Simulation of premonsoon thunderclouds over two climatic regimes: evaluation and dichotomous detection

Abstract

This study investigates premonsoon (April) thundercloud properties over Northeastern (Kohima, 25.6°N, 94.1°E) and Eastern (Rampurhat, 24.2°N, 87.8°E) India. Thunderclouds were detected using Electric Field Mills (EFM-100) and Lightning Detectors (LD-350), while the WRF model was employed to simulate thundercloud properties. Simulations were performed across all 30 days in each region, regardless of actual thundercloud occurrence, to assess the model's accuracy in differentiating thundercloud and non-thundercloud days. The regional variability of cloud-to-ground (CG) flash density was well represented by the model, with higher flash densities in Rampurhat (mean: 39 × 10⁻⁴ km⁻² h⁻¹) compared to Kohima (mean: 31 × 10⁻⁴ km⁻² h⁻¹), consistent with observations. However, the model exhibited a slight overestimation in Kohima and an underestimation in Rampurhat, with spatiotemporal deviations from observations in both regions. Despite this, the simulations effectively captured regional differences in dynamical parameters (vertical velocity, wind shear) and microphysical properties (mixing ratios: q_ice, q_graupel, q_cloud), with Rampurhat showing higher values overall. Six stability indices were evaluated to determine the most reliable indicator for distinguishing thundercloud/non-thundercloud days. In Kohima, the TT index (≥ 38 °C) was most effective, while in Rampurhat, CAPE (≥ 1680 J kg⁻¹) proved more suitable, suggesting that distinct physical mechanisms drive thundercloud development in these regions. The dichotomous detection results for all 30 days in Kohima (Rampurhat) yielded 14 (21) successful detections, 5 (1) misses, 3 (1) false alarms, and 8 (7) correct rejections. We introduced the Correct Rejection Rate (CRR) to assess the model's performance in detecting non-thundercloud days, which showed better accuracy over Rampurhat than Kohima. The study underscores the heightened complexity of predicting thunderclouds over the hilly terrain of Northeastern India compared to the flatter terrain of Eastern India and offers valuable insights for improving strategies to mitigate lightning hazards.