A study of electrical and lightning properties of the premonsoon thunderclouds is carried out over the Northeastern and Eastern parts of India with the following objectives (i) To investigate the regional variation of the electrical and lightning properties of thunderclouds, in association with their cloud top temperature, vertical structure and cloud microphysical properties, (ii) To simulate the lightning and associated cloud microphysical, dynamical and thermodynamical properties of thunderclouds, (iii) To investigate the optimum atmospheric stability indices for detection of thundercloud day. For the present study, ground observations are carried out at Kohima (25.67o N, 94.08o E) in the Northeastern India and at Rampurhat (24.17° N, 87.78° E) in Eastern India. Kohima and Rampurhat are situated at an altitude of 1500 m and 40 m from the mean sea level respectively. The present study is a comprehensive multi-sensor approach. The observations from the ground-based electric field mills (EFM-100) and lightning detectors (LD-350) are utilized in synergy with the satellite onboard radiance observations from INSAT-3D. Apart from this, the observations, from Global Precipitation Measurement (GPM) and CloudSat satellites are also utilized to study the characteristics of vertical structure of reflectivity and cloud microphysics. Simulation study is carried out with the help of Weather Research and Forecasting (WRF) model.
The electrical and lightning observations reveal that, Kohima experiences more number of, thunderclouds, albeit of smaller size, compared to Rampurhat. Over Rampurhat, there is a relatively strong diurnal cycle of the occurrence of thunderclouds compared to Kohima. The electric field observations show larger values of step change in electric field (±ΔE) over Rampurhat compared to Kohima, which appeaently suggest larger charge transfer over Rampurhat. The lightning type distribution from the LD-350 shows significant regional variation in lightning characteristics, which signify the variability in the charge structure of the thunderclouds. Overall percentage occurrence of Cloud-to-Ground (CG) lightning is more over Kohima compared to Rampurhat. On the contrary, occurrence of Intra-Cloud (IC) lightning is more over Rampurhat compared to Kohima. It is also observed that, over both the stations, the occurrence of –CG lightning is more compared to +CG lightning. Overall, negatives discharges dominated over Kohima, whereas the opposite is observed over Rampurhat. The prevalence of positive as well as negative discharge from the thunderclouds suggests the presence of tripole charge structure of the thunderclouds over both the stations, with distinct characteristics. The result is broadly in agreement with the EFM-100 observations. The thunderclouds over Rampurhat are associated with higher lightning flash density compared to Kohima, suggesting that, the thunderclouds over Rampurhat are more severe compared to Kohima. Satellite radiance observations shows thunderclouds with colder clouds over Rampurhat compared to Kohima. Thunderclouds over Rampurhat are associated with stronger mixed phase processes compared to Kohima.
The simulation of thundercloud lightning and associated cloud and atmospheric properties is carried out with the help of WRF model in the month of April. It is found that, over both the region, though up to the median value, the simulated CG flash density are reasonably in good agreement with the observations, but over both the regions, the model simulation is not able to capture the extreme CG flash density. Nevertheless, the regional variability of the CG flash density is well captured by WRF simulation, with higher flash density over Rampurhat compared to Kohima, consistent with the observations. Simulation is able to capture the strong and weak diurnal variation of thundercloud occurrence over Rampurhat and Kohima respectively. The results also suggest that the performance of simulation of space-time evolution is better over Rampurhat compared to Kohima, suggesting more challenge to forecast the impending thunderclouds over the Northeastern part of India. The mean values of ice, graupel and cloud water mixing ratio are higher in the mixed phase region over Rampurhat compared to Kohima, suggesting a prevalence of stronger microphysical processes in the mixed phase region, a favorable condition for the lightning, consistent with the observations. The simulation results are able to capture the regional variation of total wind shear, with higher value over Rampurhat compared to Kohima. The performance of six stability indices namely, Lifting index (LI), K-Index, Total Total (TT) index, Severe Weather Threat (SWEAT) index, Convective Available Potential Energy (CAPE) and Convective Inhibition (CIN) were analyzed to find the optimum stability index to detect the thundercloud days over each region. Over Kohima, the optimum stability index TT ≥ 38o C shows the best skill scores, whereas over Rampurhat, the optimum stability index CAPE ≥1680 J kg-1 shows the best skill scores. The overall performance of the optimum stability indices were found to be better over Rampurhat compared to Kohima.
Overall, the present study provides a comprehensive understanding of the regional variability of the electrical and lightning properties of thunderclouds and associated microphysical, dynamical and thermodynamical properties over the Northeastern and Eastern parts of India. The simulated results are reasonably in good agreement with the observations. The present study will help to address the issues related to the lightning hazards and their mitigation in a better way.