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review of positive lightning

Positive Lightning: Review and Update

Vladimir A. Rakov and Amitabh Nag

Department of Electrical and Computer Engineering, University of Florida, Gainesville, Florida, USA Abstract: It appears that at least six different cloud-charge-configurations/scenarios can give rise to downward positive lightning.

For four of them, tilted positive dipole, positive monopole, inverted dipole, and unusually large lower positive charge region, the primary source of charge is a charged cloud region, while for the other two, negative in-cloud leader channel cut-off and branching of in-cloud channel, the primary source of charge is an in-cloud lightning channel formed prior to the positive discharge to ground.

The highest directly measured lightning currents (near 300 kA) and the largest charge transfers (hundreds of coulombs or more) are thought to be associated with positive lightning. Positive flashes are usually composed of a single stroke, although up to four strokes per flash were observed. Positive return strokes often appear to be preceded by significant in-cloud discharge activity and tend to be followed by continuing currents. Similar to negative lightning, continuing currents in positive lightning are accompanied by M-components. There is a controversy regarding whether downward positive flashes can contain subsequent strokes following the first-stroke channel. Recent observations confirm that such positive subsequent strokes do occur.

Keywords : Positive lightning, Multiplicity, Peak Current.

1. INTRODUCTION

Downward positive lightning, which is initiated by a downward leader and effectively lowers positive charge from the cloud to ground, accounts for about 10% of all cloud-to-ground discharges (e.g., Rakov, 2003 [1]). Due to their relative paucity, positive lightning discharges are considerably less studied and understood than their negative counterparts. The charge structure and evolution of thunderclouds that produce positive lightning, as well as in-cloud processes that can lead to its initiation, largely remain a mystery. Positive lightning discharges have recently attracted considerable attention for the following reasons (see Rakov (2003) [1] and references therein):

1) The highest recorded lightning currents (near 300 kA) and the largest charge transfers to ground (hundreds of coulombs or even more) are thought to be associated with positive lightning.

2) Positive lightning can be the dominant type of cloud-to-ground lightning during the cold season and during the dissipating stage of a thunderstorm.

3) Positive lightning has been found to be preferentially related to transient luminous events known as sprites in the middle and upper atmosphere.

4) Reliable identification of positive discharges by lightning locating systems (LLS), such as the U.S. National Lightning Detection Network (NLDN), has important implications for various meteorological and other studies that depend on LLS data.

5) Several properties of positive lightning (e.g., number of strokes per flash, occurrence of continuing current, leader propagation mode, and branching) appear to be distinctly different from those of negative lightning.

Positive charge can be also transferred to ground by so-called bipolar lightning that sequentially lowers charges of both polarities to ground. Bipolar lightning is generally not considered to be a significant component of the overall lightning activity, although this type of lightning discharge may not be less common than positive lightning (Rakov, 2005) [2].

Knowledge of the occurrence and characteristics of positive and bipolar lightning is needed for studying cloud electrification mechanisms, the charge structure and evolution of thunderclouds, and lightning effects in the middle and upper atmosphere, as well as for designing adequate lightning protection schemes for various objects and systems. Further, given their very large charge transfers and their tendency to produce sprites, positive discharges may play an important role in the global electrical circuit. According to the “classical” view of atmospheric electricity, thunderstorms serve to resupply the negative charge on Earth that is constantly being lost due to the fair weather leakage current between the earth and the electrosphere. Positive lightning apparently counteracts this global-circuit mechanism. It is possible that the monitoring of such an “abnormal” component of global lightning activity can be useful in climate change studies.

2. CONDITIONS CONDUCIVE TO THE

OCCURRENCE OF POSITIVE LIGHTNING Although the overall percentage of positive lightning discharges is relatively low, there are five situations, listed below, that appear to be conducive to the more frequent occurrence of such discharges. The genesis of positive lightning in these situations is not yet fully understood.

