Comparative analysis of two squall line events under low-level warm convection forcing in Hunan

Based on conventional observations, Doppler weather radar date, NCEP/NCAR reanalysis data with 1°×1° resolution and other data, we have performed a comparative analysis between two squall line events (hereinafter referred to as the "5·11" event and the "6·5" event, respectively) occurred in Hunan on 11 May and 5 June of 2017. Results are as follows. (1) Both events occur under the condition of low-level warm convection forcing, in which the cold advection is obvious before the "5·11" event occurred, and the warm and wet airflow prevails before the "6·5" event occurred. The difference in the position of the subtropical high results in that the latter event has better water vapor condition than the former one. (2) During the "5·11" event, atmosphere in the middle layer is drier, and the height of 0℃ layer is lower (0.6 km), which is conductive to the formation of thunderstorm gale and small hail over a broad area. Meanwhile, during the "6·5" event, the free convection level (LFC) is relatively low, and humidity in the low-level is greater, which is beneficial to produce greater short-term severe precipitation. (3) The environmental conditions in which a widespread thunderstorm gale occurred in the "5·11" event is better than that in "6.5" event, while in the latter case, the strengthen of atmospheric convective instability is due to the development of surface inverted trough, the intensive warm and wet airflow and the eastward-moving of low vortex, which is more conductive to the emergence of local disastrous gale. (4) In the "6·5" event, there is only one mesocyclone which lasted only for a short time, and the characteristics of thunderstorm gale in the vertical wind profile (VWP) and radar radial velocity chart are not typical enough, which increases the difficulty of early warning to it.