Contrasting Environments and Triggering Mechanisms of Disastrous Downbursts from Isolated versus Linear-Embedded Storms in the Jianghuai Region of China

Downbursts in the Jianghuai region of East China are primarily caused by two storm types: isolated storms (ISs) and linear-embedded storms (LSs). However, systematic differences in their synoptic backgrounds, convective environments, and downburst formation mechanisms remain poorly understood, limiting the accuracy of downburst warnings. This study aims to quantify these differences and identify storm-type-specific triggering mechanisms for associated downbursts. Based on 18 downburst events (9 ISs and 9 LSs) from 2011 to 2024, we performed composite analysis using ERA5 reanalysis data (0.25°, hourly) and examined two representative downburst cases with S-band Doppler radar, automatic weather station (AWS; minute-resolution), and sounding data. Statistical significance of environmental differences was tested with t-tests. The results are as follows. (1) LSs form ahead of upper-level troughs near the 850-hPa front with strong synoptic forcing, whereas ISs develop along the subtropical high edge under weak forcing but with higher convective available potential energy (CAPE: ~3000 vs. ~2000 J kg⁻¹). LSs exhibit significantly stronger low-to-mid-level vertical wind shear (surface to 500 hPa: ~18 vs. ~7 m s⁻¹; 0–1 km: ~21 vs. ~6 m s⁻¹; p < 0.05). (2) For ISs, downburst initiation is dominated by negative buoyancy from hail melting and hydrometeor evaporation, driving a mid-level rear-inflow jet (RIJ) and a rapidly descending weak-echo gap. For LSs, a low-level RIJ, intensified by the surface cold pool and a developing mesocyclone (which generates a downward perturbation pressure gradient force), combines with hydrometeor drag to produce the downburst. (3) Surface observations show that ISs cause a larger pre-downburst temperature drop (mean 5.1 vs. 4.6°C in LSs) and a distinct dewpoint drop (3.7 vs. 1.0°C) within 10 min before the gust. In general, the two storm types develop under fundamentally different environmental controls (instability-dominated vs. shear-dominated) and rely on different primary mechanisms (thermodynamic cooling vs. dynamic forcing from RIJ and mesocyclone). These findings provide specific pre-downburst signatures (e.g., rapid temperature/dewpoint drops within 5–10 min ahead of peak winds) that can support storm-type-specific nowcasting of disastrous downbursts in the Jianghuai region of East China.