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Effects of environmental conditions on inducing charge structures of thunderstorms over Eastern India
© Pawar et al.; licensee Springer. 2014
- Received: 5 August 2013
- Accepted: 6 March 2014
- Published: 16 June 2014
It is well known that environmental conditions like convective instability, aerosol loading, and availability of moisture content affect the polarity of charge structures of thunderstorms. The electrical characteristics of thunderstorms observed during the pre-monsoon season of year 2009, over Eastern India were studied to identify the effects of different environmental conditions on charge structures of thunderstorms occurring over this region. Electric field and Maxwell current data suggest that at least one of these thunderstorms had an inverted charge structure. Doppler RADAR, radiosonde, and Moderate Resolution Imaging Spectroradiometer (MODIS) Aerosol Optical Depth (AOD) data have been used to compare the microphysical and dynamical characteristics of these thunderstorms. The thermo dynamical structure observed by radiosonde during the day on which an inverted polarity thunderstorm was observed showed very high CAPE in the mixed-phase region compared to other thunderstorm days. Furthermore, the AOD peaked 1 day before this thunderstorm. The back trajectories of winds also suggest that the aerosols might have been transported from a desert region on that day. It has been proposed that the large ice nuclei concentration can produce dominant positive charge in the lower portion of the mixed-phase region by maintaining ice saturation.
- Aerosol Optical Depth
- Lightning Discharge
- Doppler Radar
- Charge Structure
- Severe Thunderstorm
It has long since been known that some of the more severe thunderstorms can have different charge structures than generally observed in ordinary thunderstorms. From the observations of electric field changes produced by lightning during a severe thunderstorm, Vonnegut and Moore (1958) suggested that severe thunderstorms could have an inverted charge structure. In three storms that occurred during the Severe Thunderstorm Electrification and Precipitation Study (STEPS), Rust et al. (2005) have observed the positive and negative charge regions at altitudes where negative and positive charge would normally be found in ordinary thunderstorms. Lang et al. (2004) and Wiens et al. (2005) reported storms with consistent dominant upper level inverted dipole charge structure near the updraft (i.e., the upper negative charge region and the main positive charge region below it). There are many other reports of inverted polarity charge structure in severe thunderstorms which produce positive Cloud-to-Ground (CG) flashes comparable to negative CG flashes (e.g., MacGorman and Burgess 1994; Stolzenburg 1994; Carey and Rutledge 1998; Lang and Rutledge 2002; Carey et al. 2003). Over a region from the Kansas/Colorado border to Minnesota, Carey and Rutledge (2003) found that the properties of CG lightning flashes produced from severe storms during warm seasons (i.e., large hail or tornado producing) were different from those produced by non-severe storms. They found that the percentage of CG lightning flashes lowering positive charge to ground was up to three times higher in severe storms. The median positive peak current in severe storms was larger by about 25%. Furthermore, the median negative peak current in severe storms was very low (i.e., as low as 12 to 16 kA) and was noticeably less than in non-severe storms (i.e., by at least 10%).
Although earlier reports suggest most thunderstorms with inverted polarity charge structures were characterized by a high degree of severity, Qie et al. (2005) and Pawar and Kamra (2004 and 2009) have also reported some non-severe thunderstorms over the Tibetan Plateau and India, with a wide spread and strong positive charge in the lower portion of cloud. Majority of lightning activity arises from lower negative dipole of these thunderclouds. These thunderstorms can also be termed inverted polarity thunderstorms because for most of their lifetimes, lower negative dipole dominated the lightning activity of these storms.
Williams et al. (2005) hypothesized that the inverted polarity charge structure in thunderstorms is the result of superlative liquid water content in the mixed-phase region. Many observational studies support this idea (Krehbiel et al. 2000; Rust and MacGorman 2002; Lang et al. 2004). However, as emphasized by Williams et al. (2005), a strong updraft, an extraordinarily dry environment, or a high concentration of aerosols are necessary but not sufficient conditions for formation of inverted polarity thunderstorms. Many observations clearly show that thunderstorms fulfilling all the conditions above do not always form inverted polarity thunderstorms (MacGorman and Burgess 1994). Furthermore, the observations by Pawar and Kamra (2004, 2009) and Qie et al. (2005) show that some ordinary thunderstorms without strong updrafts can also have inverted polarity charge structures. All these observations clearly suggest that formation of inverted polarity is not fully understood.
In this paper, we report our observations of surface electric field and Maxwell current density made at Kharagpur (22.31°N, 87.31°E) in Eastern India, beneath three thunderstorms that occurred during the pre-monsoon season of 2009. Electric field and Maxwell current data suggest that at least one of these thunderstorms had an inverted polarity charge structure. Doppler RADAR, radiosonde, and Moderate Resolution Imaging Spectroradiometer (MODIS) Aerosol Optical Depth (AOD) data have been used to compare the microphysical and dynamical characteristics of these three thunderstorms. Possible causes that may be responsible for formation of an inverted dipole charge structure in one of these thunderstorms are also discussed.
As described in detail by Pawar and Kamra (2007), an electric field mill flushed with the ground level and a plate antenna are used for electric field and Maxwell current measurements, respectively. Measurements of both these parameters are made throughout the day at a sampling rate of 10 Hz. In our measurements, we consider the fair-weather electric field and the associated conduction current that brings positive charge to the ground, as negative polarity. In addition, we take the positive displacement current to be affected by a positive field change, lowering positive charge to the ground and vice-versa. For our analysis, AOD data is taken from MODIS, which acquires data in 36 high-resolution spectral bands between 0.415 and 14.235 μm. These data are obtained from a level-3 MODIS gridded atmosphere daily global joint product (MOD08_D3). We took the daily averaged AOD at 550 nm from MODIS-Terra Version 5. Those data for which cloud cover was greater than 20% were excluded from analysis while averaging.