Contact Address:

Vladimir A. Rakov

553 New Engineering Building

Department of Electrical and Computer Engineering

University of Florida, Gainesville, Florida 32611, USA E-mail: rakov@https://www.wendangku.net/doc/5b2216635.html,

a) The dissipating stage of an individual thunderstorm: There is a clear tendency for positive lightning to occur toward the end of a thunderstorm (e.g., Orville et al. (1983) [3]). Pierce (1955) [4] suggested that positive flashes are initiated from the upper (main) positive charge region of thunderclouds after much of the main

negative charge, located below the main positive charge, has been removed by negative ground flashes.

b) Winter thunderstorms: Brook et al. (1982) [5] observed that positive flashes constituted about 40% of the total number of cloud-to-ground flashes in winter storms in Japan. There is a clear tendency in LLS data from different countries for positive lightning to occur during the cold (non-convective) season.

c) Trailing stratiform regions of mesoscale convective systems (MCSs): The production of predominantly positive flashes by relatively shallow clouds, including the trailing stratiform regions of MCSs (Engholm et al., 1990) [6], has been observed in both winter and summer seasons. Some thunderstorm systems produce positive and negative flashes whose ground strike locations tend to be separated in space by polarity, thereby forming the “bipolar” pattern (Orville et al., 1988) [7].

d) Severe storms: The occurrence of infrequent, widely scattered positive flashes in the mature and later stages of severe springtime storms over the Great Plains in the United States was first observed by Rust et al. (1981) [8], who used electric field measurements in conjunction with optical observations. More recent LLS data indicate that positive flashes in severe storms can outnumber negative ground flashes for more than 30 minutes. The period of a storm’s lifetime in which positive flashes dominate varies, but is often during the earlier severe stages of the storm.

e) Thunderclouds formed over forest fires or contaminated by smoke: Vonnegut and Orville (1988) [9] found that 25% of about 50 cloud-to-ground flashes apparently associated with the forest fires in Yellowstone National Park lowered positive charge to earth. A relatively high percentage of positive flashes in the central United States detected by the NLDN in the spring of 1998 has been associated with cloud contamination by smoke from massive forest fires in Mexico that were up to thousands of kilometers away (Lyons et al., 1998a; Murray et al., 2000) [10, 11].

3. GENERAL CHARACTERIZATION

The following is a list of observed lightning properties that are thought to be characteristic of positive lightning discharges.

a) Positive flashes are usually composed of a single stroke, whereas about 80% of negative flashes contain two or more strokes, with three to five being typical (e.g., Rakov et al., 1994; Rakov, 2007) [12, 13]. Multiple-stroke positive flashes do occur but they are relatively rare.

b) Positive return strokes tend to be followed by continuing currents that typically last for tens to hundreds of milliseconds (e.g., Fuquay 1982; Rust et al. 1981, 1985) [14, 15, 16]. Brook et al. (1982) [5], from multiple-station electric field measurements, inferred continuing currents in positive flashes in excess of 10 kA, at least one order of magnitude larger than for negative flashes, for periods up to 10 ms. Campos et al. (2009) [17] recently showed that, similar to negative lightning, continuing currents in positive lightning are accompanied by M-components.

Directly measured positive continuing currents in the kiloamperes to tens of kiloamperes range in winter lightning in

Japan are seen following the initial current pulses in Figure 1. (For

Fig. 1. Directly measured currents in three positive lightning discharges in Japan. Note the very large peaks, 340, 320, and 280 kA, of the initial pulses followed by low-level continuing currents whose durations are of the order of (top and middle) 10 or (bottom) 100 ms. The middle and bottom panels have inserts (labeled “Expansion”) that show the same current waveform, but on an expanded (1 ms) timescale. Transferred charges (currents integrated over time) are 330, 180, and 400 C, respectively. Adapted from Goto and Narita (1995) [20].

comparison, continuing currents in negative flashes are typically in the tens to hundreds of amperes range.) Such large continuing currents are probably responsible for the unusually large charge transfers by positive flashes. Brook et al. (1982) [5], for one positive lightning in a winter storm in Japan, inferred a charge transfer in excess of 300 coulomb (C) during the first 4 ms. (For comparison, a typical negative flash transfers to ground a charge of 20 C.) Charge transfers during the first 2 ms estimated by Berger (1967) [18] for summer positive lightning in Switzerland are of the order of tens of coulombs.

