Comparative study of lightning climatology and the role of meteorological parameters over the Himalayan region

Highlights

• We investigated the lightning activities over Himalayas on decadal scale.

• Satellite based observations were used as the primary source of data.

• Of all the parameters, the relative humidity is found to play the major role on lightning activities over the Himalayas.

Abstract

Lightning activities are distributed asymmetrically over the globe. Satellite images show that the Himalayan region is one of the prone zones of lightning activity. We do not understand such an uneven distribution of lightning activities as of today. To elaborate on the present-day understanding of lightning flashes over the Himalayan region, we have analyzed various atmospheric factors in association with Lightning Flash Density (LFD). For this purpose, we divided the Himalayan Range into three sections, namely, eastern, middle, and the western Himalayas. We explored the possible association of monthly mean Convective Available Potential Energy (CAPE), Surface Air Temperature (SAT), thermodynamic temperature of the top of the cloud; Cloud Top Temperature (CTT), Relative Humidity (RH), and Specific Humidity (SH) with LFD over the three sections of the Himalayan range. We observed that CAPE and SAT play a vital role in creating instability over that region. In contrast, moderate moisture (i.e., RH) is the most suitable condition for lightning activities over all three sections. The analysis shows that 50–60% RH at 700hpa is the most favorable condition for lightning over the Himalayan region.

Introduction

Satellite images of lightning activities show that the Himalayan region is one of the world's lightning prone areas (https://ghrc.nsstc.nasa.gov/lightning/data/data_lis_trmm.html). Due to the topology, varied surface temperature, moisture, and atmospheric circulation, lightning flash density (LFD) over this region have a tremendous spatiotemporal variability. From the pre-South-East monsoon study during 2012 over Nepal, we observe the peak in the lightning flash count (LFC) to be maximum in June (Makela et al., 2014). The variation of lightning activity over the Northeastern part of the Himalayan region is semiannual with primary maxima during April. But the similar maxima occurs during July in the North-West region (Penki and Kamra, 2013). The fundamental reason behind this variation of lightning activity was an unanswered question. In this paper, we analyzed the role of moisture (in terms of relative humidity) and other meteorological parameters on lightning activity and also explained the significant influence of RH over the lightning generation process.

The scientific community has proposed several theories of the cloud electrification mechanism and separation of charges within the clouds. However, the most significant cloud electrification approach is the graupel-ice mechanism. Collisions produce electric charges between precipitation particles (graupel) and cloud particles or small ice crystals (Rakov and Uman, 2003; Hobbs and Burrows, 1966; Takahashi, 1978; Saunders and Peck, 1998). The action of gravity is mainly responsible for separating the charged particles (Rakov and Uman, 2003). The significant impact of CAPE on LFD recently been observed by (Qie et al., 2020). The decrease of the CAPE could exert the most considerable effect on decreasing the LFD trend on Southern North America (SENA), Central South America (CSA), and Eastern Australia (EA) (Qie et al., 2020). Dewan et al. (2018) observed the positive influence of CAPE on the lightning activity positively. It is a well established fact that Convective Available Potential Energy, creates instability in the atmosphere, causes updrafts and supports electrification and charge separation within the clouds. A good relationships with total lightning activity (r = 0.93) was observed if updraft volume in the charging zone (at temperatures colder than −5⁰C) with vertical velocities greater than either 5 or 10 m/s (Deierling and Petersen,2008). High lightning activity is related to CAPE's high value over Greece (Mazarakis et al., 2008). A high correlation of lightning flashes and CAPE over Maharashtra, India has been observed by Tinmaker et al. (2015); whereas, the relationship between LFC and CAPE has not been observed consistent in different regions of India (Siingh et al., 2014). Zheng et al. (2016) studied the sensitivity of lightning with CAPE and showed that lightning is more sensitive to CAPE over land than offshore water. A good correlation coefficient between LFC with CAPE and surface temperature over Northeast India was observed by (Guha et al., 2017).

Pinto and Pinto (2008) studied the sensitivity of lightning cloud-to-ground (CG) activity with the surface air temperature daily, monthly, yearly, and decadal time scale. Pinto and Pinto (2008) found a sensitivity of 40% per 1⁰C at daily and monthly timescales and 30% sensitivity per 1⁰C at decadal timescales. The lightning sensitivity of the order of 10% per 1⁰C has been observed by Williams (1994) that further supports the positive correlation between lightning activity and surface temperature for the annual and semiannual timescale. Ming et al. (2005) and Saha et al. (2019) derived the positive response of the surface air temperature (SAT) on the lightning flash rate for inter-annual and seasonal timelines. In the monthly time scale, from May to September via pre-monsoon cooling of 1⁰C in maximum SAT reduces approximately 3.5 thunderstorms per station and 73 flashes (Nath et al., 2009). The correlation between change in global monthly land wet-bulb temperature with lightning activity is strongest over the northern hemisphere and weak in the southern hemisphere (Reeve and Toumi, 1999). A 1⁰C rise in temperature increases 20–44% lightning flash density over the land mass Indian region (Kandalgaonkar et al., 2005).

For years, the effect of relative humidity (RH) on lightning became one of the burning issues in atmospheric electricity research. Ya-Jun et al. (2006) have observed that higher RH results in more lightning activity in dry regions, whereas less lightning activity in wet areas. Reeve and Toumi (1999) indicate the necessity of a high land-area to sea-area ratio for a good correlation between lightning activity and land wet-bulb temperature. But the complete understanding of the effect of RH on lightning is still lacking. The role of cloud base height and associated cloud top temperature on the lightning flash is critical in this regard. Molinie and Jacobson (2004) show an increase of lightning flash density with a decrease in cloud top brightness temperature (CTBT); the temperature of a black body that would emit the same amount of radiation as the targeted body in a specified spectral band when CTBT is less than −55⁰C. An intense condensation process and mass flux during the system's growth phase can provide favorable cloud electrification conditions and lightning (Mattos and Machado, 2011).

In the present research, we have analyzed the role of CAPE, SAT, RH, CTT, and precipitation on lightning activity in different parts of the Himalayas. We selected the regions of investigation as the Eastern Himalayas (24⁰N–28⁰N, 88⁰E-96⁰E), the middle Himalayas (27⁰N–33⁰N, 78⁰E-88⁰E), and the Western Himalayas (30⁰N–36⁰N, 68⁰ E−78⁰E). We describe the study region and its topology in Fig. 1a. The lightning density map, NDVI index map, and precipitation map are illustrated in Fig. 1b, 1c, and 1d, respectively. The middle Himalayas contains high hills and mountains followed by Western Himalayas, and the Eastern Himalayas has the least elevation.