Hydrometeor mass, number, and space charge distribution in a Hector squall line

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 109,, doi: /2004jd004667, 2004 Hydrometeor mass, number, and space charge distribution in a Hector squall line Tsutomu Takahashi Core-Education Center, Obirin University, Machida, Tokyo, Japan Thomas D. Keenan Bureau of Meteorology Research Centre, Melbourne, Victoria, Australia Received 20 February 2004; revised 10 May 2004; accepted 24 May 2004; published 24 August [1] Videosonde data from an Australian squall line were combined with a radar echo profile to construct a conceptual model of hydrometeor mass, number, and space charge evolution. Two-step hydrometeor growth modes were suggested: low-level, warm rainfrozen drop and high-level, graupel-ice crystals. A riming electrification mechanism successfully explained the charge distribution. A warm rain-frozen drop process was active in the forward area of the convective region, and frozen drop growth was highly active near the 0 C level. Ice crystals, which electrified frozen drops positively, increased in number as they were lifted. Graupel growth was enhanced, and additional charge separation occurred between ice crystals and graupel in the upper level. Large negative graupel fell at the rear of the convective region, while positive ice crystals and small negative graupel were transported into the anvil. The reason for high lightning frequencies in a squall line over a maritime continent is also discussed. INDEX TERMS: 3304 Meteorology and Atmospheric Dynamics: Atmospheric electricity; 3314 Meteorology and Atmospheric Dynamics: Convective processes; 3354 Meteorology and Atmospheric Dynamics: Precipitation (1854); 3360 Meteorology and Atmospheric Dynamics: Remote sensing; KEYWORDS: squall line electricity, squall line rain, lightning Citation: Takahashi, T., and T. D. Keenan (2004), Hydrometeor mass, number, and space charge distribution in a Hector squall line, J. Geophys. Res., 109,, doi: /2004jd Introduction [2] Knowledge about cloud particle hydrometeors is a prerequisite for the study of precipitation processes and charge separation mechanisms in thunderstorms. Estimation of the release of latent heat during cloud processes is important in large-scale modeling and also requires information about precipitation particles. In situ hydrometeor measurements by airplane flights into tropical convective clouds have limitation given they are endangered by possible excessive turbulence and heavy riming. Hence remote sensing techniques are normally used instead of in situ measurements to infer microphysical structure. Leary and Houze [1979] and Houze and Hobbs [1982] have assumed hydrometeor distributions in relation to echo intensity profiles in squall lines. More sophisticated determination of hydrometeor species has been reported using multiparameter radars [Zrnic and Ryzhkov, 1999]; however, classification of hydrometeor species by partitioning polarimetric variables is still developing. [3] May et al. [2002] reported hail in tropical clouds that developed near Darwin, Australia, using both wind profilers and polarimetric radar to compare vertical winds and Copyright 2004 by the American Geophysical Union /04/2004JD hydrometeor fall speeds. This method, however, makes it difficult to discriminate small hail from raindrop-hail mixtures and emphasizes the need for direct observation of hydrometeors in clouds. [4] During the transition period between the dry and wet seasons over northern Australia, deep convection, called Hector, locally develops over the Tiwi Islands. The island thunderstorms develop in a regime with low-shear and moderate convective available potential energy (CAPE) with high moisture, as well as an unstable upper troposphere and a high tropopause [Keenan et al., 1990; Simpson et al., 1993]. Initial storm forcing is strongly influenced by local sea breezes and the growth of a mesoscale system is governed by interaction between convective scale downdrafts. Echo tops reach 20 km in height, and the system shows squall line character, having both convective and stratified cloud areas [Keenan et al., 2000]. [5] During November and December 1995 an international project, the Maritime Continent Thunderstorm Experiment (MCTEX), was conducted over Tiwi Island at 12 S and 130 E [see Keenan et al., 2000]. A Japanese group was involved in MCTEX using 14 videosondes to obtain in situ samples of cloud hydrometeors and their related charge characteristics [Takahashi, 1990]. The observations were accompanied by polarization radar observations of the clouds by the Bureau of Meteorology Research Centre [Keenan et al., 2000]. Of the 14 sondes launched, 7 suc- 1of11