Charge transfers of the order of 1000 C were reported, from direct current measurements, by Miyake et al. (1992) [19] for both positive and negative winter lightning in Japan. However, these latter events may well be unusual forms of lightning discharges because the grounded strike-object tip was very close to or inside the cloud.

c) From electric field records, positive return strokes often appear to be preceded by significant in-cloud discharge activity lasting, on average, in excess of 100 (Fuquay, 1982) [14] or 200 ms (Rust et al. 1981) [15]. This observation suggests that a positive discharge to ground can be initiated by a branch of, or otherwise produced by, an extensive cloud discharge (see Section 4). Negative cloud-to-ground discharges are less often preceded by such long-lasting in-cloud discharge activity.

d) Several researchers (e.g., Fuquay, 1982; Rust, 1986) [14, 21]

reported that positive lightning discharges often involve long horizontal channels, up to tens of kilometers in extent. This might be due to their more intimate relation to cloud discharges.

e) It appears that positive leaders can move either continuously or intermittently (in a stepped fashion), as determined from time-resolved optical images. This is in contrast with negative leaders, which are always optically stepped when they propagate in virgin air. Further, distant (radiation) electric and magnetic field waveforms due to positive discharges are less likely to exhibit step pulses immediately prior to the return-stroke waveform than are first strokes in negative lightning.

Finally, positive leaders usually do not radiate at very high frequency (VHF) and at ultra high frequency (UHF) as strongly as negative leaders and therefore are usually not detected by VHF–UHF lightning imaging systems. In the case of a positive leader, electrons present or produced ahead of the leader tip move toward the tip because they are attracted to the positive charge on it, and the resultant ionization occurs in the strong field near the tip. In the case of a negative leader, electrons tend to “run ahead” of the moving leader tip to where the field is relatively low because they are repelled by the negative charge on the leader tip. Thus, ionization occurs under less favourable conditions for the negative leader than for the positive leader. As a result, streamer zone formation in the negative leader requires a higher tip potential than for the positive leader, which may be related to the apparently different intensity of VHF–UHF radiation produced by these two types of leaders.

It follows from this list and accompanying discussion that positive discharges to ground are usually composed of a single stroke, often appear to be preceded by significant in-cloud discharge activity, and tend to be followed by continuing currents. In contrast to negative leaders, positive leaders seem to be able to move either continuously or in a stepped fashion, although the stepping mechanism is different for these two types of leaders.

4. CONCEPTUAL CLOUD-CHARGE-

CONFIGURATIONS/SCENARIOS LEADING

TO PRODUCTION OF POSITIVE LIGHTNING The gross charge structure of a "normal" thundercloud is often viewed as a vertical tripole consisting of three charge regions, main positive at the top, main negative in the middle, and an additional (typically smaller) positive below the main negative (Williams, 1989) [22]. Such a charge structure appears to be not conducive to production of positive cloud-to-ground lightning. In this section, we describe five conceptual cloud-charge-configurations/scenarios that were observed or hypothesized to give rise to positive lightning.

