A balloon-borne instrument was designed to measure the electric field in
thunderstorms. One case of thunderstorm was observed in the Pingliang region
(35.57∘ N, 106.59∘ E; and 1620 m above sea level, a.s.l.)
of a Chinese inland plateau, through penetration by the balloon-borne
sounding in the early period of the mature stage. Results showed that the
sounding passed through seven predominant charge regions. A negative charge
region with a depth of 800 m located near the surface, and a positive charge
region appeared in the warm cloud region; their mean charge densities were
-0.44 ± 0.136 and 0.43 ± 0.103 nC m-3, respectively.
Five charge regions existed in the region colder than 0 ∘C, and
charge polarity alternated in a vertical direction with a positive charge at
the lowest region. The mean charge densities for these five regions were
0.40±0.037 nC m-3 (-9.5 to -4 ∘C), -0.63±0.0107 nC m-3 (-18 to -14 ∘C), 0.35±0.063 nC m-3 (-27 to -18 ∘C), -0.36±0.057 nC m-3 (-34 to -27 ∘C), and 0.24±0.06 nC m-3 (-38 to -34 ∘C). We speculated that the two
independent positive charge regions in the lower portion are the same charge
region with a weak charge density layer in the middle. The analysis showed
that the real charge structure of the thunderstorm is more complex than the
tripole model, and the lower dipole is the most intensive charge region in
the thunderstorm.
Meteorology and atmospheric dynamics (atmospheric electricity)Introduction
At the beginning of the 20th century, based on the observation of a field
mill instrument, Wilson (1929) proposed that the charge structure of a
thunderstorm is a positive dipole, i.e., a main positive charge region
is located upon a main negative charge region in a thunderstorm. By using E
field soundings, Simpson and Scrase (1937), and Simpson and Robison (1941)
proposed a tripole charge structure, namely a negative charge region that
exists in the middle with two positive charge regions at the upper and lower
parts, and this model was accepted widely. More studies confirmed the
existence of a lower positive charge center (LPCC) in thunderstorms
(MacCready and Proudfit, 1965; Marshall and Winn, 1982; Marshall and
Stolzenburg et al., 1998; Bateman et al., 1999; Mo et al., 2002). However, an
increasing number of E field soundings proved that the charge
structure is more complicated than the tripole model (Marshall and Rust,
1991). Stolzenburg et al. (1998a, b, c) presented a new conceptual model of
charge structure that consisted of four charge regions in the updraft region
and at least six charge regions in the region outside of the updraft; the
lowest charge region was positive and changed polarity alternately. Some
thunderstorms showed a different charge structure with those already
mentioned. Rust et al. (2005) and Tessendorf et al. (2007a, b) found an
inverted tripole charge structure in some thunderstorms.
Since the 1980s, Chinese researchers have found some unique electrical
features of thunderstorms in a Chinese inland plateau, such as low lightning
flash rate, larger-than-usual LPCC and frequent intra-cloud flashes between
the LPCC and main negative charge region (Liu et al., 1987; Wang et al.,
1987; Zhao et al., 2004; Zhang et al., 2004; Qie et al., 2005a, 2009, 2015).
Multi-station observations on lightning flashes were employed to study the
charge structure in the region (Qie et al., 2000; Zhang et al., 2009; Cui et
al., 2009), and they found that some of the thunderstorms fit the model of
the tripole charge structure with a larger-than-usual LPCC. Qie et
al. (2005b) found that the thunderstorms with a larger-than-usual LPCC appear
to begin with the lower dipole, rather than with the upper dipole followed by
the development of a weaker lower positive charge. In addition, they found
the larger-than-usual LPCC is unfavorable for negative cloud-to-ground
flashes in the mature stage. Zhao et al. (2004) studied one case of
thunderstorm with E field sounding, and they confirmed the tripole
charge structure with a large LPCC, but with an additional negative screen
layer below the LPCC region. By using a lightning mapping system, Li et
al. (2013) found that only the main negative charge region and LPCC existed
in the initial and mature stage thunderstorm. However, in the decay stage,
there were four charge layers and the lowest was negative. Zhang et
al. (2015) found that the charge structure also fit the tripole model in the
decay stage and the LPCC is the earliest to disappear. Most of the previous
studies on charge structure of thunderstorms in a Chinese inland plateau were
based on the surface electric field, electric field changes, or lightning
mapping system, and the charge structure retrieved by these techniques is the
main charge region or charge regions that frequently participate in
discharge. However, for those weak charge regions or screen layers, they
cannot be observed because they hardly participate in
discharge.
