discharge event. A MATLAB function fminsearch, which uses
the simplex search method [Lagarias et al., 1998] to find theminimum of a multivariable function, is employed to minimize the function (4). In this way, 3‐D location of CIDs andvirtual ionospheric height are obtained.
3.3.2. Height of Occurrence of +CIDs and −CIDs[20] In order to validate this method, results of locationsof CIDs are compared with composite radar reflectivity.
Figure 6 shows an image of composite radar reflectivity
with CIDs that are located using their ionospheric reflection pairs. The storms are nearly 200 km away from the
network. Although some of CIDs are located out of the

Figure 5. Illustration of the propagation paths of ionospheric reflection pairs of CIDs. A flat Earth modelis used.
5 of 9

D03111WU ET AL.: COMPARISON OF POSITIVE AND NEGATIVE CIDS

Figure 6. Composite radar reflectivity with locations of
CIDs within 6 min of the radar time. Black triangles represent CIDs, which are located by their ionospheric reflection
pairs. Red dots represent observation stations. Red plus sign
represents the radar station.

high‐reflectivity core, the result is generally satisfactory
considering the long distances between the storms and the
network and the small area that the network covers.
[21] Only results of source height (h) and ionosphericvirtual height (H) for those CIDs that are located within thehigh‐reflectivity cores are used because these results aremost probably accurate. For example, CIDs located in whiteand blue areas in Figure 6 are not selected for statistics. In
this way, a total of 555 +CIDs and 174 −CIDs are pickedout manually by comparing with composite radar reflectivity.
Their source heights and corresponding ionospheric virtual
heights are scatterplotted in Figure 7. +CIDs and −CIDs
show surprisingly distinctive source heights. Heights ofmost +CIDs range from 8 to 16 km with a geometric meanof 12 km, and heights of most −CIDs range from 16 to
19 km with a geometric mean of 17 km. This result agrees
well with that of Smith et al. [2004], further demonstratingthat +CIDs and −CIDs occur in distinctly different altitudes.
It is worth noting that the highest CID in this study is 20.1 km,
larger than the tropopause height of 16–17 km in the summer according to local sounding data. Several previous
studies have also observed many CIDs occurring above
20 km and even as high as 30 km [Smith et al., 2004;
Jacobson and Heavner, 2005; Nag et al., 2010]. Ionosphericvirtual heights in this study are mostly between 85 km and
95 km. Since CIDs with ionospheric reflection pairs were allobserved at night in this study, from about 1800 to 0600
local time, this result generally agrees with Smith et al.
[2004] as well.
3.4. Percentage of −CIDs and Relationship With
Convective Strength
3.4.1. Cell Creation Procedure[22] In this study, the observation area is quite limited
compared with previous studies [e.g., Wiens et al., 2008]

D03111

based on the data of LASA. In order to relate the CIDs withthe storm producing them, only those storms and CIDs thatare within 150 km from the central station are considered
because within this range the location error is relatively
small (see Figure 1). In this area, 9 storm processes areconveniently picked out according to the radar data. It is
worth noting that since the analysis area is relatively small,the processes considered are not always going through theentire storm life; that is, a storm may not be formed within
the area or may move out of the area in its later stage. In
order to statistically study the relationship between CIDs
and convective strength, the duration of each storm isdivided into time windows of 6 min, equivalent to a complete volume scan of radar. However, the area is not divided
into equivalent grids like in previous studies; instead, it isconsidered as a single zone unless it comprises multiple
independent storms and each storm produces CIDs. Suchtreatment can ensure that each zone covers a separate storm.
Each zone is treated as a data cell, and the number of both
+CIDs and −CIDs and the maximum reflectivity of this datacell are calculated. For example, in Figure 1a, only one datacell is considered, while in Figure 1b, two data cells are
considered (illustrated by black circles in Figure 1). In thisway, a total of 698 data cells are obtained.
[23] Unlike previous studies, such a procedure createsdata cells that do not have identical areas, but the areas
usually do not differ a lot, and when the ratio of +CIDs to−CIDs is considered, difference of areas is not a problem.
An obvious advantage of this procedure is that each data cell
contains a separate storm and a single storm would not be
divided into multiple data cells.
3.4.2. Percentage of −CIDs[24] In all of 698 data cells, 416 contain at least one CID.
Figure 8 shows the distribution of percentages of −CIDs(ratio of −CIDs to both polarities of CIDs) in each of thesedata cells. A pronouncing feature in Figure 8 is that there are
218 data cells containing only +CIDs, accounting for 52%
of all data cells with at least one CID. Besides, 29 data cells

Figure 7. A scatterplot of source height of CIDs versus
ionospheric virtual height. The results are computed by ionospheric reflection pairs. Only those CIDs located within high‐reflectivity cores are included.