(a) Tilted positive dipole (Figure 2a): This cloud charge structure was proposed by Brook et al. (1982) [5] who suggested that positive flashes can originate from the upper positive charge of a vertical dipole that is displaced horizontally by vertical wind shear from the lower negative charge and thereby exposed to the ground, as shown in Figure 2a. The titled dipole configuration was inferred from multiple-station electric field change measurements and radar observations during winter thunderstorms in Japan. (b) Positive monopole (Figure 2b): Kitagawa and Michimoto (1994) [23], using a network of electric field mills and radar observations, inferred the existence of positive monopolar charge structure during the dissipating stage of Japanese winter thunderclouds. During the developing stage, electric charge distribution is a positive dipole, net positive charge being found in the top portion of the cloud and net negative charge at lower regions. In the mature stage the thunderclouds exhibit a "normal" tripolar charge structure; that is, a positive dipole with an additional lower positive charge region below the negative charge region. The negative charge and the lower positive charge are carried mainly by graupel particles which are present in the cloud for a relatively short period of time. As a result the lifetime of the dipolar and tripolar charge structures is very short (usually less than 10 minutes). During the dissipating stage positive charge is dominant in the whole cloud, this charge being carried by ice crystals and snowflakes. It is this stage that is characterized by a positive monopolar charge structure. The dissipating stage has a much longer duration than the developing and mature stages and thus accounts for most of the lifetime of Japanese winter thunderclouds. According to Kitagawa and Michimoto (1994) [23], the monopolar cloud charge structure (see Figure 2b) explains why positive lightning is the dominant lightning type in winter thunderstorms in Japan.

The depletion of negative charge in the dissipating stage of a thunderstorm, hypothesized by Pierce (1955) [4] (see Section 2), may be also leading in effect to the positive monopole configuration.

(c) Inverted dipole (Figure 2c): The highest incidence of storms producing predominantly positive ground flashes in the United States is in the High Plains region (northwestern Kansas and eastern Colorado) (Orville and Huffines, 2001; Lang et al., 2004) [24, 25]. Rust et al. (2005) [26] used balloon-borne electric field soundings and lightning VHF mapping in the Severe Thunderstorm Electrification and Precipitation Study (STEPS) program to examine the electrical structure of such storms. They found the positive and negative charge regions to be at altitudes where, respectively, negative and positive charge would be found in "normal", positive-dipole thunderclouds. Specifically, the positive charge region in their storms was at altitudes between 5 to 8 km above mean sea level and the negative charge region between 11 and 13 km. They also detected a positive charge region between 13 and 14 km, which was possibly the screening layer charge near the top of the thundercloud. Accordingly, this charge structure was termed an inverted dipole (main negative charge region above main positive charge region). Cloud flashes bridging the upper negative and the lower positive charge regions (inverted-polarity cloud flashes) and predominantly positive ground flashes (originating from the positive charge region, as shown in Figure 2c) are common in such storms. Rust et al. (2005) [26] described two inverted-polarity storms that produced predominantly positive ground flashes and one inverted-polarity storm which produced only inverted-polarity cloud flashes, but no flashes to ground.

In addition to main positive and main negative charge regions, Figure 2c shows an additional negative charge region near the bottom of cloud, which was observed by Tessendorf et al. (2007) [27] to facilitate positive cloud-to-ground lightning initiation; that is, to play the same role in inverted-dipole clouds as the lower positive charge region in "normal"-dipole ones.

Fig. 2. Conceptual cloud-charge-configurations/scenarios leading to production of downward positive lightning. An additional scenario involving a tower and a long upward connecting leader intercepting a positively charged in-cloud channel is found in Figure 4b.

(d) Unusually large lower positive charge region (Figure 2d): Cui et al. (2009) [28] inferred from seven-station electric field measurements that thunderstorms in the Tibetan Plateau region have a "normal" tripolar charge structure (positive at the top, negative in the middle, and an additional positive below the negative). However, these storms are characterized by a larger-than-usual lower positive charge region. They produce mostly intracloud discharges (between the upper positive and negative charge regions as well as between the negative and lower positive charge regions) and a relatively small numbers of negative and positive cloud-to-ground flashes (Qie et al., 2009) [29]. For one storm, Cui et al. (2009) [28] reported a total of 112 flashes, of which 98 were cloud flashes, 12 negative ground flashes, and 2 positive ground flashes. It is not clear why the unusually large lower positively charged region appears to be unlikely to launch a positive leader toward ground. The upper positive charge region was inferred to be located at a height of about 7.5 km, negative charge region at about 6.0 km and lower positive charge region at about 4.0 km above mean sea level (the altitude of the local terrain was about 2 km above mean sea level). It appears that if the upper positive charge were not detected, the Tibetan Plateau cloud charge structure could be interpreted as an inverted dipole. It is also possible that the detected upper positive charge was associated with the screening layer at the cloud top.