2 Figure 1. The videosonde system used to collect in situ observations of cloud particle type and associated charge [Takahashi, 1990]. cessfully entered a Hector squall line that was producing major rainfall. 2. Videosonde [6] The videosonde (Figure 1) has been designed to measure both the shape and the charge of hydrometeors in clouds [Takahashi, 1990]. The significant features of the sonde are a videocamera that records particle images and an induction ring recording particle electric charges. Particles large enough to trigger the flash are photographed at a 10 magnification. A 1680 MHz carrier wave (bandwidth 4 MHz, transmission power 0.5 W) is modulated according to brightness of image from 10 Hz to 1 MHz. At the ground station an antenna with an eight-element array automatically tracks the signals. At about 15 km in height the balloon bursts and the sonde falls slowly on a parachute. Only data acquired during ascent is used for the analysis. The images and the charge signals are transmitted to a ground station where each particle is classified as a raindrop, a frozen drop, graupel including hail, an ice crystal or a snowflake. [7] Raindrops, frozen drops, graupel, and ice crystals are distinguished by differences in transparency and shape. Frozen drops show opaque, spherical shapes and raindrops show deformed shapes with unique light reflection patterns on their surfaces. Graupel are irregular, white shapes, keeping image intensities for multiple frames, in contrast to ice crystals in single frames. Since only particles 0.5 mm or larger, that trigger the flash lamp, are recorded, unless they are mixed with larger particles, ice crystal concentrations are underestimated by as much as one order of magnitude as compared with Braham s [1990] reported aircraft data. Ice crystal size-number distributions, derived from videosonde data, overlapped Braham s aircraft data for particles larger than 0.5 mm diameter, and from this the total ice crystal number was estimated. [8] Electric charges are detected in a range from 0.1 to 200 pc. To increase charge information, those cases where more than one particle of the same crystal shape appeared in one frame were also saved. They have been plotted in the diagram by partitioning charge in proportion to radius among particles. The charge-size measurements at Ponape, Micronesia, reported by Takahashi [1978b], seem to support this concept. [9] Space charge is the sum of the charge in frames with one particle and frames with multiple particles of the same shape. For each 500 m height segment, if particles of different shapes are included in a frame, they are assumed to carry the same average charge measured on particles in an unmixed frame, irrespective of size. The net space charge obtained by using actually observed charge in a mixed particle frame was quite similar to the net space charge calculated by summing the average charge on particles in mixed frames [Takahashi et al., 1999]. Particles in unreadable charge frames, around 20% in videosonde 5, are also assumed to carry the average charge. Precipitation particles less than 0.5 mm diameter, which were not considered here, will contribute to the total space charge by 36% at 10 mm/h rainfall intensity, and by 19% at 100 mm/h assuming a Marshall-Palmer size distribution and the charge distribution is proportion to size. [10] If precipitation particles are uniformly distributed and fall continuously, concentration will be overestimated, especially for raindrops, because large particles continue to fall from above as the balloon ascends. If raindrops follow the Marshall-Palmer distribution and a rainfall intensity of 30 mm/h is assumed with charge distributed with radius, overestimations for the number, mass, and charge will be approximately 50, 120, and 80%. Since rain distribution in 2of11

