The Warm-Sector Thunderstorms Triggered by Mesoscale Boundary-Layer Convergence over the DPRK

Abstract

Warm-sector thunderstorms (WSTs), characterized by weak synoptic forcing and extreme precipitation rates, pose a major global forecasting challenge. This study investigates the mesoscale processes initiating WSTs over the complex terrain of the Democratic People's Republic of Korea (DPRK), a region where triggering mechanisms remain poorly understood. We analyze three extreme rainfall events (Hoichang 2016, Unpa 2017, Pyongyang 2018), each producing rainfall rates exceeding 60 mm h⁻1 under the weak forcing typical of the northwestern periphery of the West Pacific Subtropical High. While operational global models failed to predict these events, the convection-permitting WRF model skillfully replicated the initiating mechanisms and subsequent convection. Model performance was quantitatively assessed using multiple verification metrics, including Probability of Detection (POD), False Alarm Ratio (FAR), Bias, and Critical Success Index (CSI). Integrated analysis of observations and high-resolution (3 km) Weather Research and Forecasting (WRF) model simulations reveals a consistent trigger: mesoscale boundary-layer convergence lines. These zones formed through the interaction of synoptic southwesterlies with localized, terrain-modulated flows and were collocated with horizontal moisture gradients. Crucially, the three-dimensional structure of Convective Available Potential Energy (CAPE) manifested as narrow, vertical towers of high instability, delineating regions of deep convection initiation 2–4 h in advance. A pre-convective, deep moist layer (relative humidity >80% in the 850–700 hPa layer) was identified as a necessary precondition. This study establishes terrain-forced boundary-layer convergence as a primary trigger for WSTs over the DPRK, providing a valuable framework for improving prediction in other monsoonal regions with complex topography.