A contrastive analysis on the causes of two regional snowstorm processes influenced by the southern branch trough in Hunan in early 2022

In early 2022, there were four low-temperature weather processes with rain and snow in Hunan Province, China. Two processes occurred on January 28–29 (referred to as the “0128” process) and February 6–7 (referred to as the “0206” process), and they have overlapping areas of heavy snowfall and high intensity of short-term snowfall. Multi-source observation data and the National Centers for Environmental Prediction (NCEP) reanalysis data are used to analyze the characteristics of circulation background and mesoscale. In addition, the causes of heavy snowfall processes under the influence of the southern branch trough are discussed based on the dual-polarization radar products at Changsha station. The results show that two processes are characterized by the rapid phase transformation of rain and snow, concentrated snowfall periods, and heavy snowfall at night. The short-term snowfall intensity of the “0206” process is greater than that of the “0128” process. The high-latitude blocking high of the “0206” process is stronger than that of the “0128” process, and the water vapor transport of the southerly jet in low levels in the “0206” process is also stronger. The organized development of cold cloud clusters from the meso-β scale to the meso-α scale indicates that the snowfall intensifies, and the maximum blackbody temperature gradient corresponds well to the center of heavy snowfall. The propagation that is similar to the train effect is an important reason for the heavy snowfall process. The vertical variation of the Z_H and the bright band of dual-polarization parameters can determine the phase transformation between rain and snow. When the Z_H and Z_DR bright bands are 1–3 km away from the ground, the phase state is rain if the Z_H near the ground is greater than 0 dBZ and the CC is close to 1; the phase state is the rain-snow mixed phase if the CC is less than 0.95. When the bottom of the Z_H bright band decreases, the CC/Z_DR bright band disappears, the near-surface CC is greater than 0.99 and the Z_DR is less than 1 dB, the rain turns to snow. Compared with the “0128” process, the characteristics of the bright ring during the rainfall period of the “0206” process are more obvious, the precipitation intensity judged from the larger Z_H and K_DP is larger, and the phase transformation is faster due to more significant cooling effect caused by precipitation.