Role of orography in inducing high lightning flash rate at the foothills of Himalaya
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Surface electric measurements obtained beneath thunderstorms with almost similar characteristics at a station located close to the Himalayan foothills in northeastern India have been analyzed. All these thunderstorms had some similar features - occurred after midnight and lasted for a short duration of less than an hour, and an active stage of these thunderstorms lasted for 10 to 25 min. All these thunderstorms exhibited very high peak flash rates ranging from 40 to 80 flashes per minute during the active stage. A lightning jump of about 65 flashes per minute (fpm) was observed during two occurrences of these thunderstorms. Surprisingly, in spite of very high peak lightning flash rates and lightning jumps, no severe weather phenomena were observed at the ground during these thunderstorms. The formation of such small duration thunderstorms with very high lightning flash rates is attributed to the conversion of moisture in the valley during nighttime.
KeywordsSevere thunderstorms Lightning jump Himalayan foothills
Many field experiments and laboratory studies have shown that the magnitude of updraft of a thundercloud is a major factor that is responsible for large-scale electrification of thunderclouds and hence the lightning flash rate and updraft velocity inside a thundercloud are closely related (Williams 1985; Williams et al. 2005). Vonnegut (1963) has proposed that the lightning flash rate is proportional to the fifth power of the thunderstorm’s height which is generally known as ‘fifth power relationship’. Williams (1985) has observed that the fifth power relationship holds fairly good over the continent. Price and Rind (1992) have shown that due to large difference in updraft speeds over land and ocean, this relationship does not hold true for ocean thunderstorms. However, more recently, Yoshida et al. (2009) have shown that the number of lightning flashes per second per convective cloud is approximately proportional to the fifth power of the cold-cloud depth (the height from the melting level to the storm height) and this relationship is not regional dependent. All these observations clearly indicate that the updraft velocity inside thunderclouds play important role in determining the lightning flash rate during thunderstorms.
The role of aerosols on microphysical and dynamical characteristics of cloud has been studied for a long time (Rosenfeld 1999, Rosenfeld and Woodley 2003, Khain et al. 2008, Rosenfeld et al. 2008). Many observations suggest that the aerosols influence the charge generation processes inside thunderclouds (Orville et al. 2001, Steiger et al. 2002, and Steiger and Orville 2003). Williams et al. (2002) have suggested that the observed contrast between lightning over continent and ocean may be due to differences in aerosol concentrations over land and ocean. They have suggested that the increase in aerosol concentration could reduce mean droplet size and thereby suppress warm rain coalescence and enhance the cloud water reaching the mixed phase region, which can increase the lightning flash rate. Moreover, it has been recognized that ice content inside thunderclouds plays an important role in lightning activity (Dye et al. 2000, Blyth et al. 2001, Lal and Pawar 2009). Therefore, an increase in ice nuclei also can enhance the lightning flash rate.
Another factor, which affects the lightning flash rate, is the orography of the region. Many studies have shown that the local orography can generate intense vertical velocity and affect a change in lightning flash rate by interacting with prevailing wind and/or large-scale processes (Zajac and Rutledge 2001; Bourscheidt et al. 2009; Pawar et al. 2010; Bourscheidt et al. 2009) in their study over South Brazil have shown that the terrain slope has more influence than altitude on thunderstorm occurrence and lightning activity. Pawar et al. (2010) have found a very high peak-lighting flash rate in spite of very low convective available potential energy (CAPE) over the northeast part of India. They attributed this high lightning flash rate to the short duration of an active phase of a thunderstorm.
Many parts of India experience thunderstorms during the pre-monsoon season (April to May) and such thunderstorms are generally severe in the northeastern part of India. Reasons for the formation of severe thunderstorms in this part are discussed in detail by Koteshwaram and Srinivasan (1958), Raman and Raghavan (1961), and Krishna Rao (1966). In this paper, we report observations made during three short-duration thunderstorms occurred in this region with very high peak flash rates (about 80 flashes per minute). Possible causes of such high lightning flash rates in these thunderstorms are discussed.
A vertical field mill, which can measure field up to ±12 kV/m, is used for measurement of atmospheric electric field observations. Details of the same are given in Kamra and Pawar (2007). Sensor plates of the field mill are kept flushed to the ground. Details of sensitivity and the procedure followed for observations are given in Pawar et al. (2010). We have followed the convention that the fair-weather electric field would result in negative polarity field change. The field mill has a range of around 25 km, and field changes occurring as far as 25 to 30 km can leave a signature in our field records.
We have included, for discussion here, three thunderstorms with similar characteristics. Pawar et al. (2010) have reported the electrical characteristics of one such severe thunderstorm (which is included for analysis here). All these thunderstorms developed in late night hours and lasted for a short duration, and active phases of these thunderstorms lasted for less than 30 min. None of these thunderstorms produced severe weather - such as heavy winds, hails, heavy precipitation - at the ground. Upper air radiosonde observations made at 1730 hours around 30 km from the observational site on all these days showed that the CAPE was around 2,000 J kg−1 and level of free convection was at 800 m which are conducive for development of a thunderstorm. Although no radar observation is available, we could infer to some extent from observed electric field whether the storm remained stationary or not. Our observations indicate that the electric field induced by lightning remained similar up to the last observed lightning which suggests that these thunderstorms remained stationary overhead for most of their lifetime. By small thunderstorms, we mean storms of small duration.
