Storm splitting process and the associated mechanisms for a long-lived hailstorm

Abstract

Storm splitting poses a great challenge in storm tracking and forecasting. However, most of previous studies have focused on the effect of warm rain microphysics on storm splitting, that of ice microphysics, especially hail microphysics on hailstorm splitting remain unknown. The storm splitting and associated mechanisms for a long-lived severe hailstorm occurred on 19 June 2017 in Beijing are investigated in this study based on radar observations and simulated results from a three-dimensional cloud model with hail-bin microphysics. The results show that the hailstorm experienced twice splitting. Both right-moving and left-moving cells strengthened in the first splitting, while only the right-moving cell strengthened in the secondary splitting. It is found that the clockwise-curved hodograph environment initially creates asymmetric negative pressure perturbations at the low levels. With the development of the storm, the negative pressure perturbations are strengthened, lifted, and evolved as vertically clockwise- or anticlockwise-rotating low-pressure centers, which significantly enhance both upward and outward pressure gradient forces at the levels where the low-pressure centers are located, and selectively induce the updraft development on the flanks of the storm, resulting in the low-level upward-curved updraft splitting structure. In the mature and decaying stage of the storm, strong downward propagating gravity waves excited by high graupel/hail loading through buoyancy oscillations are found to have a critical role in the subsequent rapid updraft splitting prior to the graupel/hail-induced downdraft. Both the gravity waves and graupel/hail-induced downdraft have important roles in the hailstorm splitting rather than rain-induced downdraft as proposed in previous studies.