In the summer of 2012, a custom-designed instrument called an “E field
sounding observation system” has been used to conduct field experiments on
thunderstorms in the Pingliang region. One set of soundings was released into
a thunderstorm that occurred on 25 August, and the vertical E field was
obtained. This paper mainly introduces the characteristics of the E field
in the thunderstorm.
Observation and data
Observations on the E field in the thunderstorm were conducted at the
Pingliang station of the Chinese Academy of Science (35.57∘ N,
106.59∘ E; 1620 m a.s.l.) in a Chinese inland plateau. X-band
radar at the station was used to monitor the activity of the thunderstorm,
and a field mill was employed to record the surface E field of the
thunderstorm. The radar was located approximately 200 m from the balloon-launching site;
therefore, we considered the two sites to be the same
location in this paper. The signals of balloon-borne soundings were
transmitted to a ground-receiving system on a frequency of 2.4 GHz. Signals
include GPS information, temperature, humidity, and corona current.
Based on the corona discharge, the electric field instrument was designed to
measure the vertical component of the electric fields in the thunderstorm.
Zhao et al. (2008) established the relationship between the E field
and corona current in the laboratory and field test, as shown in Eq. (1).
Where E is in units of kV m-1, E0 is the threshold of the
E field for the corona current in kV m-1, and I (µA)
is the corona current with the measurement range of ± 16 µA.
Chou et al. (1965) found – under the assumption that the inclination of the
corona probe is 15∘ and the wind
rate is 20 m s-1, with the influence of cloud droplets on the corona
current (10 %), relative humidity changes (10 %) and air pressure
(10 %) – that the corona current measurement error is approximately
21 %, and in some extreme conditions, measurement error is likely to
reach 50 %. As a result, the maximum errors of E field retrieved
by corona current can reach 9.8 and 23 % for the corona current
measurement errors of 21 and 50 %, respectively. Such an error might be
too large to accurately study the electric field, but it is acceptable for
discussing the charge structure of a thunderstorm. Considering the
contribution of atmospheric pressure, the relationship between corona current
I and E can be expressed as Eq. (2) (Chou et al., 1965). Where
P0 is the air pressure at the ground, and P is the air pressure at a
sounding point. The charge polarity and density along the sounding path can
be calculated by the vertical component profile of the electric field. The
charge density ρ can be expressed as Eq. (3), in which ε
is the permittivity of air (8.86 × 10-12 F m-1). Here,
the electric field was deduced from the corona current whose direction is the
same as the direction of the electric line of force. If the downward current
is defined as positive, then the associated electric field is also defined as
positive.
I=4.48×10-3(E-E0)I=4.48×10-3(E-E0)(P0/P)1.5|ρ|=ε|ΔEz/ΔZ|
Evolution of surface E field and lighting flash rate of the
thunderstorm. (a) Surface E field; (b) Flash rate.