3 Figure 2. MCTEX observation site. Videosondes were launched at Maxwell Creek. The C-POL radar operated at Nguiu. space varies greatly, adjustment by fall speed has not been made. The balloon ascent rate was typically 300 m/min. The electric potential gradient has been measured by a field mill at the balloon launching site. 3. Observations [11] The C-band polarimetric radar was located at Nguiu on Tiwi Island, 25 km south and 5 km east of the videosonde launch site at Maxwell Creek. The locations are shown in Figure 2. As Keenan et al. [2000] have documented, in late morning a sea breeze forms over the islands, forcing small convective clouds to develop. At about noon, some of the clouds develop and merge and heavy precipitation begins. The mesoscale cloud organization evolved from north to south, showing squall line characteristics, moving slowly west with frequent lightning and continued heavy rain. [12] Fourteen sondes were released using radar information, seven of which entered the so-called Hector squall line (Table 1). The videosonde radio receiving set tracked the sondes, enabling the trajectories to be superimposed on the observed radar echo structure. Some examples launched in convective, transition, and stratified regions are given below Videosonde 13, Launched Into a Front-Convective Region [13] A squall line was approaching from the south when, just above the launch site, a low-level, black pedestal cloud suddenly appeared and large raindrops started to fall. The videosonde was launched at 1431 and entered cloud at 1435 (17 November). As is shown in Figure 3a, the videosonde ascended forward in the squall line. Following modification for the storm s movement, the videosonde trajectory was traced on the radar echo image closest to the ascent time. At the beginning of the storm, the electric potential gradient (EPG) was increasingly negative, as opposed to the positive fair weather field, however, the sonde was launched just as the EPG shifted to positive. In the cloud s lower levels the videosonde data showed raindrops as large as 8 mm diameter and many large, frozen drops at the 0 C level (Figure 4). Above the 15 C level in the cloud, there were many graupel and an increasing number of ice crystals. Raindrops in the lower levels were positively charged at a few tenths of a pc. Frozen drops were charged both positively and negatively. The videosonde signal ceased at 10.6 km, probably because of a lightning strike Videosonde 3, Launched Into a Transition Zone [14] Soon after sonde 13 had been launched, near the site there was heavy rain and frequent lightning, the strikes sounding a series of loud claps. The EPG again shifted to negative, remaining so for about two hours. Videosonde 3 was launched in weak rainfall with radar indicating ascent near the transition zone (1515, 17 November, Figure 3b). The videosonde data showed raindrops smaller than 3 mm diameter with abundant graupel and ice crystals above the freezing level and a tendency for greater graupel growth close to that level. Both positive and negative charges were noted with ice crystals more positively charged at higher levels and, at lower levels, highly negative raindrops. The most interesting results were the abrupt increase in the magnitude of the negative charge on graupel and ice crystals below 30 C level and on the positively charged graupel below 15 C level. The charge on ice crystals in the upper level was a few pcs while, in the lower level, the charge on raindrops increased to as high as 100 pc Videosonde 12, Launched Into a Thick Anvil Region [15] The convective region of the squall line moved away to the east and the launch site was completely covered by thick anvil cloud. When the EPG shifted again from highly negative to positive, the sonde was released (1640, 3 December), the radar data showed its ascent into the anvil. The videosonde data showed both small ice crystals and graupel with the crystals positively charged at about 1 pc (Figure 3c). 4. Mass, Number, and Charge in Space [16] On different days, Hector squall lines have common features: echo tops of km, convective region widths of km and an anvil thickness of 8 10 km. Since the radar echo structures were very similar on different days, the videosonde data could be combined into an idealized squall line echo. This was accomplished by using the echo from the convective region (1540 on 17 November) with the anvil region echo at the transition zone. (1617 on 27 November). [17] The resulting echo profile was characterized by an intense, vertically extended echo forward in the storm, at 17 km, a strong, gradually decreasing echo toward the transition zone at 40 km, and a thick, layered echo stretching horizontally east that was 70 km long, and 7 8 km deep. An occasional convective cloud developed under the Table 1. Hector Cloud Videosonde Observations Videosonde Date Data Collection Nov Nov Nov Nov Nov Dec Dec of11

4 Figure 3a. Radar echo over Tiwi Island (1430, 17 November) and videosonde (number 13) trajectory (dotted line), launched at (top left) Precipitation particles. Open circles, raindrops; solid circles, frozen drops; triangles, graupel; crosses, ice crystals. (top right) Electric charge. Echo intensities are given in the right corner. anvil and individual cumuli grew without developing a squall line. By using the edge of the forward echo as a reference for the flights in the convective region and the transition zone for the anvil flights, all of the flight trajectories could be projected on the idealized echo pattern. [18] In summary, three videosondes, 5, 13, and 6, ascended into the convective region, and videosonde 3 ascended near the transition zone. Number 12 went through the middle of the anvil, and number 2 went into the anvil after passing through a low-level cumulonimbus, while number 11 traveled through an individual cumulonimbus cell. Diagrams of mass, number, and space charges were first constructed (Figures 5a, 5b, and 5c), and values were then plotted along the trajectories. Smoothed contours were drawn for each hydrometeor type Mass Density [19] The mass density profile was characterized by high water content with a maximum of 3 g m 3 for raindrops in the lower levels of the cloud and for the frozen drops spread horizontally around the melting level in the regions of the convective cores (Figure 6a). Above this level, graupel increased in mass density with height. The maximum density for ice crystals occurred at 40 C level. Large graupel fell; smaller ones and ice crystals were transported into the anvil. In the anvil where low-level convective cells developed, the graupel mass density increased slightly. Even in the isolated cumulonimbi, the hydrometeor mass density profile was similar to that in the squall line convective region Number Density [20] The number density profile was similar to that of the mass density, but in the upper level of the convective region, ice crystals increased in number with height much more rapidly than graupel (Figure 6b). There was a split in the graupel number density near the transition zone which was probably a result of size sorting Space Charge [21] Positively charged raindrops predominated in a narrow area forward in the cloud, surrounded by broadly distributed, negatively charged drops (Figure 6c). Frozen 4of11