Electric field changes induced by lightning discharges
Maxwell current density measurements
Electrical structure of the thunderstorm
Previous studies have revealed that in thunderstorms with non-inverted polarity charge structures (with midlevel negative and upper level positive charge regions), most of the electric field changes produced by lightning are of negative polarity, indicating removal of negative charge from overhead (Jacobson and Krider 1976; Livingston and Krider 1978; Mohanty and Pradeep Kumar 2004; Pawar and Kamra 2004). Our observations of electric field changes produced by lightning, of Maxwell current densities, and of recovery curves of the electric field during the thunderstorms of 6 May 2009 and 12 May 2009, indicate the removal of negative charge from overhead by lightning and buildup of negative charge after lightning. Therefore, it is clear from the electric field and Maxwell current density data for those 2 days that these thunderstorms had non-inverted polarity charge structures.
Possible causes of inverted polarity charge structure
Many laboratory and field experiments have clearly shown that inverted polarity in thunderstorms is the result of superlative liquid water content in the mixed-phase region (Williams et al. 2005). One or more of the following factors can account for superlative liquid water content in the mixed-phase region, i.e., strong instability, strong updraft, greater cloud base height, and high aerosol concentration. In our observation, as shown in Figure 2, CAPE was very high 3,124 J/kg on 11 May 2009. In addition, it should be noted here that the vertical distribution of CAPE was very peculiar on that day. As can be seen in Figure 2, most of the CAPE is concentrated in the 5- to 10-km altitude range. Even though the CAPE was very high (3,054 J/kg) on May 6, it was distributed over a larger region compared to May 11. The high CAPE and its vertical distribution clearly suggest that the updraft could be high in the mixed-phase region on May 11. As shown in Figure 8, the aerosol concentration was also very high on May 11 compared to the other days.
Our observations not only show high concentrations of aerosols but also that the degree of concentration of ice nuclei may be playing an important role in the formation of inverted polarity charge structures in thunderclouds. Our study site located in the IGB region is known for its very high aerosol concentrations owing to its unique topography. During the pre-monsoon or summer seasons, this region receives large amounts of natural dust aerosols, transported from neighboring desert regions (i.e., from the Thar Desert) (Dey et al. 2004; Pandithurai et al. 2008). These dust particles can act as ice nuclei. Our observations on 11 May 2009 are similar to the observations of Pawar and Kamra (2009), during which back trajectories were shown to be indicative of aerosol transport from desert regions on the day when an inverted polarity thunderstorm was observed. Small inverted polarity thunderstorms reported by Gopalakrishnan et al. (2010) in this region and by Pawar and Kamra (2002, 2004, and 2009) at Pune, India, support the idea that large ice nuclei concentrations can play an important role in the alteration of charge structures of thunderstorms with moderate or low instability. Many laboratory experiments (Baker et al. 1987; Williams et al. 1991; Caranti et al. 1991) suggest that dominant positive charge will appear on larger precipitation particles in the lower portion of the mixed-phase region if ice saturation is maintained.
The authors are aware that the aerosol and/or ice nuclei concentrations cannot influence the polarity of thunderstorms by themselves. More observations of such thunderstorms are required to confirm our proposal that increased optical depth leads to changes in ice nuclei, which in turn affect the charging mechanism. Although many severe thunderstorms occur over this area during pre- and post-monsoon seasons, occurrences of thunderstorms with inverted polarity are rare. We have made observations of thunderstorms over this region and over Pune, for three seasons and for about 2 decades, respectively. However, only three to four thunderstorms with inverted polarity have been observed over these regions. Gopalakrishnan et al. (2010) have already reported one such thunderstorm over this region. Occurrences of such inverted polarity thunderstorms over other parts of India are also few. Pawar and Kamra (2002, 2004, and 2009) have reported some of such thunderstorms over Pune. There are no other reports of such thunderstorms from any other part of India. This complicates the task of finding a robust statistical test to corroborate our claim. In the absence of such a test, our proposal is merely speculative. Nevertheless, observations of greater optical depth prior to the occurrence of a severe thunderstorm in the present case do lend support to our speculation.
Electric field and Maxwell current density measurements clearly illustrate that the 11 May 2009 thunderstorm had a strong and dominant positive charge region in the lower portion of cloud. Analysis of back trajectories (Figure 10) and satellite-derived AOD observations (Figure 8) lend support to our argument that the May 11 thundercloud formed in an environment with high ice nuclei concentration, i.e., high compared to other thunderstorms. Therefore, our studies suggest that a large ice nuclei concentration can produce dominant positive charge in the lower portion of the mixed-phase region by maintaining ice saturation.
The authors express their sincere gratitude to Dr. M Mandal of IIT, Kharagpur for providing the logistics necessary to conduct fieldwork, and are also grateful to the Department of Science and Technology, Government of India, for funding this study under the Severe Thunderstorm-Observation and Regional Modeling (STORM) program. A part of this work was carried out under the DST-RFBR Indo-Russian program (DST/INT/RFBR/P-158). The authors thank Dr. D. Pradhan of IMD, Kolkata for providing RADAR data, and are also grateful to the NOAA Air Research Laboratory (ARL) for providing the HYSPLIT transport and dispersion model and the READY website (http://www.arl.noaa.gov/ready.html). Finally, the authors thank the Department of Atmospheric Science, University of Wyoming, for allowing access to their data.
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