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Charges on Graupel and Snow Crystals and the Electrical Structure of Winter Thunderstorms

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Charges on Graupel and Snow Crystals and the Electrical Structure of Winter Thunderstorms 1JUNE 1999 TAKAHASHI ET AL. 1561 Charges on Graupel and Snow Crystals and the Electrical Structure of Winter Thunderstorms TSUTOMU TAKAHASHI School of International Studies, Obirin University, Machida-shi, Tokyo, Japan TAKUYA TAJIRI Kyushu University, Hakozaki, Fukuoka, Japan YASUO SONOI Kansai Electric Company, Amagasaki, Osaka, Japan (Manuscript received 1 December 1997, in ®nal form 22 May 1998) ABSTRACT The shape and electric charge on particles in Hokuriku winter cumulus clouds have been measured using videosondes. The sign of the charge on graupel reversed at about 2118C. Charges on graupel and ice crystals are responsible for the tripole structure. The magnitude of the space charge increased as the particle concentrations increased. Graupel concentrations in excess of 1 L21 and an average charge on the precipitation particles of a few tenths of pC produced an accumulated space charge suf®cient to initiate lightning. These ®ndings support the model results reported by Takahashi in which riming electri®cation mechanisms were emphasized as the primary charge separation process. It was also observed that the most active particle-charging process occurred at around the 2208C level. 1. Introduction discharge of a lightning ¯ash is 20 C, the average space charge density for a thunderstorm will be about 5 nC Although recent advances in understanding the phys- m23 or5pCL21. The maximum electric ®eld may then ics of lightning have been important, knowledge of the be calculated as about 1800 V cm21, which is the same mechanisms through which rain clouds are electri®ed order measured by Gunn (1948) and Marshall and Rust still remains elusive (Williams 1988). The lack of in- (1991) as the maximum electric ®eld in the thunder- cloud measurements that can be correlated with labo- storm. Although Workman et al. (1942) reported that ratory and model experiments may be a contributing the upper, positive space charge moves clockwise in a factor. horizontal plane, both the midlevel negative charge and Electric ®eld pro®les taken at the ground during thun- the lower-level positive one exist in the same vertical derstorm passages are best explained by a tripole struc- column (Winn et al. 1981). The question then is how ture. From examination of many in-cloud electric ®eld the midlevel, negative charge remains at that level with- pro®les, Simpson and Scrase (1937) and Simpson and in the precipitation shaft. Robinson (1941) proposed the existence of three Having examined all of the many proposed charge charged regions: an upper, positive charge at about separation mechanisms, Takahashi emphasized riming 2308C; a middle, negative charge at about 2108C; and electri®cation as the primary process (Reynolds et al. a lower, positive charge at about 0 C. By assuming that 8 1957; Takahashi 1978, 1984). In this model, reversal of charges are distributed spherically, the diameter of the the sign of the charge during collisions between graupel middle, negative charge was estimated to be about 2 and ice crystals occurs at about 2108C and is critical km. If, as Workman et al. (1942) have reported the total not only in producing the tripole structure but also in keeping the midlevel charge at a ®xed height (Takahashi 1978, 1984). At about 2108C, negatively charged ice crystals are carried aloft in a gentle updraft and combine Corresponding author address: Dr. Tsutomu Takahashi, School of International Studies, Obirin University, 3758 Tokiwa-machi, Ma- with negatively charged graupel falling from above (Fig. chida-shi, Tokyo 194-0294, Japan. 1), thus enhancing the midlevel charge with increasing E-mail: [email protected] precipitation. The model results also suggest that in or- q 1999 American Meteorological Society Unauthenticated | Downloaded 09/29/21 06:28 AM UTC 1562 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 56 regions were determined as well as the regions peak electric charges. The measurements were made in winter clouds over Hokuriku, Japan. Cold air from Siberia blows over the warm waters of the Japan Sea producing bands of cu- mulus clouds that, in the winter monsoon, often result in heavy coastal snowfall. Although the clouds may be as shallow as 5 km, they still may produce lightning and have been characterized by Takeuchi et al. (1978) as having higher lightning discharge currents than in sum- mer storms and producing frequent positive discharges to the ground. However, these clouds also have electrical structures similar to the summer thunderstorms reported by Brook et al. (1982), suggesting that the same charge separation mechanism may occur in both instances. The major advantage in choosing the Hokuriku winter clouds is the simplicity of the microphysics involved. They sel- FIG. 1. Model of triple charge formation and accumulation