5 Figure 3b. Radar echo (1520, 17 November), videosonde (number 3), launched at 1515 into transition region. Trajectory in cloud is shown by the dotted line. (top left) Precipitation particles and (top right) particle electric charge. drops were of both signs but tended to be more negatively charged near the melting level. Above about the 10 C level, both ice crystals and graupel were primarily negative. Ice crystals, from the upper transitional area to the eastern edge of the anvil, carried positive charge although there was a narrow layer of negatively charged graupel. 5. Discussion [22] Knowledge of the distribution of mass, number, and space charge in a Hector squall line aids in the determination of hydrometeor growth modes. The wide distribution of frozen drops near the freezing level and a similar magnitude of mass in raindrops indicates an active, warm rain-frozen drop growth process in the low-level, convective region. The warm rain process must work effectively in forming raindrops before the parcel reaches the freezing level. At this level the raindrops freeze and, since they do not break up when frozen, may continue to grow by collecting the surrounding supercooled drops. [23] The distribution of charges may be explained by riming electrification [Takahashi, 1978a, 1984; Williams, 1988]. The results of a recent investigation of winter clouds over Hokuriku, Japan, reported by Takahashi et al. [1999] further supports the importance of riming electrification as the major charging process in thunderstorms. In riming electrification in a cloud, a high electric charge is generated during collision between graupel-frozen drops and ice crystals with the charge sign, changing at around 10 C. [24] Since most frozen drops are formed below 10 C level in clouds, collisions with ice crystals further enhances their positive charge and they melt as they fall, forming positively charged raindrops. Above 10 C level, small frozen drops will be negatively charged. The data show that frozen drops tend to distribute positive charge below this level and negative charge above (Figure 5c). Negatively charged ice crystals, however, will rise and some may grow into graupel through riming, forming negatively charged graupel. Where it is colder than 10 C, new ice crystals are also generated, collide with graupel and will be positively charged. Large, negatively charged graupel and negatively charged, small frozen drops fall, forming negative raindrops. Small negative graupel and positively charged ice crystals are transported into the anvil (Figure 7). 5of11

6 Figure 3c. Radar echo (1645, 3 December). Videosonde (number 12) trajectory, launched at 1640 into the anvil. (top left) Precipitation particles and (top right) particle electric charge. [25] A two-step growth process is suggested: warm rainfrozen drop growth in the lower cloud levels and graupel growth in the upper ones. Frozen drop growth has been reported in the western Pacific by Takahashi [1990], Takahashi et al. [1995a]. May et al. [2002], using wind profilers and polarimetric radar, also reported rapid, solid particle growth near the freezing level in tropical convective clouds near Darwin, Australia. The warm-rain-frozen drop process is probably common in tropical convective clouds. [26] From results from the MCTEX project, Carey and Rutledge [2000] reported large accumulations of water near the cloud freezing level. CPOL data analysis suggested that most of it may have been from graupel but the hydrometric data indicates that it was from misclassified, frozen drops. In addition, polarimetric radar data does not distinguish small graupel hidden in a group of ice crystals. Mixtures of ice crystals and graupel in a thick anvil have been modeled by Leary and Houze [1979]; however, in the Hector anvils there were no reports of snowflake formation. The observed surface electric field profile during the passage of the Hector squall is similar to that observed with COPT 81 squall lines which were four times more intense than the Hector [Chauzy et al., 1985]. [27] Hydrometeors carry the major space charges. These charges are of similar magnitude as those estimated by electric field sounding in thunderstorm [Winn et al., 1981]. Figure 4. (left) Videosonde (number 5) image of frozen drop at 4.6 km height and 2.3 C, launched to the front of the squall line. (right) Videosonde (number 2) images of ice particles at 7.5 km and 20 C, launched into anvil cloud. 6of11

7 Figure 5a. Mass density averaged for each kilometer. Videosonde data are arranged from the front to the rear of the squall line. Raindrops, large dots; frozen drops, dark shaded; graupel, small dots; and ice crystal, crosses. Figure 5b. Number density profiles as in Figure 5a. 7of11

