Comparison of causes of two regional rainfall/snowfall and freezing events with low temperatures in Hunan in early 2024

In February 2024, Hunan encountered two regional rainfall/snowfall and freezing events with low temperatures, one in northern Hunan from February 2 to 6 (hereinafter referred to as the“2.02”event), and the other across the whole province from February 21 to 26 (hereinafter referred to as the“2.21”event). Using conventional upper-air and surface weather observations, regional automatic weather station data, and NCEP reanalysis data, we conducted a comparative analysis of the circulation background, cooling mechanism, and water vapor transport characteristics of the two events. The results are as follows. (1) There are obvious differences in the circulation background between the two events. The“2.02”event occurred in the background with eastward movement of both the southern branch trough and westerly trough overlapping in phase. However, the“2.21”event occurred under the cold wave caused by the collapse of the blocking high, with the transition of rain and snow phases appearing multiple times due to the small influence on the southern branch trough. (2) There were the strong low-level frontal zone in both events. A rare north-south temperature gradient greater than 26 ℃ was observed in the“2.21”event, which may be an important indication of this extreme cold wave weather. (3) Temperature advection played an important role in the transition of rain and snow phases. In the“2.02”event, the strong cold advection intrusion in the middle level was driven by the westerly trough, causing a temperature decrease across the whole layer. In the“2.21”event, the continuous input of the ultra-low-level strong cold advection was one of the important cooling mechanisms. In addition, the strongest transport layer of warm advection corresponded to the negative center of vertical velocity, and the stronger vertical wind shear in the middle and lower troposphere resulted in stronger rain and snow intensity. (4) The Arabian Sea was the main source of water vapor for both events. Southwest warm and wet airflow in the middle troposphere moved with the low trough at 500 hPa or the small trough from it, with the highest contribution of specific humidity and water vapor flux to both events. The low troughs provided both favorable dynamical conditions and abundant water vapor for the occurrence of rain and snow weather. In the “2.21”event, the warm and wet air carried by three combined channels from the South China Sea channel, the Arabian Sea channel and the low latitude inland regions provided water vapor and warm layer conditions for the freezing area in the southern Hunan, which was an important reason for the severe freezing in this area.