ResultsEvolution of thunderstorm and surface E field
In the afternoon of 25 August, at 14:00 CST (China Standard Time, UT + 8), two weak convective cells, one approximately 15 km to the
southwest of the station and the other approximately 30 km away to the
north, were observed by the radar. At 15:00, the cell in the southwest moved
toward the station and its scale expanded. At the same time, the cell in the
north remained motionless, but expanded horizontally. The radar reflectivity
of both cells was weak. Half an hour later, as the two cells developed
further, they connected into a curved cloud band with a cloud edge near the
station. A maximum reflectivity of 40 dBZ appeared in the northwest
approximately 25 km away. At 16:00, the reflectivity showed a decreasing
trend as the cloud body moved closer to the station, and only a small portion
of it reached 35 dBZ. Due to power failure, the radar halted at 16:15 and
recovered at 17:18. During this period, the cloud body further expanded in
scale and completely covered the station with a maximum reflectivity of
50 dBZ. Starting at 18:00, the thunderstorm began to recede while moving
eastward and its horizontal scale also shrank gradually. At 19:00, the
thunderstorm completely moved away from the station until it disappeared to
the east of the station.
Balloon-borne sounding path, (a) surface projection;
(b) distance and altitude from the station.
The surface E field was negative and the total flash rate was
1–2 fl min-1 before the thunderstorm arrived at the station at 16:03,
as shown in Fig. 1. We knew that the surface E field is positive,
i.e., it is controlled by the LPCC when the thunderstorm arrived at the
station. No other lightning flash data can be used to study the total
lighting flash rate and to distinguish cloud-to-ground flashes from
intra-cloud discharges. Therefore, the flash rate was counted from the
surface E field changes caused by lightning flashes. The surface
E field changed from negative to positive at 16:15 and lasted 29 min
(16:44). The flash rate increased rapidly and reached a peak value of
6 fl min-1 during this period. Based on the surface precipitation at
the station and time differences between sound and light of flashes, we
inferred when the thunderstorm arrived at the station and that the surface
E field was controlled by LPCC. A lightning flash at 16:44:58 caused
the surface E field to change from positive to negative, and the
negative E field was maintained for 28 min. During this period, the
mean flash rate was 2 fl min-1. We speculated that the intra-cloud
discharges continued to consume positive charge of the LPCC. As a result, the
surface E field was controlled by the main negative charge region.
The low flash rate was in favor of recovering LPCC and the E field
reversed to positive at 17:12. Seven minutes later, the polarity of the
surface E field reversed again to negative and lasted for 11 min.
The mean flash rate increased to 3–4 fl min-1 as the value of the
negative E field was enhanced. The fluctuations of the surface
E field were mainly influenced by the intensity of each charge region
in the thunderstorm and distance between the charge center and station. At
18:00, the flash rate began to decrease because the thunderstorm was
dissipating, and continuous fluctuations of the E field occurred in
the decay stage. Marshall et al. (2009) considered that the EOSO (end of
storm oscillation) is related to the formation and dissipation of charge
regions in the thunderstorm.
Composite reflectivity of radar, (a) 16:14;
(b) 17:25.
Sounding results: (a) ascending velocity of the balloon;
(b) corona current; (c)E field, temperature (T),
and relative humidity (RH). Black lines are vertical E fields
retrieved by corona current and they have been manually connected by light
grey lines to form a complete profile; (d) mean charge density.
Vertical E field in thunderstorm
The balloon-borne sounding was launched when the thunderstorm covered the
station at 16:08. Observations showed that the sounding traversed the cloud
top for the duration of 33 min. Influenced by a horizontal wind, the balloon
flew horizontally eastward approximately 11 km before traversing the cloud
top, as shown in Fig. 2a, b. Because of electric power malfunction, the last
radar volume scans occurred at 16:15 and the composite reflectivity (CR) was
provided in Fig. 3a. The radar worked in a specified mode for business
operation, thus its maximum elevation was 19.5∘. In addition, the
launching site of the sounding was so close to the radar that the sounding
path was mainly located in the dead zone of the radar. Therefore, here we
chose CR as a key reference index to estimate the evolution of the
thunderstorm. Compared with the CR of the thunderstorm in the mature stage at
17:25 (Fig. 3b, the first volume scan of the radar after the electric power
recovery), we found that the thunderstorm had a small scale with weak CR.
