利用常规高空资料、地面加密自动站、双偏振多普勒天气雷达、微波辐射计与ERA5再分析数据等多源资料,分析了2021年2月下旬黄河中下游两次暴雪过程的相态演变及形成机理。结果表明:两次过程的大尺度影响系统基本一致,只是影响系统的强度和位置不同导致两次过程存在些许差异。两次过程均存在相态转换,过程Ⅰ中存在相态逆转(由雨转雪再转雨),而过程Ⅱ中只存在雨转雪的转换。在太行山以西的山区,当地面2 m气温低于0.5 ℃时,降水相态以雪为主,在0.5~1 ℃之间时,多为雪或雨夹雪并存;在平原地区,当2 m气温为1~2 ℃时,降水相态为雨或雨夹雪,在0~1 ℃之间时,则为雪与雨夹雪并存,低于0 ℃时,降水相态为雪。在降雨阶段,双偏振雷达产品相关系数(CC)值约在0.98以下,差分反射率(ZDR)在0.6 dB以上,差分传播相移率(KDP)值约在0.2 (°)·km-1以上;在降雪阶段,CC值在0.98~0.99之间,ZDR值在0.2~0.8 dB之间,KDP 值约在0.2 (°)·km-1以下;但在降水相态由雨转雨夹雪时,ZDR、CC与KDP 值没有明显变化。
The phase evolution and formation mechanism of two snowstorm processes in the middle and lower reaches of the Yellow River in late February 2021 are analyzed by using multi-source observation data including conventional high-altitude data, data of densified ground automatic stations, dual-polarization Doppler radar, and microwave radiometer as well as ERA5 reanalysis data. The results show that the large-scale synoptic systems of the two processes are basically the same, but the intensity and position of the systems are different, leading to some differences between the two processes. There are phase transitions in both processes, phase reversal (from rain to snow and then to rain again) occurs in Process Ⅰ, while only the transition from rain to snow occurs in Process Ⅱ. In mountainous areas west of Taihang Mountain, the precipitation phase is mainly snow when the temperature at 2 m is lower than 0.5 ℃, and snow or sleet coexist in the range between 0.5 ℃ and 1 ℃; while in plains, the precipitation phase is rain or sleet when the temperature at 2 m is between 1 ℃ and 2 ℃, snow and sleet coexist in the range between 0 ℃ and 1 ℃, and the precipitation phase is snow when the temperature is lower than 0 ℃. In the rainfall stage, the dual-polarization radar correlation coefficient (CC) is below 0.98, the differential reflectivity (ZDR) is above 0.6 dB, and the specific differential phase shift (KDP) is above 0.2 (°)·km-1; while in the snowfall stage, CC is between 0.98 and 0.99, ZDR is between 0.2 dB and 0.8 dB, and KDP is below 0.2 (°)·km-1; however, when the precipitation phase changes from rain to sleet, the values of ZDR, CC, and KDP do not change significantly.