Liu et al. (1994) [30] described the results of triggered-lightning experiments in Gansu province in northwestern China with the ground-level electric field being apparently dominated by a large lower positive charge region. They reported 10 triggered lightning flashes initiated during three summers from 1989 to 1991 with all of them being composed of the initial stage only (upward moving negative leader followed by initial continuous current), effectively transferring positive charge to ground.

(e) Negative in-cloud leader channel cut-off (Figure 2e): Positive flashes in mesoscale convective systems are often preceded by intracloud discharges with negatively-charged channels (leaders) originating in the convective region and propagating horizontally over several tens of kilometers, apparently into the upper positive charge layer (5 to 8 km above ground) in the stratiform region. When a negatively charged horizontal leader becomes cut-off from the older (near the origin) channel and the newly-formed rear end of this leader gets positively charged, a positive leader is launched from the rear end of the advancing negative leader channel, resulting in a positive cloud-to-ground flash, as shown in Figure 2e. This positive flash typically occurs tens of kilometers away from the origin of the parent cloud discharge, under the stratiform, but may occur under the

convective. Such a scenario was apparently first suggested by Krehbiel (1981) [31]. Perhaps the best examples of positive cloud-to-ground flashes belonging to this category are presented by Lu et al. [2009] who used lightning mapping array (LMA) observations of very-high-frequency (VHF) sources in conjunction with NLDN data and ultra-low-frequency (ULF) magnetic field observations. They examined eight positive cloud-to-ground strokes in a mesoscale convective system in Alabama along with their parent intracloud discharges and observed that VHF sources associated with the positive CG return stroke propagated only toward the advancing end of the negative in-cloud leader (not to its origin in the convective region) and viewed this fact as an evidence of the negative leader channel cut-off. It has been suggested that the so-called spider lightning (heavily branched channel system crawling along the lower cloud boundary), known to be associated with positive discharges to ground, follows the same scenario (Boccippio et al., 1995; Mazur et al., 1998) [32, 33].

(f) Branching of in-cloud channel (Figure 2f): Positive cloud-to-ground discharges can be produced by branching of in-cloud discharge channels, probably most often when these channels occur near or below the cloud base, as shown in Figure 2f. High-speed video images of such flashes are found in Kong et al. (2008) [34] and Saba et al. (2009) [35].

It is important to note that in scenarios (e) and (f) the primary source of charge for positive cloud-to-ground discharge (at least its impulsive component) is an in-cloud lightning channel (a conductor) that was formed prior to the positive discharge, as opposed to a charged cloud region (initially an insulator) in scenarios (a) through (d). The in-cloud-channel source, which is likely to result in very large continuing currents and charge transfers, was also hypothesized by Rakov (2003) [1] to explain millisecond-scale positive lightning current pulses measured by K. Berger at instrumented towers in Switzerland (see Figure 4b).

5. MULTIPLICITY

The term multiplicity is often used to denote the number of strokes per flash, not necessarily along the same channel to ground. Positive flashes are usually composed of a single stroke, whereas about 80% of negative flashes contain two or more strokes (e.g., Rakov et al., 1994; Rakov, 2007) [12, 13]. Multiple-stroke positive flashes do occur but they are relatively rare. The overall electric field of the three-stroke flash in Florida and time-expansions for individual strokes are shown in Figure 3. The first and second strokes have similar overall waveshapes which suggests that the second stroke followed the first-stroke channel. The third stroke apparently formed a separate channel to ground. There was no preliminary breakdown pulse train or other accompanying in-cloud discharge activity observed in the electric field record of this flash.