Moreover, the lightning flash rate was still quite low, although it increased
to its peak value at 16:29. All of these facts indicated that the period of
sounding was during the early period of the mature stage of the thunderstorm.
Figure 4 showed the corona current, temperature, relative humidity, balloon
ascending velocity, and the mean charge density. The values of balloon
ascending velocity ranged from 1 to 8 m s-1 with a mean value of
5 m s-1 and the extreme values of ascending velocity appeared in the
region of 5.81–6.15 km. Balloon ascending velocity was the result of a net
uplift force of the balloon and updraft in the thunderstorm; therefore, it
should be larger than the velocity of the updraft. The strongest positive
E field appeared at 2.3 km and the strongest negative E
field appeared at 6.1–6.7 km. Comparing the balloon ascending velocity and
E field, we found that the maximum value of balloon ascending
velocity was located in the lower part of the saturated negative E field,
and such a result is basically coincident with physical principles, i.e.,
when the charge regions with opposite polarity are distributed alternately,
the extreme value of the vertical E field is in the transition zone
between the two adjacent charge regions. When the charged particles with
opposite polarity are separated rapidly by the stronger updraft in a certain
region, the extreme value of the E field can be observed in the same
region. As a result, the particles separated by the updraft should have
different sizes or weights. This deduction basically conforms to the
ice-graupel charging mechanism proposed by Takahashi (1978).
Based on a one-dimensional approximation to the Gauss law, the mean charge
densities of the charge regions were retrieved by considering the E
field errors caused by a corona measuring error of 50 %. The results
showed that the balloon sounding completely passed through seven predominant
charge regions in its path. Two charge regions located in the region warmer
than 0 ∘C and the remaining five charge regions existed in the
region colder than 0 ∘C.
The polarity of the charge region near the surface was negative with a depth
of 800 m. The other charge region was a positive one in the warm cloud. The
mean charge densities of the two charge regions were
-0.44 ± 0.136 nC m-3 (1.7–2.5 km) and
0.43 ± 0.103 nC m-3 (2.5–3.6 km). Such a negative corona
ion layer with larger charge density and depth heavily influences the
measurement of surface E field. Soula and Chauzy (1991) measured the
E field near the surface at several levels up to 800 m, and they
found a positive corona ion region with charge density close to
1 nC m-3 within each layer. Qie et al. (1994) simulated the corona
ions beneath a thunderstorm and found the layer of corona ions can reach
several hundreds of meters above the ground. Qie et al. (1998) studied the
electric field and charge density under one thunderstorm in the Pingliang
region, and they also found the corona ion layer was negative with its depth
larger than 600 m.
In the region colder than 0 ∘C, a total of five charge regions
existed and they were distributed alternately in a vertical direction with
the lowest charge region being positive. From bottom to top, the mean charge
densities of the five charge regions were 0.40 ± 0.037 nC m-3
(5.2–6.1 km, -9.5 to -4 ∘C),
-0.63 ± 0.0107 nC m-3 (6.7–7.5 km, -18 to
-14 ∘C), 0.35 ± 0.063 nC m-3 (7.5–8.6 km, -27 to
-18 ∘C), -0.36 ± 0.057 nC m-3 (8.6–9.5 km, -34
to -27 ∘C), and 0.24 ± 0.06 nC m-3 (9.5–10.2 km,
-38 to -34 ∘C). In the region around 0 ∘C
(3.6–5.2 km), no charge layer was detected, it indicated that the electric
field in this region was too weak to cause the corona current on the probe,
the corona current and the retrieved E field were equal to zero as a
result. In the case of the continuous evolution of the E field, it is
possible that the charge regions of R2 and R3 are the same charge region with
a weak charge density layer in the middle. If it was true, the mean charge
density of this lower positive charge region was
0.24 ± 0.046 nC m-3 (2.5–6.1 km, 14 to
-9.5 ∘C). Additionally, in the
region of 6.1–6.7 km, the E field was saturated with two positive
charge regions at the upper and lower parts, and the boundary of the two
charge regions was unknown. So the real range of the two charge regions
should be underestimated.