of neg- dom contain raindrops or hail and because they are shal- ative charge proposed by Takahashi (1984). low, the videosonde ascent times are short, minimizing the possibility of modi®cation of the charge distribution during the ascent in the cloud. der to create a charge suf®cient to produce lightning, A description of the videosonde system is given be- the particles must be charged to a few tenths of a pi- low. The observational results were carefully examined, cocoulomb and ice crystals and graupel must be in con- after which typical space charge pro®les were deduced centrations exceeding about 50 and 1.5 L21. The model in relation to the stages of cloud lifetimes. The graupel results also indicate that large particles carrying large charge sign reversal, the peak space charge, and the peak charges are the dominant contributors. particle charge were determined with respect to the Testing the hypothesis requires simultaneous mea- cloud-top temperature and the concentration of graupel. surements of the particle shapes (raindrops, graupel, or A conceptual model is proposed, based upon the ob- ice crystals) and the charges they carry. Gaskell et al. servations. (1978) reported aircraft measurements using an induc- tion ring and an array of photocells and found 1-mm particles carrying 100 pC in the main negative space 2. Observations charge region. Graupel often carry such large charges a. Videosonde (Weinheimer et al. 1991). Marshall and Winn (1982) used a balloon-launched device that involved two in- The videosonde has been designed to measure both duction rings. Bateman et al. (1995) added the optical the shape and the charge on particles in clouds (Taka- sensor to it. They reported particles 1±3 mm in diameter hashi 1990). The signi®cant features of the sonde are carrying charges of a few hundred picocoulombs in the given in Figs. 2a, 2b, and 2c. The video camera records lower positive space charge region within the convective the particle images and an induction ring measures the shower. Takahashi (1983) used a radiosonde system to electric charges they carry. An infrared light is mounted make extensive measurements of charges on particles above the camera with its beam parallel to the camera's in Ponape, Micronesia, by combining induction rings line of sight. Interruption of the beam by any particle and particle identi®cation devices. The results indicated larger than 0.5 mm in diameter triggers the ¯ash lamp the existence of a tripole structure when the top of the mounted just above the camera's lens. The volume seen clouds were above the freezing level and graupel were is 20 3 15 3 29 mm3 with a maximum ¯ash rate of present. However, the results reported above were ob- two per second. In calculating the true sampling volume, tained in the absence of clearly identi®ed precipitation the dead time during which the batteries are recharging particles and a new videosonde system has been de- is subtracted (Takahashi et al. 1995). The sonde has an veloped to correct this de®ciency. additional lamp with a lower priority that also ¯ashes The primary purpose of what follows is to test the twice a second. This lamp aids in the determination of results from a model (Takahashi 1984) using observa- instances of malfunction in the triggered ¯ash as well tions obtained with the video system, namely, 1) for- as taking pictures of particles smaller than those that mation of a tripole structure by graupel and ice crystals, trigger the other lamp. The induction ring, 70 mm in 2) graupel sign reversal at about 2108C, and 3) the diameter and 10 mm high, is mounted at the top of the ability of charges on graupel and ice crystals to form a sonde. Particles enter through a cone above the sonde space charge suf®cient to initiate lightning. The number and the signals are logarithmically ampli®ed. Charges concentrations of these particles in the highly charged are detected in the range of 0.1±200 pC. To minimize Unauthenticated | Downloaded 09/29/21 06:28 AM UTC 1JUNE 1999 TAKAHASHI ET AL. 1563 FIG. 2. (a) Videosonde system. (b) Data display on television screen. Upper LEDs show pressure, temperature, and humidity; lower ones give sign and magnitude of electric charge. (c) Cross section of sampling area. splashing, the outer surface of the cone is covered with Very low concentrations of highly charged precipitation sponge. Corona from the radiosonde may modify the particles from nonthunderstorm clouds (Fig. 14b) may precipitation particle charge; however, this will not be exclude the measurement of bounced particle charges. a serious problem since the radiosonde ascent rate is To protect it from ambient electrical disturbances, the typically 5 m s21. Ions ejected from the radiosonde will entire sonde is encased in aluminum foil. be left behind when the electric ®eld is less than 50 kV Two sets of light-emitting diodes (LEDs) are posi- m21. In the thunderstorm, particle charge of a few hun- tioned on the bottom of the sonde and within the dred picocoulombs is often observed.
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