Occurrence of positive flashes with different number of strokes from different studies is summarized in Table 1. As noted earlier, strokes may belong to the same flash, but develop in different channels to ground. In Florida, about 50% of negative flashes produce multiple terminations on ground.

Ishii et al. (1998) [36], using five-station electric field records, examined 11 two-stroke and 3 three-stroke positive flashes in winter storms in Japan. Observations were performed in summer as well, but no positive flashes were recorded. All the subsequent strokes created new terminations on ground. In 71% of cases, a new termination was more than 10 km away from the first-stroke termination. The average distance between ground terminations for positive flashes (all in winter) was 13.4 km versus 2.1 km (2.1 km

in summer and 2.2 km in winter) for negative flashes.

Fleenor et al. (2009) [37] examined video records of nine two-stroke positive flashes in the U.S. Central Great Plains (Kansas and Nebraska) and found that in four flashes, second strokes created new terminations on ground, while in five flashes second strokes remained in the previously-formed (first-stroke) channel.

Saba et al. (2010) [38], using high-speed video records and lightning locating system data, found that, out of 21 subsequent strokes in 20 multiple-stroke positive flashes, 1 stroke followed previously-formed channel and 20 strokes created new terminations on ground.

Nag et al. (2010) [39] inferred that three (38%) subsequent strokes in positive lightning likely followed the previously-created (first-stroke) channel and five (62%) likely created new ground terminations. Their inferences are based on NLDN locations and detailed examination of electric field waveform features.

Occurrence of positive subsequent strokes in a previously-created channel is summarized in Table 2. It follows from this Table that most of the experimental data are not in support

of Mazur's (2002) [40] conjecture that "positive CG flashes cannot have multiple return strokes" traversing the same channel to ground.

6. PEAK CURRENT

A reliable distribution of positive lightning peak currents applicable to objects of moderate height on the flat ground is presently unavailable. The sample of 26 directly measured positive lightning currents analyzed by Berger et al. (1975) [41] is commonly used as a primary reference both in lightning research and in lightning protection studies. However, this sample is apparently based on a mix of 1) discharges initiated as a result of junction between a descending positive leader and an upward-connecting negative leader within some tens of meters of the tower top and 2) discharges initiated as a result of a very long (1–2 km) upward negative leader from the tower making contact with an oppositely charged channel inside the cloud. These two types of positive discharges, which differ by the height above the tower top of the junction between the upward-connecting leader and the oppositely charged overhead channel (descending positive leader or positively charged in-cloud channel), are expected to produce very different current waveforms at the tower, as illustrated in Figures 4a and 4b. The “microsecond-scale” current waveform shown in Figure 4a is probably a result of processes similar to those in downward negative lightning, whereas the “millisecond-scale” current waveform shown in Figure 4b is likely to be a result of the M-component mode of charge transfer to the ground (Rakov et al. 2001) [42]. (The M component is a transient process, an increase in current and associated luminosity, that occurs in a lightning channel carrying continuing current.) It is

RT = 2.3 μs

RT = 5.5 μs

RT = 5.0 μs Fig. 3. (a) Electric field record of a three-stroke positive cloud-to-ground flash in Florida shown on a 75-ms time scale. (b), (c), and (d). Electric field waveforms of the first, second, and third return strokes (RS), respectively, on a 1.5-ms time scale. GPS timestamps and NLDN information were not available for this flash. RT = 10-to-90% risetime. The first and second strokes have similar overall waveshapes which suggests that the second stroke followed the first-stroke channel. The third stroke apparently formed a separate channel to ground. Adapted from Nag et al. (2010) [39].

