Cloud to Ground Lightning Strike Density Analysis Based on Geographic Information System in Tuban Regency Area

This study aims to analyze the density of Cloud to Ground lightning strikes in the Tuban Regency area from September 2022 to January 2023 using the help of a Geographic Information System (GIS). After processing and analyzing the data, the results were that the highest lightning strike density occurred in October 2022, with 40240 strikes, while the lowest occurred in 2790 strikes. The total number of strikes from September to January is 135249 strikes. Based on aspects of the earth’s surface, it was obtained that the density of cloud to ground lightning strikes that occurred in the Tuban Regency area increased along with increasing height. This can be seen where the southern part of Tuban Regency, namely in the sub-districts bordering Bojonegoro Regency, including Parengan, Soko, and Senori sub-districts, has the highest level of vulnerability.


Introduction
Indonesia is a tropical country that has a high potential to experience lightning strikes.This is because the territory of Indonesia has a relatively high rate of warming throughout the year and a huge water area, resulting in considerable evaporation and has the potential for the growth of lightning-producing clouds, namely cumulonimbus clouds [1].In meteorological and geographical aspects, Indonesia has complexity in weather and climate phenomena.The atmosphere over Indonesia is complex and cloud formation is unique [2].
Tuban Regency is one of the regencies in East Java located in the westernmost region, the province with an area of 183,994,561 Ha.Astronomically, Tuban Regency is located at coordinates 111°30'-112°35' East Longitude and 6°40'-7°18' South Latitude.The length of the coastal area in Tuban Regency is 65 km, stretching from the East in Palang District to the West in Bancar District, with an ocean area covering 22,608 km².This causes Tuban Regency to have a high potential for lightning strikes compared to other regions because its area is directly adjacent to waters in Indonesia, namely the Java Sea, with the sea being the largest source of evaporation in the process of cumulonimbus cloud formation [3] Lightning is one of the natural electrical phenomena in the form of discharge from cumulonimbus clouds and this event cannot be prevented.Lightning occurs due to atmospheric dynamics influenced by global, regional and local factors in a region that cause atmospheric conditions to become unstable, thus triggering the formation of cumulonimbus clouds [4].Based on the process of occurrence, lightning can be classified into four types, Cloud to Ground (CG), Inter Cloud (IC), Cloud to Cloud (CC), and Cloud to Air (CA) [5].This study focuses on the Cloud to Ground (CG) type of lightning because this type of lightning is always associated with lightning disasters.CG lightning originates from clouds, such as cumulonimbus (CB) clouds, moving through the atmosphere and into the ground.This type of lightning is dangerous and destructive because it is directly related to human activity [6][7].Altitude and land use factors on the earth's surface trigger lightning strike activity in an area [8].
Research on lightning strike event analysis is needed as an effort to reduce and prevent losses that will occur, by conducting Cloud to Ground lightning strike density analysis using a Geographic Information System (GIS) [9].Based on this, the author is interested in researching the density of lightning strikes in the Tuban Regency area.

Method
This research was conducted with the Physics study program, Faculty of Mathematics and Natural Sciences, Billfath University with the Tuban Meteorology, Climatology and Geophysics Station (BMKG).
The data used is in the form of lightning data limited to areas in Tuban Regency and rainfall data from September 2022 to January 2023.Lightning data comes from the Nganjuk Meteorology, Climatology and Geophysics Agency (BMKG) which is obtained using a "Lighting Detector", while rainfall is obtained from the Tuban BMKG Station.The lightning strikes are recorded using a tool called Lighting Detector (LD).As a recording device, the LD consists of an antenna connected to a PCL card that produces lightning discharge (10 kHz to 200 kHz) and is detected directly by the Storm tracker [4].
Lightning data obtained in *csv format is the result of raw recordings from real-time lightning detector sensors that the BMKG Nganjuk Geophysical Station has processed.Then lightning strike data in *csv format is analyzed spatially using ArcGIS software, a package consisting of geographic information system software products.In this case, the kriging interpolation method produces the lightning strike density per square kilometer.Kriging is a method used to analyze geo-statistical data, to interpolate the values of the variables used based on sample data, taken at irregular locations or points.
This method estimates the magnitude of the characteristic value at the non-sampling point based on information from the characteristics of the sample surrounding it, considering the spatial correlation present in the data.The Clip method is also used for special data selection in Tuban Regency.