Model results of vertical E field by using the mean charge
density in Fig. 4d, (a) vertical section across model center;
(b) profiles of E field at different paths.
The product term of the mean charge density ρ and thickness h was
used to estimate the intensity of each charge region, and the horizontal
distribution of charge density was regarded as uniform (Marshall and Rust,
1991). The values of ρh for the seven regions from surface to cloud top
were -344.5 ± 109.1, 431.6 ± 113.5, 356.0 ± 68.4,
-532.5 ± 85.6, 356.6 ± 69.3, -356.7 ± 52.2, and
177.9 ± 42.1 nC m-2. The results indicated that the most
intensive charge region was the negative charge region at the height of
6.7–7.5 km, and the weakest was the positive charge region at the top of
the cloud. The remaining three charge regions had nearly the same intensity.
If the two positive charge regions of R2 and R3 are the same charge region,
the value of ρh is 864 ± 165.6 nC m-2. Our calculation
showed that the summation of ρh of the five charge regions in the
region colder than 0 ∘C was nearly equal to zero. Therefore, the net
charge in the thunderstorm is also zero if the charge density is distributed
uniformly across the horizontal plane. According to the law of conservation
of charge, the quantities of negative charge and positive charge produced by
the charging mechanism in the cloud should be equal even if the intra-cloud
discharges occur frequently. However, cloud-to-ground (CG) flashes can
consume charges with particular polarity, and the equal net charge with
opposite polarity will be reserved in the thunderstorm as a result. We
assumed that CG flashes rarely occur in the thunderstorm because the sounding
period is in the early period of the mature stage; hence, the net charges in
the thunderstorm stay at zero.
The influence of balloon path on E field measurement
The balloon drifted horizontally almost 11 km before passing through the
cloud top. It was still unknown if the E fields obtained in such a
path can be used to study the charge structure accurately. Here, a model was
adopted to analyze the influence of the balloon path on the E field
measurement.
The adopted model assumes that the charges are distributed uniformly in a
disk with a radius of R and thickness of H (Burke and Few, 1978). Based
on the vertical distribution of the mean charge density, the vertical
E field can be calculated in each grid point of the model. Here, the
charge structure of the model adopted the real sounding result of Fig. 4c to
test the E field profile in the different sounding paths. The radius
of the disk is another parameter that influences the vertical E
field, and several values for radius were selected to compare the difference
between the real E field profile and calculated results at the center
of the cylinder. We found that the smaller the radius, the larger the
difference between profiles and the real E field, when the disk
radius is larger than 10 km, the real E field profile and the
calculated results are quite similar. As a result, the spatial distribution
of the vertical E field was calculated by using a disk radius of
10 km. In fact, the real radius of the thunderstorm is larger than 10 km in
the mature stage. Figure 5a showed the vertical cross section through the center
of the model. The vertical E fields of four vertical paths (P1, P2,
P3, and P4) and one slant path (P5) were figured out in Fig. 5b. When the
vertical path is closer to the center, the E profile is more accurate
and there is a larger difference in the fringe region of the model. However,
all E field profiles of different paths have almost the same trend
and polarity. This indicated that the polarity and location of the charge
regions retrieved by the E field of the five paths are consistent
with each other, with the exception of the mean charge density. The real
sounding path of observation is more complicated than the five cases
discussed. These simulation results indicated that only observations in the
fringe region can cause non-ignorable errors.
Conclusion and discussion
Based on observations of the vertical E field and particles in a
thunderstorm that occurred on 25 August 2012, the vertical electrical field
and charge structure in the thunderstorm were analyzed.