6 of 9

D03111WU ET AL.: COMPARISON OF POSITIVE AND NEGATIVE CIDS

D03111Figure 8. Distribution of percentages of −CIDs in each data cell.
contain only −CIDs. The number of data cells with only one
polarity CIDs is 247, accounting for 59% of all data cellswith at least one CID. From another perspective, the percentages of −CIDs in 9 storms in this study are 41%, 54%,
13%, 18%, 15%, 41%, 27%, 35%, 10%. Clearly moststorms have more +CIDs than −CIDs, and both polarities of
CIDs occur in the same storm. These results indicate that ona small time scale (6 min in this study), CIDs tend to occur
as the same polarity; that is, CIDs of the same polarity tend
to cluster in a short period. However, on the time scale of astorm, CIDs of both polarities are produced.
[25] Although +CIDs are more prevalent, the biggest
number of −CIDs in one data cell is larger than that of +CIDs.
The three largest numbers of −CIDs in one data cell are 95,
71, and 51. And the three largest numbers of +CIDs in one
data cell are 67, 48, and 46. Three data cells have total
duration of only 18 min, but they account for 19% and 7% of
all −CIDs and +CIDs, respectively. This clearly demonstrates
the temporal compactness of CID occurrences, which has also
been pointed out by previous studies [e.g., Jacobson andHeavner, 2005]. And this result indicates that −CIDs are
more pronounced than +CIDs in such characteristic.
3.4.3. Relationship With Convective Strength[26] The number of CIDs and the maximum reflectivity ofeach data cell are scatterplotted in Figure 9. The red line
connects the average number of CIDs with the same value ofmaximum reflectivity. The maximum reflectivity is obviously correlated with convective strength to some degreeand has been employed by Wiens et al. [2008] to infer the
relationship between CIDs and convective strength as well.
Note that radar‐derived storm top is not utilized here
because the value appears inaccurate when the storm is very
close to the radar station. Figure 9 shows that the average
CID rate increases with the maximum reflectivity of below
58 dBZ. The large variation of CID rates when the maximum reflectivity exceeds 58 dBZ is probably because ofsmall samples of data cells. This result is in accordance withprevious studies [e.g., Wiens et al., 2008] and also justifies
the cell creation procedure in this study.
[27] Percentage of −CIDs in each storm varies considerably, from 10% to 54%. It seems that the percentage of−CIDs in a storm is connected with the convective strength.
Figure 10 depicts the mean maximum reflectivity versus thepercentage of −CIDs in each storm, and they are linearly
fitted with a correlation coefficient of 0.62. The mean
maximum reflectivity is calculated by averaging the maximum reflectivity of all data cells in a given storm. The trend
for the mean maximum reflectivity to increase with thepercentage of −CIDs is obvious. This result probably has
certain associations with the fact that the heights of −CIDs aremostly larger than that of +CIDs. Since maximum reflectivity
is related with convective strength and height of cloud top tosome degree, this result may indicate that the higher a thunderstorm develops the greater possibility it has to produce−CIDs.