Lightning flash rate
Figure 2 shows the lightning flash rate estimated from electric field record during three thunderstorms. As shown in Figure 2, these thunderstorms are divided into three stages - initial, active, and dissipating stage based on lightning activity. In the first thunderstorm, the initial stage lasted for about 20 min and lightning flash rate remained between 10 and 20 fpm. The active stage started at about 0012 hours and lasted only for 7 min up to 0019 hours. The lightning flash rate increased sharply to about 84 fpm within 1 to 2 min and remained between 75 and 85 during active stage. In the active stage, flash rate increased at the rate of about 65 fpm. As described by Williams et al. (1999), such a sudden jump in lightning flash rate is most of the time associated with increase in updraft and severe weather at the ground. In the dissipation stage, the flash rate decreased rather slowly and reached near to zero at about 0043 hours. Thunderstorm on 21 April 2007 also showed a high flash rate during the active phase. In the initial phase, flash rate was between 10 and 20 fpm, which increased to 20 to 25 fpm in the active phase. The active stage lasted for about 17 min in this thunderstorm. In this thunderstorm also, decrease in lightning flash rate during dissipation stage is slow. Thunderstorm developed on 23 April showed a low flash rate of about 10 to 15 fpm in the initial phase. This thunderstorm also showed a lightning jump in the active stage similar to that observed on the thunderstorm of 20 April 2007. The lightning jump in this case is about 60 fpm. The peak flash rate observed in this thunderstorm was 90 fpm. In this thunderstorm also, the active stage lasted for only 10 min. The flash rate started decreasing slowly in the dissipation stage.
Electric field changes induced by lightning and recovery curves
Observations of these three thunderstorms over the northeastern part of India demonstrate that small thunderstorms can have very high lightning flash rates of about 40 to 90 fpm. A sharp increase in lightning flash rates, known as ‘lightning jumps’, of about 30 to 75 fpm is also observed in the beginning of the active stages of these thunderstorms. Earlier observations by Williams et al. (1991), McCaul et al. (2002), Wiens et al. (2005), and Schultz et al. (2009) show that very high flash rates (more than 60 fpm) and lightning jumps are always found to be associated with severe and big convective systems. However, our observations demonstrate that high lightning flash rates and lightning jumps can also be associated with small single cell non-severe thunderstorms. It should be noted here that the surface winds were low or calm during the whole thunderstorm period. Thunderstorms were overhead during their active stage. Though handicapped by absence of radar data, a pattern of electric field changes (Figures 3 and 4) and steady decrease of lightning flash rate during dissipation stage (Figure 2) suggests that thunderstorms have not moved away from the observational site during their lifetime. Electric field changes induced by lightning and recovery curves of electric field between two lightning flashes suggest that all the three thunderstorms were having a normal positive dipole-type charge structure. However, all the electric field changes induced by lightning starting with positive electric field change indicate that all the lightning discharges destroy some positive charge initially. This may be an indication of presence of widespread positive charge in the lower portion of cloud. As stated earlier, there are no radar observations and only this can give the exact distance of thunderstorms from the observational site. To overcome this, we have employed time-to-thunder technique to calculate horizontal distance of thunderstorms from the observatory. Despite the fact that this method may not be very accurate, this technique gives approximate distance and movement of thunderclouds. Further, storms that were either overhead or very close to observatory have only been considered for analysis. With this, we can study the lightning activity or charging processes inside thunderclouds of stationary thunderstorms with fairly good approximation by having surface measurements of electric field at only one point (Pawar and Kamra 2004; Pawar and Kamra 2009).
Electrical characteristics of thunderstorms occurring over this region are sparsely studied. Recently, RameshKumar and Kamra (2012) have studied the spatiotemporal variations of lightning over the foothills of the Himalayas and their relationship with CAPE and surface temperature. They attributed the observed higher flash rate to diurnal cycle of mountain breeze. However, they have not examined the electrical nature of these lightning flashes. To the authors’ knowledge, it is for the first time that observations demonstrate even small thunderstorms can exhibit very high peak lightning flash rates and lightning jumps. As shown in Figure 1, the observational site is situated in the Brahmaputra valley with 4- to 5-km-high mountains about towards the north and south. Barros et al. (2004) have shown that high mountains of Himalaya interact with prevailing winds in monsoon season and can produce deep convections over that region at late night hours. They have also suggested that the radiative cooling at high mountaintops can generate moisture conversions at the foothills and they may be responsible for deep midnight convections over that region. Our observations also suggest that moisture conversion at foothills due to radiative cooling at mountaintops during nighttime may be responsible for triggering of such deep convections over Guwahati during the pre-monsoon season.
It is to be noted here that our hypothesis that conversion of moisture in the valley during nighttime leads to thunderstorms with high flash rates and lesser number of active phases needs to taken with caution as this is based on a limited number of observations. As stated above, observations of electrical field are not conducted regularly at this place and this observation was made during a campaign. The authors are aware that more such observations are required to strengthen our argument. Nevertheless, one needs to take into consideration that 3-year averaged data of lightning obtained from WWLLN lends support to our hypothesis.
Our observations of electric field made beneath thunderstorms at Guwahati, known for occurrence of severe thunderstorms during pre-monsoon season - locally known as ‘Nor’wester’, show that high lightning flash rates and lightning jumps can happen in small thunderstorms without causing any severe weather phenomenon at ground. This study also brings out the fact that many severe thunderstorms occur over this region during late evening/night hours, which can be attributed to moisture conversions at the foothills due to radiative cooling.
This experiment is conducted as a part of the Severe Thunderstorm-Observation and Regional Modeling (STORM) program of Department of Science and Technology, Government of India. Data from World Wide Lightning Location Network (http://wwlln.net), a collaboration among over 40 universities and institutions, for providing the lightning location data used in preparation of Figure 5.
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