# A 3D Numerical Model to Estimate Lightning Types for PyroCb Thundercloud
## 1 Introduction  
Pyrocumulonimbus (pyroCb) thunderclouds commonly form above bushfires, expanding to substantial heights and influencing the chemical composition as well as the concentrations of aerosol in the upper part of the troposphere [1]. In the formation of pyroCb thunderstorms, the presence of aerosol particles produced by smoke serves as cloud condensation nuclei (CCN), exerting a significant influence on the advancement of the storms. One of the most crucial impacts of pyroCb is the initiation of additional wildfires by cloud-to-ground (CG) lightning. Similar to conventional thunderclouds, pyroCb clouds can result in both positive and negative cloud-to-ground (+CG and \(-\)CG) lightning, with +CG being more likely to trigger secondary spot fires and cause extensive harm to nature [1; 2; 3]. Over 90% of all lightning strikes are of negative polarity, which is frequently observed in thunderstorms that occur in humid conditions [4]. Conversely, +CG strikes are oftenobserved in dry environments with higher cloud bases [5]. These +CG flashes also occur in thunderstorms such as pyroCb, which release significant amounts of smoke into the atmosphere [6]. Multiple investigations have shown that CG strokes of either positive or negative having large continuous currents could start most of the fires [2; 7; 8; 9]. The emergence of secondary bushfires ignited by CG lightning has become a significant concern for firefighting management on a global scale. Although there has been a notable rise in the number of pyroCb events in the literature, only a few investigations have been conducted in recent years. In August 2013, Western North America experienced the occurrence of 88 massive wildfires, which gave rise to 26 highly intense pyroCb storms [10]. Over the period of 1973-2020, the southeastern part of Australia faced several distinct pyroCb storms where recent events reported as Black Saturday (Victoria, 2009) and Black Summer (New South Wales, 2019-2020) observed the ignition of new spot fires by lightning as well as the longevity of the pyroCb storms [11; 12; 13; 14]. The Black Saturday wildfire events (Figure 1a) produced three distinct pyroCb storms, with the largest reaching an altitude of 15 km and generating hundreds of lightning strikes in Southeast Australia, as depicted in Figure 1b [11]. According to the Victorian State Government records, the \"East Tyers-Thomson\" fire event began at 1800 LT and was attributed to lightning strikes occurring on Black Saturday. During the 2019-2020 Black Summer fire season, dry lightning strikes in East Gippsland ignited three fires, eventually expanding into a widespread bushfire disaster that raged for months [12]. On 8 January 2003, lightning strikes ignited 87 separate fires in northeastern Victoria, leading to the Alpine Fire [14]. These fires were exacerbated by several consecutive days of unprecedented high temperatures and dry weather conditions. Numerous studies in the past have investigated the electrical conditions that promote the formation of CG lightning strikes could serve as a crucial indicator for understanding specific phenomena associated with pyroCb events. Multiple hypotheses have been developed recently to explain the electrical configurations of charge regions and situations that result in both \(-\)CG and +CG lightning strikes and their association with thunderstorms [15; 16]. The overall charge distribution of the conventional thundercloud developed in a moist environment is commonly visualized as a vertical tripole having three charge layers: a predominantly positive layer at the upper portion, a dominant negative layer in the middle, and an auxiliary positive layer below the main negative, typically of smaller magnitude [17]. This lower positive charge layer, in particular, has a crucial role in generating \(-\)CG flashes [18; 19]. On the other hand, the presence of an enlarged region of positive charge at the bottom of thunderclouds may begin the progression of positive downward leaders to initiate +CG strikes [15; 16; 19; 20; 21; 22].