Observational Study of a Rare Winter Supercell Hailstorm and Its Strong Updraft

On the afternoon of February 14, 2020, an extremely rare large hail event occurred in Enshi, Yichang, southwestern Hubei Province, producing hailstones larger than 5 cm in diameter along with numerous smaller hailstones. Based on surface automatic weather stations, radar and satellite observations, and the ECMWF ERA5 reanalysis data, this study analyzes the environmental characteristics, triggering mechanisms, storm structure, and causes of large hail in this hailstorm event. The results show that before convective initiation, the 0–6 km vertical wind shear reached 19 m/s, while the convective available potential energy (CAPE) was only 438 J/kg, indicating a rare large hail event in a high-shear, low-CAPE (HSLC) environment. At 850 hPa, lift associated with the peripheral shear of the southwestern vortex led to strong convergence between low-level southeasterly warm-moist airflow and northeasterly cold-dry airflow in the mountainous area of northern Enshi. Combined with orographic lifting, this resulted in the development of two convective cells into supercells (referred to as cell A and cell B). Cell A was an isolated classic supercell, while cell B was an embedded supercell within a multicell storm; both produced large hail. Estimates of updrafts in the two supercells based on divergence and perturbation pressure methods show that after the mesocyclones intensified, the dynamic perturbation pressure gradient force in the mid-lower levels of the storms contributed more significantly to the updrafts than in the mid-upper levels. The maximum updraft occurred near 6–7 km above ground level. In the high-shear environment, both linear and nonlinear dynamic vertical perturbation pressure gradient accelerations were equally important for storm development. In the early stage of the storms, the mesocyclone was weak, the contribution of the nonlinear term was small, and the low-level updraft was dominated by the linear component. As the mesocyclone intensified, the nonlinear term became the dominant factor in the updraft. The mesocyclone prolonged the residence time of hail embryos in the region rich in supercooled water, allowing them to grow into large hailstones. More importantly, the perturbation pressure generated by the mesocyclone compensated for the insufficient buoyant vertical acceleration in the low-CAPE environment, enhancing the updraft. This, in turn, enabled the updraft to support larger hailstones and further extend their residence time.