Deen Mani Lal

This study examines the influence of transboundary dust on lightning activity in the Mumbai region, India, during an extreme pre-monsoon event on May 13, 2024, which resulted in multiple fatalities and injuries. A severe dust intrusion from the Middle East and Arabian deserts coincided with a westward-moving convective system originating from inland Nashik, India, and moving toward the southwestern coast of India. A multi-platform dataset combining ground-based and satellite observations was used. Lightning data obtained from the Indian Lightning Location Network (ILLN), PM₁₀ concentrations from the Central Pollution Control Board (CPCB), cloud parameters from satellite and reanalysis, and dust transport pathways from HYSPLIT. Results show that transboundary dust led to an eightfold increase in PM₁₀ (~800 µg m⁻³), accompanied by more than 31,000 lightning flashes, with a peak of over 8,717 flashes coinciding with the PM₁₀ peak at 16:00 IST. Dust particles altered cloud microphysics by reducing cloud liquid water and enhancing cloud ice water content. This process strengthened deep convection, lowered cloud-top temperatures to ~205K, and elevated cloud tops to ~15-16km. These conditions promote strong non-inductive charge separation, resulting in intense lightning over metropolitan Mumbai. The findings provide robust evidence that long-range transported dust significantly enhances thunderstorm electrification by promoting ice microphysics and increasing lightning activity.

During the Indian winter, the southerly propagation of cold air at middle and upper altitudes is caused by a trough in westerly jet streams due to the effect of Western Disturbances (WDs). Sometimes, this advection of cold and dry air above near‐surface moist and warm air can make the atmosphere conditionally unstable. Therefore, these WDs are associated with thunderstorm formation and lightning activity over northern India (north of 20° north); however, occasionally, the effect of WDs extends up to the entry point of southern India. The satellite and ground‐based observations of lightning over western India (15.5°–22°N, 72.5°–81°E) over two decades (1997–2023) observed a falling trend of winter lightning at the rate of −0.02 (LIS/OTD), and −0.06 (ILLN) flashes km⁻² year⁻¹, respectively. Our observation suggested that the majority of lightning and thunderstorm activity is primarily initiated by WDs during winter; however, the incursion of moisture from the surrounding oceans also plays a crucial role in producing severe weather and precipitation over the study region. Many researchers and our observations have shown a decreasing trend of WD frequencies and winter precipitation over the Indian region. This paper addresses important aspects of climate change as lightning activity and thunderstorm days decreased steadily over western India, suggesting the falling trend is possibly linked to the weakening and reduced frequency of WDs from 1997 to 2023. While global trends suggest increasing lightning with climate change, this study reveals a decreasing trend in lightning activity, highlighting the uneven regional impacts of climate change.

The study analysed the impact of aerosol (represented by aerosol optical depth, AOD) and relative humidity (RH) on lightning activities (LA) for distinct five locations around the world: South Africa (R1, 5⁰N to -1⁰S, 9⁰E to 18⁰E), North-western South America (R2, 2⁰N to 5⁰S, -77⁰W to -67⁰ W), Indonesia (R3, 10⁰N to 17.5⁰N, 100⁰E to 109⁰E), South America (R4, 31⁰S to 21⁰S, -61⁰W to -53⁰W) and North America (R5, 29⁰N to 34⁰N, -96⁰W to -81⁰W). Based on the study, it was found that the LA is not uniformly associated with AOD or RH individually, rather their synergic impact defines the LA. Results reveal that the association between RH and LA is weaker when AOD is high, such as for regions R1 and R3, whereas it was substantially intense when RH dominates (regions R4 and R5). Based on the study, it was concluded that the humidity per aerosol is critical in lightning formation and is more effective in assessment of the LA than aerosol or humidity alone.

The association between aerosol and lightning has been investigated with long-term decadal data (2005–2014) for lightning, aerosol optical depth (AOD), relative humidity, and effective cloud droplet size. To understand the complex relationship between aerosol and lightning, two different regions with different climatic and weather conditions, a humid region R1 (22°–29° N, 89°–92° E) and an arid region R2 (23°–28° N, 70°–76° E) of northern India, were chosen for the study domain. The results show that lightning activity was observed to occur more over the humid region R1, i.e., 1141 days (1/3 of total days), than over the arid region R2, i.e., 740 days (1/5 of total days). Also, over the humid region R1, the highest lightning flash density was recorded as nearly 4.6 × 10^–4 flashes/km²/day observed for 18 days (1.5%); on the contrary, over the arid region R2, the maximum lightning flash density was observed to be 2.5 × 10^–4 flashes/km²/day and occurred for about 22 days (2.9%). The analysis shows that a nonlinear relationship exists between aerosol and lightning with a highly associated influence of relative humidity. A very significant positive and negative co-relation that varies with relative humidity has been observed between AOD and lightning for both humid and arid regions. This shows relative humidity is the key factor in determining the increase or decrease of lightning activity. This study also shows that the larger the cloud droplet size, the higher the relative humidity and vice versa. This study emphasizes that aerosol concentration in the atmosphere influences cloud microphysics by modulating the size of cloud droplets and thereby regulating the lightning frequency. The atmospheric humidity is the driving factor in deciding the positive or negative co-relationship between aerosol and lightning.