4.1 Monthly Variation of Flash rate density, Flash Count, Flash Days, and Flash Radiance
Figure 2 shows the area average monthly mean values of lightning flash characteristics (flash rate density, flash count, flash days, and flash radiance) of the study region Telangana and Figs. 3, 4, 5 and 6 represents lightning flash characteristics over Andhra Pradesh. In the study regions Telangana and Andhra Pradesh flash rate densities exhibits semi-annual variation with primary maximum during the month of May and secondary maximum during the month of September. Similarly, semi–annual variation of flash count is observed over Telangana with primary maximum in the month of May and the secondary maximum in the month of September (Fig. 2). Similar trend of monthly variation of flash count is observed over the districts of Rayalaseema region with primary maximum in the month of May and secondary maximum in the month of September (Fig. 3 (a)). Flash count over the districts of coastal Andhra Pradesh also exhibit semi-annual variation with primary maximum during April–May and secondary maximum during the month of September (Fig. 3(b)). The increase in flash frequency during the September month can be explained on the basis of thermodynamic instability developed during withdrawal phase of the monsoon and onset of northeast monsoon in this region during this month. While considering the monthly variation of meteorological parameters over Telangana, surface temperature (Fig. 7 (a)) is found to increase during pre–monsoon season (March–May) reaching maximum value in the month of May. Thereafter the surface temperature is found to decrease during the monsoon season (June–September) and post monsoon season (Oct–Nov) reaching minimum value in winter season (Dec–Jan). Similar trend is observed in the districts of Rayalaseema and coastal Andhra Pradesh. The high frequency of lightning and maximum values of surface temperature during the pre–monsoon season suggests association of lightning with surface temperature (Williams et al., 1994). The latitude and longitude of all the districts of Andhra Pradesh state were collected from Andhra Pradesh state portal (ap.gov.in).
The frequency of flash days over Telangana (Fig. 2) shows semi-annual variation with primary maximum during the month of June and secondary maximum during the month of September. Rayalaseema and coastal Andhra Pradesh also exhibit semi-annual variation of flash days (Fig. 5 (a) and (b)) with primary peak during the month of May and secondary peak during the month of September.
Lightning flash radiance over Telangana (Fig. 2) and Rayalaseema region is found maximum during the monsoon season (June–September) (Fi 6a) and minimum during the winter season (Dec–Jan). TRMM LIS lightning flash radiance represents the strong physical processes of thunderstorm, its dependence on depth of the cloud and energy of lightning over different regions (Qie et al. 2003). The study by (Petersen et al. 2017a) shows that superbolt is associated with high radiance values. This suggests lightning over Telangana and Rayalaseema regions during monsoon season are more energetic compared to other season lightning. High radiance values are observed over coastal Andhra Pradesh during pre-monsoon and post-monsoon season (Fig. 6b) with fluctuating values and minimum values are observed during winter season.
4.2 Correlation of Flash Count with Meteorological Parameters
Simple linear regression is performed to find out the correlation between flash count, and meteorological parameters (surface temperature, CAPE, convective precipitation, and AOD). Table 2 represent the values of correlations between flash counts, surface temperature, CAPE, convective precipitation, and AOD with the corresponding p-values with a significance level of 0.05. High correlations (0.76 < CC < 0.82) were observed between lightning flash count and surface temperature over Telangana, in all the districts of Rayalaseema except Chittoor where moderate CC (0.67) is observed. High correlation (0.70 < CC < 0.89) is found in all the districts of coastal Andhra Pradesh except in the districts of Guntur, Krishna, and Nellore where moderate correlation (0.51 < CC < 0.58) is observed. High correlation between flash count and surface temperature indicates that the development of lightning is due to the convection developed because of excessive heating of land by the incoming solar radiation. Previous studies (Price and Asfur, 2006b; Sekuguchi et al., 2006) also emphasizes the strong association of lightning with surface temperature.
Similarly, significant correlation (0.90 < CC < 0.96) is observed between lightning flash count and CAPE in the districts of Telangana, Rayalaseema (except Chittoor where CC = 61) and in the districts of coastal Andhra Pradesh (0.87 < CC < 0.96) except in Guntur, Krishna and Nellore where moderate correlation (0.34 < CC < 0.53) is observed.
Very poor correlation of flash count with convective precipitation is observed over Telangana and Rayalaseema regions whereas moderate correlation (CC < 0.50) (Table 2) is observed in the districts of coastal Andhra Pradesh except Guntur, Krishna, and Nellore districts where significant correlation (CC) (varies from 0.72 to 0.76) is observed (Table 2). This confirms the earlier studies (Piepgrass et al. 1982; Williams et al. 1989; Tapia et al. 1998; Lang and Rutledge 2002; Lal and Pawar 2009) that the lightning is regionally dependent on rainfall. Observations in the previous studies (Rutledge and MacGorman 1985; Sun et al. 2019) show that –CG flashes are mainly concentrated on convective precipitation regions. Hence lightning flash count during post–monsoon season over Guntur, Krishna, and Nellore is mainly due to convective precipitation. Very poor correlation is observed between lightning flash count and AOD in the study regions except in Chittoor where significant correlation (0.74) is observed.