4. Discussion[28] Le Vine [1980], when he observed CIDs for the firsttime, noticed that they were “relatively isolated.” Willett etal. [1989] had the similar result, concluding that CIDs
were not associated with any identified lightning process.
Smith et al. [1999a] studied waveforms of CIDs producedby three thunderstorms and stated that CIDs were temporally isolated from other discharges on a time scale of several
seconds to several tens of seconds within the same thunderstorm; that is, CIDs occurred as single discharges. There
are also many studies concluding that CIDs are accompanied
by other discharge processes. Rison et al. [1999], utilizing
LMA (Lightning Mapping Array), observed 13 +CIDs andfound that each +CID initiated an intracloud discharge
within 10 ms. They stated that previous studies observed
CIDs unaccompanied with any other discharge process
because those measurements were primarily at large distances and they tended to miss small‐amplitude processes of
intracloud discharges. Smith et al. [1999b] also presents an

7 of 9

D03111WU ET AL.: COMPARISON OF POSITIVE AND NEGATIVE CIDS

D03111

both analyses by Jacobson and Heavner [2005] and Wienset al. [2008] were based on grid cells having duration of
10 min. We have already demonstrated that at a short timescale CIDs tend to be of single polarity. In this study, 59%
of 6 min data cells have only one polarity of CIDs, which islikely corresponding to the result of Jacobson and Heavner[2005]. It seems that both of the above analyses could only
show the ratio of +CIDs to −CIDs at a time scale of 10 min
instead of a given storm. For 9 storms in this study, every
storm produces both polarities of CIDs, and it is very likelythat for a given storm in both Florida and Great Plain, it also
produces both polarities of CIDs. Further research on somespecific storms in these areas are needed to solve this issue.

5. Conclusions

Figure 9. A scatterplot of CID rate versus maximum reflectivity for each data cell. The red line connects the meanvalue of CID rate of each value of maximum reflectivity.
example of a +CID followed by intracloud discharges. Betz etal. [2008] analyzed the first in‐cloud events by both VLF/LF
and VHF networks. They found that up to half of those eventswere almost simultaneously detected by both VLF/LF and
VHF networks. The time differences between VLF/LF andVHF signals were often smaller than 100 ms. With large
amplitudes of VLF/LF signals that were comparable to thatof RSs, these time‐coincident events were very likely CIDs.
If this is true, then at least a significant portion of CIDs do
initiate normal lightning. Results in our study also support
that a portion of CIDs are indeed followed by other discharge processes. CIDs analyzed by Smith et al. [1999a]were mostly hundreds of kilometers away from the observation stations and it is very likely that some small‐amplitudedischarge processes following CIDs were not detected. It isinteresting to note that instances of CIDs that are followedby other discharge processes presented by previous studies
are all positive polarity, and the result of this study also
shows that +CIDs are more likely to be followed by otherdischarge processes than −CIDs. Perhaps it is just a coincidence, but it is possible that discrepancy between +CIDs
and −CIDs is larger than their preceding‐discharge percentages indicate. It is even possible that when CIDs are foundto be followed by other discharge processes, they are mostly+CIDs, while −CIDs rarely have such behavior. At last, aconclusion should be made, as indicated by the following‐discharge percentages, that both +CIDs and −CIDs are much
more likely to be initiation processes than to be initiated byother discharge processes.
[ 29 ] Jacobson and Heavner [2005] and Wiens et al.
[2008] have both statistically studied the ratio of +CIDs to−CIDs in a given storm. Jacobson and Heavner [2005], on
the basis of LASA observation in Florida, found that a given
storm rarely produced both polarities of CIDs. However,
Wiens et al. [2008] found that both polarities of CIDs often
occurred in the same storm in Great Plains. Hamlin et al.
[2009] stated that the difference of CID production in
Great Plains versus Florida possibly indicated the differencein the storms themselves. However, it should be noted that[30] This study focuses on differences between +CIDs and−CIDs which have rarely been investigated by previous
studies. It has been discovered that, compared with +CID,−CID is a more special type of discharge in the following
respects.
[31] 1. −CIDs produce, on average, larger electric fieldchanges than +CIDs do.
[32] 2. Larger portion of −CIDs than that of +CIDs aretemporally isolated with other discharge processes on a timescale of 10 ms.
[33] 3. −CIDs mostly occur at higher altitudes than +CIDs
do. Most of −CIDs occur between 16 and 19 km, while mostof +CIDs occur between 8 and 16 km.
[34] 4. −CIDs are generally rarer than +CIDs. Eight out of
9 storms in our analysis produce more +CIDs than −CIDs.
But both +CIDs and −CIDs occur in every storm.
[35] 5. Considering some short periods that have largest
rates of +CIDs or −CIDs, there are significantly larger
portion of −CIDs than that of +CIDs occurring in these
periods. It indicates that −CIDs’ occurrences are more
temporally compact.
[36] 6. The percentage of −CIDs in a given storm has thetrend to increase with the convective strength.