T ABLE 1 O CCURRENCE POSITIVE FLASHES WITH DIFFERENT NUMBER OF STROKES

Occurrence (percentage) of flashes with different number of strokes

Reference Location

Sample

size Single-stroke Two-stroke Three-stroke Four-stroke Average multiplicity Heidler and Hopf (1998) [44] Germany (1988 - 1993) 44 33 (75%) 8 (18%) 2 (5%) 1 (2%) 1.3 Heidler et al. (1998)

[45] Germany (1995 - 1997) 32 28 (88%) 4 (13%) 0 0 1.1 Fleenor et al. (2009)

[37] U.S. Cental Great Plains (Kansas and Nebraska) 204 195 (96%) 9 (4%) 0 0 1.0 Saba et al. (2010) [38] Brazil, Arizona, Austria

103 83 (81%) 19 (18%) 1 (1%) 0 1.2 Saba et al. (2010) [38] Brazil 70* 54 (77%) 15 (21%) 1 (1%) 0 1.2 Nag et al. (2010) [39]

Florida

42 (81%)

9 (17%)

1 (2%)

1.2

* Subset for Brazil only of the 103 events recorded in Brazil, Arizona, and Austria.

possible that such millisecond-scale waveforms are characteristic of tall objects capable of generating very long upward-connecting leaders. On the other hand, the distribution of positive lightning peak currents inferred from electric or magnetic fields recorded by multiple-station LLSs, such as the NLDN, are influenced by the uncertainties of the conversion of the measured field to current. The NLDN formula that is used for this conversion is based on the linear regression equation relating the NLDN-measured field peak to the directly measured current peak for negative triggered lightning strokes and is extrapolated to natural positive strokes. Additionally, the lower end of the positive lightning peak current distribution based on LLS data is contaminated by misidentified cloud-flash pulses (e.g., Cummins et al., 1998) [43].

T ABLE 2. O CCURRENCE OF SUBSEQUENT STROKES IN POSITIVE FLASHES THAT FOLLOW A PREVIOUSLY-CREATED CHANNEL

Reference Location

Occurrence (percentage) of subsequent

strokes in a previously-created channel Sample size (total number of

subsequent strokes)

Remarks

Ishii et al. (1998) [36] Japan 0 (0%)

Winter storms; five-station electric field records Fleenor et al. (2009)

[37]

U.S. Cental Great Plains (Kansas and Nebraska) 5 (56%) 9

Summer storms; video records, electric field records

(LASA), NLDN Saba et al. (2010) [38] Brazil, Arizona,

Austria

1 (4.8%) 21

Probably summer storms; high-speed video records, lightning locating systems Nag et al. (2010) [39] Florida 3 (38%) 8

Summer (2 flashes) and winter (1 flash) storms; electric field

records, NLDN

LASA = Los Alamos Sferic Array

Fig. 4. Examples of two types of positive lightning current waveforms observed by K. Berger: (a) (right-hand side) microsecond-scale waveform, similar to those produced by downward negative return strokes, and (left-hand side) a sketch illustrating the lightning processes that might have led to the production of this waveform; (b) (right-hand side) millisecond-scale waveform and (left-hand side) a sketch illustrating the lightning processes that might have led to the production of this current waveform. Arrows indicate directions of the extension of lightning channels. Adapted from Rakov (2003) [1].

7. SUMMARY

In spite of recent progress, our knowledge of the physics of positive lightning remains considerably poorer than that of negative lightning. Many questions regarding the genesis of positive lightning and its properties cannot be answered without further research. Although positive lightning discharges account for 10% or less of global cloud-to-ground lightning activity, there are five situations that appear to be conducive to the more frequent occurrence of positive lightning. These situations include (1) the dissipating stage of an individual thunderstorm, (2) winter thunderstorms, (3) trailing stratiform regions of mesoscale convective systems, (4) some severe storms, and (5) thunderclouds