4.3 Inter-annual Variation of Flash Count
Inter-annual variation of lightning characteristics is studied to understand the variation of lightning activity over longer time period. Figure 11 represent the inter–annual variation of lightning flash count during the period 1998–2013. Lightning flash count in the study regions Telangana, Rayalaseema, and in the districts of coastal Andhra Pradesh shows increasing trend. Lightning flash count over Srikakulam district shows significant increasing trend during the period 1998–2013 compared to other regions. The lightning flash count is highly correlated with surface temperature (CC = 0.89), CAPE (0.95) and moderately correlated (CC = 0.65) (Table 2) with AOD in the Srikakulam district. The increasing trend of lightning flash count in this region is attributed to the change in the surface temperature during the period 1998–2013 because of global warming. Annual mean flash count (Table 1) over Vishakhapatnam district is maximum (87.645) compared to all other regions. Annual mean flash rate density is maximum over Srikakulam district compared to all other regions.
4.4 Discussion
Maximum flash count, flash rate density and high correlation of lightning flash count with surface temperature and CAPE is observed (Table 2) during pre–monsoon season (March to May) over Telangana, Rayalaseema (Except Chittoor), and in coastal Andhra Pradesh (except Guntur, Krishna, and Nellore). This confirms several earlier studies (Price 2003; Kandalgaonkar et al. 2005; Penki and Kamra 2012a, 2013) which establish the sensitivity of lightning activity with surface temperature and CAPE. It is observed that a change in wet–bulb temperature of 1o K would result in the increase in lightning activity by 40% (Reeve and Toumi 1999). CAPE is found to influence nonlinearly the growth and accumulation of ice particles in the upper part of the cloud and also helps in the charge separation by the differential motions of the particles (Williams 1992). Hence, buoyancy force developed due to heating of land, because of incoming solar radiation, is mainly responsible for the electrical activity of the thunderstorm during pre-monsoon season in Telangana, Rayalaseema (except Chittoor), and in coastal Andhra Pradesh (except Guntur, Krishna, and Nellore). The secondary maximum is attributed to the instability developed due to the withdrawal phase of the monsoon and onset of the northeast monsoon over this region. Maximum lightning over these regions during pre–monsoon and post–monsoon seasons is not only due to the sensitivity of lightning with surface temperature and CAPE but also because of the thunderstorms developed due to the passage of the tropical cyclones emerged over Bay of Bengal (Penki and Kamra 2012b).
Monthly variation of convective precipitation (Fig. 8a, b) over Telanagana and Rayalaseema shows maximum value during post–monsoon season. The monthly variation of convective precipitation (Fig. 8c) over coastal Andhra Pradesh shows maximum value during monsoon and post–monsoon season. Aerosol optical depth doesn’t show any definite trend over coastal Andhra Pradesh and Rayalaseema regions (Fig. 9b, c). While maximum values of AOD are observed during monsoon season over Telanagana (Fig. 9a). Very poor correlation of lightning with convective precipitation (except Guntur, Krishna, and Nellore) and AOD (except Chittoor) is observed (Table 2) in the study regions. In Chittoor high correlation is observed between lightning flash count and AOD. Hence the lightning activity over Chittoor is due to high AOD values during the pre–monsoon season. High concentration of aerosols inhibits collision and coalescence process results reduced droplet size and suppression of warm rain process (Rosenfeld 1999). High aerosol concentration is responsible for the formation of large hail (Rosenfeld and Woodley 2000) and increase in the total number of lightning flashes (Orvilley et al. 2001; Stallins and Rose 2008; Wang et al. 2011; Tao et al. 2012). This clearly establishes the earlier studies finding that the sensitivity of lightning with precipitation and aerosols varies from region to region (Rosenfeld 2003; Kamra and Ramesh Kumar 2021). The monthly variation of lightning flash radiance over Telangana shows maximum during the month of June (Fig. 2). The radiance values of lightning flash characterise the convective intensity of thunderstorm (Jin You et al. 2019). This shows that inspite of decreasing trend of lightning flash count during monsoon season the lightning flashes are highly energetic because of the presence of deep convective systems over Telangana. Similar pattern of lightning flash radiance is observed in the districts of Ryalaseema (Fig. 6a). The lightning flashes due to low radiance values in the pre–monsoon season are less energetic compared to monsoon season lightning over Rayalaseema, whereas over coastal Andhra Pradesh lightning flash radiance is maximum during pre-monsoon season.
Very poor correlation of flash count and convective precipitation is observed over coastal Andhra Pradesh except Guntur, Krishna, and Nellore districts where high correlation is observed. This clearly shows that lightning over Guntur, Krishna, and Nellore districts is mainly due to the deep convective systems because of the passage of the synoptic systems developed over Bay of Bengal through these regions during pre–monsoon and post monsoon seasons. The maximum annual mean value of flash count shows that lightning frequency is high over Vishakhapatnam district and maximum flash rate density shows that frequency of intense thunderstorms and lightning frequency over Srikakulam district are high compared to other regions. The inter–annual variation of flash count shows increasing trend over Telangana and Andhra Pradesh regions with a significant increasing trend over Srikakulam district during the period 1998–2013. The significant increasing trend over Srikakulam is due to the change in surface temperature over this region. Earlier studies by (Kandalgaonkar et al. 2005) show that a small change in surface temperature leads to a greater increase in lightning activity.