Figure 10. A scatterplot of the mean maximum reflectivity
versus percentage of −CIDs in each storm. The mean maximum reflectivity is calculated by averaging the maximum
reflectivity of all data cells in a given storm.

8 of 9

D03111WU ET AL.: COMPARISON OF POSITIVE AND NEGATIVE CIDS[37] These differences are related with each other. Supported by the result of Smith et al. [2004], it is almost certainthat −CIDs occur at higher altitudes (the third difference).
Some −CIDs even occur above the nominal tropopause[Jacobson and Heavner, 2005]. This requires the thunderstorm producing them to be extremely vigorous and thecloud top to be very high, which is likely related with thesixth difference. Besides, the region where −CIDs occur
probably corresponds to the region between upper positive
charge layer and the screening charge layer, where normal
lightning discharges can rarely occur, while the region
where +CIDs occur is almost the same as the region where
normal intracloud discharges are usually observed. This may
determine that −CIDs are more temporally isolated withother discharge processes (the second difference).
[38] Since −CIDs are often accompanied by extremely
vigorous storms, such vigorous storms usually provide large
power, which may be responsible for −CIDs’ larger electricfield changes (the first difference) and extremely high rates
in some short periods (the fifth difference). At the sametime, such vigorous state can hardly last for a long time,
which makes −CIDs relatively rarer (the fourth difference).
[39] The above explanation is of course more of a conjecture. Since the mechanism of CIDs is not quite clear yet,
it is impossible to explain these characteristics explicitly.
The differences we have found, however, surely distinguish−CIDs from +CIDs in many aspects. More work is needed
to reveal the physical mechanisms responsible for thesedifferences between +CIDs and −CIDs.
[40] Acknowledgments. This work was supported in part by the
National Natural Science Foundation of China under grants 41075022
and 40875003 and the Ministry of Science and Technology under grant
2008EG137292. The authors thank Xuan‐Min Shao for constructive comments on this paper. Valuable suggestions from three anonymous reviewers
are also acknowledged.

References
Betz, H.‐D., T. C. Marshall, M. Stolzenburg, K. Schmidt, W. P. Oettinger,
E. Defer, J. Konarski, P. Laroche, and F. Dombai (2008), Detection ofin‐cloud lightning with VLF/LF and VHF networks for studies of theinitial discharge phase, Geophys. Res. Lett., 35, L23802, doi:10.1029/
2008GL035820.
Cooray, V., and S. Lundquist (1985), Characteristics of the radiation fields
from lightning in Sri Lanka in the tropics, J. Geophys. Res., 90(D4),6099–6109, doi:10.1029/JD090iD04p06099.
Hamlin, T., T. E. Light, X. M. Shao, K. B. Eack, and J. D. Harlin (2007),
Estimating lightning channel characteristics of positive narrow bipolar
events using intrachannel current reflection signatures, J. Geophys.
Res., 112, D14108, doi:10.1029/2007JD008471.
Hamlin, T., K. C. Wiens, A. R. Jacobson, T. E. Light, and K. B. Eack(2009), Space‐ and ground‐based studies of lightning signatures, inLightning: Principles, Instruments and Applications, edited by H. Betz,
U. Schumann, and P. Laroche, pp. 287–307, Springer, Berlin.

