Ensemble Sensitivity Study of Two Warm-Sector Heavy Rainfall Events Associated with Low-Level Jets in North China

Abstract

Based on ECMWF ensemble forecast, this study examines two warm-sector heavy rainfall (WSHR) events in North China (3 July and 18 August 2022), which were characterized by distinct synoptic patterns, pre-trough type and edge of the Western Pacific Subtropical High (WPSH) type, yet both featured low-level jets (LLJs). Through ensemble sensitivity analysis and member-based comparisons, we identify key influencing factors and explore their implications for operational forecasting. Main findings are as follows: (1) The critical sensitive factors vary with synoptic pattern. The “7.3 event” exhibits high sensitivity to geopotential height at 500-850 hPa and mid-to-low-level temperature fields, with weak cold air intrusion acting as a trigger for convection. In contrast, the “8.18 event” shows strong sensitivity to LLJ intensity and deep-layer thermal structure, where multi-directional moisture convergence serves as the primary convective trigger. (2) In both cases, member-based grouping reveals that consistently stronger jet intensity distinguishes relatively better-performing members from worse-performing members. Members with the LLJ axis located to the left of the ensemble mean yield better precipitation predictions. (3) The rainfall mechanism of the two WSHR events follows a “displaced triggering” pattern, in which convection typically initiates downstream of regions with high moisture and CAPE sensitivity, specifically where LLJ-driven moisture transport overlaps with boundary-layer convergence. Key operational indicators for forecasting include: identifying high-sensitive regions 36–42 hours in advance; focusing on the overlap of the LLJ and boundary-layer convergence lines. These findings offer a scientific foundation for enhancing the prediction of WSHR over North China.