A total of seven predominant charge regions were penetrated by the
balloon-borne sounding. In the region warmer than
0 ∘C, a pair of charge regions existed with a positive charge
region upon a negative one. In contrast to the classic tripole model
summarized by Williams (1989), there were five charge regions in the region
colder than 0 ∘C and the lowest charge region and upper boundary
were all positive. This result showed that the real charge structure of
thunderstorms in a Chinese inland plateau is more complex than the tripole
model retrieved by multi-station observation of lightning flashes in the
study region (Qie et al., 2000; Zhang et al., 2009; Cui et al., 2009).
The values of ρh for the five charge regions in the mixing phase region
were 356.0 ± 68.4, -532.5 ± 85.6, 356.6 ± 69.3,
-356.7 ± 52.2, and 177.9 ± 42.1 nC m-2. In the region
colder than 0 ∘C, the most intense charge region is the main
negative charge region located at 6.7–7.5 km and the weakest is the
positive charge region at the top of the cloud. The regions with saturated
E field were not considered in calculating the values of ρh,
the real values of ρh for the lower positive charge region
(5.2–6.1 km) and main negative charge region (7.5–8.6 km) were
underestimated as a result. If it is true that the two positive charge
regions in the lower portion of the thunderstorm are the same charge region,
then its mean charge density and intensity are
0.24 ± 0.046 nC m-3 (2.5–6.1 km, 14 to -9.5 ∘C) and
864 ± 165.6 nC m-2, respectively. The strong lower dipole can
well explain the frequent intra-cloud discharge occurring between the two
charge regions as reported by early observation results (Qie et al., 2005a,
b). Furthermore, the
charge layers near surface cannot be ignored when considering the influence
of charge structure on lightning activities.
Stolzenburg et al. (1998b) proposed a conceptual model of the charge
structure in isolated thunderstorms, i.e., four charge regions appear in the
updraft and six in the downdraft; a common feature is that the lowest charge
region is positive and the negative charge region at the upper boundary is
considered a screening layer. Zhao et al. (2009) found that the charge
structure for one case of a thunderstorm in the Pingliang region is similar
with that of the conceptual model in the updraft shown by Stolzenburg et
al. (1998b). In addition, the mean charge density of the lower positive
charge region is the largest for all four charge regions. Based on
observations of a three-dimensional lightning mapping system, Li et
al. (2013) studied a thunderstorm in Qinghai, on eastern verge region of
the Tibetan Plateau, and noted that the thunderstorm had two charge regions with
a negative charge region (6–7 km a.s.l.) upon a stronger positive one
(4–6 km a.s.l.) during its developing and mature stages. This sounding
lasted a total of 33 min and started from the late period of the initial stage to
the end of the mature stage; therefore, our study may be a good presentation
of the charge structure of the thunderstorm in this transitional period. In
comparison to the results of Li et al. (2013), we found that there were three
additional charge regions upon the lower dipole in our case study. Li et
al. (2013) found that the charge structure converted into four charge regions
during the dissipating stage. However, it is unreasonable that two additional
charge regions suddenly appeared in the thunderstorm. It is more likely that
they had existed before and did not participate in discharging. Therefore, we
speculated that more charge regions should exist upon the lower dipole of the
thunderstorm studied by Li et al. (2013).
All data used in this paper can be found on the following
website: https://pan.baidu.com/s/13MnvwP4Jrgee1H4TNtdIgA (Yu, 2018).
TZ organized the electric field sounding observations of
thunderstorms in the Pingliang region in 2012 and obtained the key data of
the study case in this paper. The raw data analysis and error estimation were
finished by HY and FZ, and the model was completed by JC and MZ. Finally,
based on discussions by the authors, TZ prepared the
manuscript.
The authors declare that they have no conflict of
interest.
Acknowledgements
The research is supported by National Natural Science Foundation (grant no:
41375011, 41775011). Special thanks are owed to Pingliang Station of
Lightning and Hail Research, Chinese Academy of Sciences for providing radar
service and technical support. The
topical editor, Vassiliki Kotroni, thanks two anonymous referees for help in
evaluating this paper.
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