8 Figure 5c. calculated. Charge density profiles as in Figure 5a. Positive and negative space charges are separately After comparison of space charge from hydrometeors with space charges estimated by electric fields in clouds, Marsh and Marshall [1993] reported that both low-level positive and midlevel negative space charges reside primarily on hydrometeors. The present observations, with more sensitive charge measurements, not only support their report but also suggest that even high-level positive space charges may also reside on hydrometeors. The data presented herein make it possible to identify the hydrometeors carrying the major charges. They are low-level, positive raindrops, negative graupel and ice crystals in the main convective area, and positive ice crystals in the higher levels. [28] The space charge distribution determined from the videosonde data is basically the same as the classical tripole structure from Simpson and Scrase [1937], but the high level, positive space charge is extended from the convective region to the anvil. The existence of strong westerlies at the anvil heights may explain this modification. The vast pool Figure 6a. Composite, hydrometeor mass density (g/m 3 ). Superimposition is the composite radar reflectivity (dbz) of an ideal squall line. Videosonde trajectories are given by light green dotted lines. Purple lines are for raindrops, black lines are for frozen drops, blue lines are for graupel, and red lines are for ice crystals. 8of11

9 Figure 6b. Composite, hydrometeor number density as in Figure 6a. of ice crystal carried, positive, space charge may also be advantageous in sprite initiation [Lyons et al., 2003]. [29] The magnitude of the charge on raindrops is similar to that reported during thunderstorm by Takahashi [1973], Weinheimer et al. [1991], and Marsh and Marshall [1993] and anticipated from riming electrification [Takahashi, 1984]. Although the measured particle space charge distribution in anvil cloud is much simpler than those measured by the electric field vertical profile in stratified cloud [Marshall and Rust, 1993], negative graupel space charge may overcome the positive ice crystal charge locally, leading to a complicated space charge distribution if it is measured by electric field. [30] One mystery is the extremely high numbers of ice crystals, as many as 10 cm 3 at some levels (Figure 4). It is suggested that these concentrations may be related to a strong updraft and many small nuclei. According to Keenan et al. [2000] and May et al. [2002], the updraft speed in sea breeze convective clouds over the maritime continent is from 10 to 20 m/s, twice as high as measured over the ocean by Jorgenson and LeMone [1989]. Podzimek [1980] and Miura et al. [1993] reported a higher number of small nuclei over the maritime continent. [31] The scenario for high ice particle production is as follows. As a parcel of air is lifted, large nuclei are activated, producing large drops. The low number concentration aids Figure 6c. Composite, hydrometeor space charge. Positive (solid lines) and negative (dashed lines). 9of11

10 [35] The basic concept of growth modes and particle charge evolution obtained in a Hector squall line may be applied to other squall lines, although different microphysics in different areas may modify the distribution. It is highly desirable to conduct similar measurements in other areas. [36] Acknowledgments. This research was conducted as a part of the MCTEX project. The authors would like to thank D. Jasper for his tremendous help during this expedition. Many students, K. Suzuki, T. Kawano, T. Tajiri, M. Sugiyama, and Y. Kushiyama, from Kyusyu University participated. Research was supported by the Japan Aerospace Exploration Agency, the Ministry of Education, Science, Sports, and Culture of Japan, and the Climate Center of Tokyo University. Figure 7. Conceptual model of hydrometeor growth and space charge. Red circles are for raindrops originating on frozen drops. Blue circles are for raindrops, originating on graupel. Green circles are frozen drops, blue triangles are graupel, and red crosses are ice crystals. A two-step growth process is suggested by warm rain-frozen in front and graupel growth in the upper level. in enhancing the warm rain process [Takahashi, 1976; Takahashi and Lee, 1978]. Above the freezing level a strong updraft creates high supersaturation and small nuclei are activated. Some of them will act as new ice forming nuclei due to high supersaturation [Hussain and Saunders, 1984]. Small drops collide with small frozen drops and ice crystals carried from a lower level, and graupel are formed. Because the drops are small, low-density graupel grows, with brittle surface branches [Macklin, 1977]. Collision with a large frozen drop or large graupel leads to ejection of ice fragments [Takahashi et al., 1995b]. Additional release of latent heat by the nucleation of small nuclei also enhances the updraft. 6. Conclusions [32] During the MCTEX project, 14 videosondes were launched into clouds over Tiwi Island near Darwin, Australia. Seven entered a Hector squall line, returning data related to the distribution of hydrometeor species and their electrical charges. Knowledge of their positions in relation to the storm allowed the data to be combined in the construction of a composite map of the mass, number, and space charge distributions. [33] The composite images thus obtained showed a twostep hydrometeor growth process: one at low levels at the front in warm rain freezing and the other at upper levels within main convection by graupel and ice crystal growth. The space charge evolution has been successfully explained by riming electrification. [34] The space charge distribution exhibited a basic tripole structure, but with a positive space charge extended to the anvil. 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