D03111

Holden, D. N., C. P. Munson, and J. C. Devenport (1995), Satellite observations of transionospheric pulse pairs, Geophys. Res. Lett., 22(8), 889–892,
doi:10.1029/95GL00432.
Jacobson, A. R. (2003), How do the strongest radio pulses from thunderstorms relate to lightning flashes?, J. Geophys. Res., 108(D24), 4778,
doi:10.1029/2003JD003936.
Jacobson, A. R., and M. J. Heavner (2005), Comparison of narrow bipolar
events with ordinary lightning as proxies for severe convection, Mon.
Weather Rev., 133, 1144–1154, doi:10.1175/MWR2915.1.
Kitagawa, N., and M. Brook (1960), A comparison of intracloud and cloud‐to‐ground lightning discharges, J. Geophys. Res., 65(4), 1189–1201,
doi:10.1029/JZ065i004p01189.
Lagarias, J. C., J. A. Reeds, M. H. Wright, and P. E. Wright (1998), Convergence properties of the Nelder‐Mead simplex method in low dimensions, SIAM J. Optim., 9(1), 112–147, doi:10.1137/S1052623496303470.
Le Vine, D. M. (1980), Sources of the strongest RF radiation from lightning,
J. Geophys. Res., 85(C7), 4091–4095, doi:10.1029/JC085iC07p04091.
Light, T. E. L., and A. R. Jacobson (2002), Characteristics of impulsive
VHF lightning signals observed by the FORTE satellite, J. Geophys.
Res., 107(D24), 4756, doi:10.1029/2001JD001585.
Nag, A., V. A. Rakov, D. Tsalikis, and J. A. Cramer (2010), On phenomenology of compact intracloud lightning discharges, J. Geophys. Res.,
115, D14115, doi:10.1029/2009JD012957.
Rison, W., R. J. Thomas, P. R. Krehbiel, T. Hamlin, and J. Harlin (1999), A
GPS‐based three‐dimensional lightning mapping system: Initial observations in central New Mexico, Geophys. Res. Lett., 26(23), 3573–3576,
doi:10.1029/1999GL010856.
Shao, X. M., M. Stanley, A. Regan, J. Harlin, M. Pongratz, and M. Stock(2006), Total lightning observations with the new and improved Los Alamos
sferic array (LASA), J. Atmos. Oceanic Technol., 23, 1273–1288,
doi:10.1175/JTECH1908.1.
Smith, D., X. M. Shao, D. N. Holden, C. T. Rhodes, M. Brook, P. R.
Krehbiel, M. Stanley, W. Rison, and R. Thomas (1999a), A distinct classof isolated intracloud lightning discharges and their associated radio emissions, J. Geophys. Res., 104(D4), 4189–4212, doi:10.1029/
1998JD200045.
Smith, D. A., R. S. Massey, K. C. Wiens, K. B. Eack, X. M. Shao, D. N.
Holden, and P. E. Argo (1999b), Observations and inferred physicalcharacteristics of compact intracloud discharges, in Proceedings ofthe 11th International Conference on Atmospheric Electricity, editedby H. Christian, NASA Conf. Publ., 209261, 6–9.
Smith, D. A., K. B. Eack, J. Harlin, M. J. Heavner, A. R. Jacobson, R. S.
Massey, X. M. Shao, and K. C. Wiens (2002), The Los Alamos sfericarray: A research tool for lightning investigations, J. Geophys. Res.,
107(D13), 4183, doi:10.1029/2001JD000502.
Smith, D. A., M. J. Heavner, A. R. Jacobson, X. M. Shao, R. S. Massey, R. J.
Sheldon, and K. C. Wiens (2004), A method for determining intracloudlightning and ionospheric heights from VLF/LF electric field records,
Radio Sci., 39, RS1010, doi:10.1029/2002RS002790.
Suszcynsky, D. M., and M. J. Heavner (2003), Narrow bipolar events as
indicators of convective strength, Geophys. Res. Lett., 30(17), 1879,
doi:10.1029/2003GL017834.
Wiens, K. C., T. Hamlin, J. Harlin, and D. M. Suszcynsky (2008), Relationships among narrow bipolar events, “total” lightning, and radar‐inferred convective strength in Great Plains thunderstorms, J. Geophys.
Res., 113, D05201, doi:10.1029/2007JD009400.
Willett, J. C., J. C. Bailey, and E. P. Krider (1989), A class of unusual lightning electric field waveforms with very strong high‐frequency radiation, J. Geophys. Res., 94(D13), 16,255–16,267, doi:10.1029/
JD094iD13p16255.
W. Dong, T. Wang, T. Wu, and Y. Zhang, Laboratory of Lightning
Physics and Protection Engineering, Chinese Academy of Meteorological
Sciences, Beijing 100081, China. (wuting@cams.cma.gov.cn)

9 of 9