Results and Discussion
Research related to lightning strikes has been carried out several times in various regions with the aim of mapping areas prone to lightning strikes.For example, Paski conducted research in Jabodetabek in 2016.The results of the analysis showed that the peak of CG strikes occurred in the MAM period around the Bogor area [10].The second was conducted by Yulia, who studied lightning strike density in the East Tanjung Jabung region in 2020.The analysis showed that the highest lightning density occurred in October and December, while the lowest occurred in February [11].The third study was conducted by Sekarmayang, who researched the relationship of CG lightning with rainfall in 2018 around BMKG Sanglah.The results showed a relationship between daily and monthly CG precipitation and lightning with Spearman of 0.372 and 0.834 [12].
Tuban Regency generally has a monsoonal rain pattern type because it has a clear difference between the rainy and dry seasons periods.The rainy season occurs from October to April and the dry season lasts from May to September.Rainfall will be under the seasonal climate in Tuban Regency.The rainfall will be high during the rainy season, while the dry season the rainfall is low.As for lightning events not always affected by rain, this study has not found a relationship between the amount of rainfall and the number of lightning events.
It can be seen in Table 1 in September that rainfall and lightning strikes were at the lowest level with 2790 strikes and 116.2 mm of rainfall.In October there were 40240 strikes and 410.4 mm of rainfall.In November there were 35521 strikes and 416.2 mm of rainfall.In December there were 27855 strikes and 664 mm of rainfall.In January there were 29023 strikes and 296 mm of rainfall.
(e) Figure 1.Maps of rainfall and lightning strikes in the Tuban region.
September is the transition season from the dry season to the rainy season.In this season, rainfall and lightning strikes are still relatively low.In December there is an increase in Asian monsoon activity which can increase the growth of rain clouds significantly in western, central and southern parts of Indonesia.In addition, in December there is also an increase in Asian cold calls that can increase the potential for rain clouds around Kalimantan, Sumatra, Java, Bali to Nusa Tenggara.
Based on data from the Central BMKG, the forecast for the 2022/2023 Rainy Season in Indonesia shows that most regions are predicted to experience the beginning of the 2022/2023 Rainy Season from September to November 2022.It can be seen in Figure 1 that there are five maps, each representing the level of lightning strike density in Tuban Regency per month based on the start of the 2022/2023 rainy season, which is September, which also uses the rigging interpolation method.Of the five maps, Tuban Regency is included in the low-medium category in all sub-districts.
Based on the results of mapping the level of lightning vulnerability in Tuban Regency, East Java for five months from September 2022 to January 2023 using the kriging interpolation method (See Figure 2), it can be seen that the southern part of Tuban Regency, namely in the sub-district bordering Bojonegoro Regency, among others, Parengan, Soko, and Senori sub-districts are the areas with the highest level of vulnerability.Because it borders Bojonegoro Regency, Parengan, Soko and Senori have the highest lightning density compared to Tuban district.This is because Bojonegoro regency has an altitude of 19 meters above sea level while the district.Tuban has an altitude of 4 meters above sea level.Because lightning itself usually strikes higher ground because the location contains the largest and closest positive.

Conclusion
Based on the research results, it can be concluded that lightning strikes are generally in the form of Cloud to Ground type lightning strikes, namely positive CG and negative CG with a total of 135,429 strikes over the last five months.Another thing that shows its characteristics is the difference in monthly strike rates, ranging from low, medium and high.The highest rate of lightning strikes occurred in October 2022 and the lowest was in September 2022.

Table 1 .
Number of lightning strikes and rainfall in Tuban Regency.