formed over forest fires or contaminated by smoke. It appears that at least six different cloud-charge-configurations/scenarios can give rise to downward positive lightning. For four of them, tilted positive dipole, positive monopole, inverted dipole, and unusually large lower positive charge region, the primary source of charge is a charged cloud region, while for the other two, negative in-cloud leader channel cut-off and branching of in-cloud channel, the primary source of charge is an in-cloud lightning channel formed prior to the positive discharge to ground. The highest directly measured lightning currents (near 300 kA) and the largest charge transfers (hundreds of coulombs or more) are thought to be associated with positive lightning. Two types of impulsive positive current waveforms have been observed. One type is characterized by risetimes of the order of 10 μs, comparable to those for first strokes in negative lightning, and the other type by considerably longer risetimes, up to hundreds of microseconds. The latter waveforms are apparently associated with very long, 1 to 2 km, upward negative connecting leaders. Positive flashes are usually composed of a single stroke, although up to four strokes per flash were observed. Positive return strokes often appear to be preceded by significant in-cloud discharge activity and tend to be followed by continuing currents. Similar to negative lightning, continuing currents in positive lightning are accompanied by M-components. There is a controversy regarding whether downward positive flashes can contain subsequent strokes following the first-stroke channel. Recent observations confirm that such positive subsequent strokes do occur.

8. ACKNOWLEGDEMENT

This research was supported in part by NSF grant ATM-0852869.

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外研版三起英语三年级下册知识点总结全

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who谁 only只，仅仅 year年龄，岁数 work工作地点 go to work上班 Saturday星期六 shopping购物 go shopping去购物 dad爸爸 Module6 do 做 Sunday 星期日 swimming游泳（运动） go swimming去游泳 eat吃 in在…期间，在…时候 sleep睡觉 watch观看 TV电视节目 watch TV看电视 have做，进行，从事 class课，班级 today今天 music 音乐 has（have的第三人称单数形式）做；进行；从事 Chinese 语文，汉语 maths 数学 art 美术PE 体育 science 科学 Module7 we我们 fly放（风筝） spring春天 summer夏天 season季节 nice迷人的，令人愉快的 warm暖和的 hot热的 autumn秋天 cool凉爽的 winter冬天 cold寒冷的 skating滑冰 go skating去滑冰 play玩，玩雪 snow雪；下雪 rain雨；下雨 sunny晴朗的，阳光充足的 windy有风的，刮风的 very很，非常 Module8 toy玩具 under在…下面 for为，给，对 box盒子 behind在…后面 bedroom卧室 flies（fly的第三人称单数形式）放（风筝） park公园 lake湖 tree树 fishing捕鱼，钓鱼 go fishing去钓鱼 walk行走，步行 Module9 have got 拥有 sweater 毛线衫 bed床 line线，绳，金属丝 about关于 animal 动物 sport运动 haven’t(got)=have not(got)没有 dress 连衣裙 coat 外套 T-shirt T恤衫 has got 拥有（have got的第三人称单数形式）有 hasn’t(got)=has not(got)没有 by 乘（交通工具） bus 公共汽车 car汽车，轿车 - 2 -

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door 门 sit 坐 down向下 stand站 up 向上 window 窗户blackboard 黑板 bird 鸟 tweet（鸟）啾啾的叫声desk 桌子 chair 鸟 MODULE4 it 它 it’s= it is它是 red 红色的 look 看 wow 呀，哇 yellow 黄色的 blue 蓝色的 a（an）一个，一chameleon变色龙 my 我的 panda 熊猫 now现在 green 绿色的 black 黑色的 dog 狗 cat 猫 cap 帽子 MODULE 5 how many 多少 one 一 two 二 three 三 four 四 five 五 six 六 seven 七

eight 八 nine 九 oh 噢，哦 ten 十 eleven 十一 twelve 十二 MODULE6 happy 快乐的 birthday 生日 here (here’s= here is)这里，(这是) present 礼物 this 这个 pencil 铅笔 pen 钢笔 cake 蛋糕 old ……岁的 how old 多大 yes 是的 you’re= you are 你是 MODULE 7 teacher 教师 pupil 小学生 school 学校 classroom 教室 English 英语 that 那个 say 说 again再一次 schoolbag 书包 ball 球 book 书 MODULE 8 monster 怪物 new 新的 kite 风筝 or 或者

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