云南南部冰雹形成的天气背景与云微物理特征

Characteristics of synoptic background and microphysics for hail events in the south Yunnan

中文关键词:  冰雹事件,天气背景,微物理特征,云南南部

英文关键词:Hail events, synoptic background, microphysical properties, South Yunnan

中文摘要:

      基于天气、双偏振雷达和激光降水粒子谱仪等观测数据，研究了云南南部红河州2022~2023年期间16次冰雹过程形成的天气背景、大气层结和云微物理特征。结果表明，云南南部冬、春季与夏季冰雹形成的天气背景不同，冬、春季冰雹的形成主要与青藏高原盛行的南支西风槽波动和南亚副热带高压环流有关，而夏季冰雹主要与青藏高原高空反气旋性环流和南亚季风环流有关，这些环流背景有利于冰雹发生区域的大气不稳定层结增强和水汽增加。降雹主要发生在午后，可能与午后强烈的太阳辐射加热造成层结不稳定增强有关，另外复杂地形影响下的地表非均匀加热也有利于局地对流的触发。各季节的冰雹云均为暖底云，云底温度变化范围为10 ~ 20 ℃，暖层厚度范围为1.8 ~ 3 km，云顶海拔高度最大为15 km，最大回波强度为65 dBZ。地面冰雹尺度以10 mm以下为主，最大可达到20 ~ 30 mm。在雷达回波强度大于50 dBZ的冰雹形成区，不同季节的偏振雷达参数值存在明显特点，冬春季差分反射率（ZDR）和比相位差（KDP）比较相似，ZDR值一般在-2 ~ +0.2 dB范围，KDP在-0.8 ~ +0.5 o /km，但相关系数（CC）在冬季为0.95 ~ 0.98，而在春季减小为0.93，说明冰雹形成区以较小尺度的锥状、球状冰雹、过冷雨滴等混合粒子组成。随着季节变暖，冰水粒子组成趋于复杂化，导致CC减小。夏季ZDR和KDP 显著增大，分别为-2 ~ +5 dB和-0.4 ~ +2.4 o/km。但CC值进一步减小为0.85，说明夏季冰雹形成区冰水粒子组成更为复杂，以尺度比较大、水平取向更明显的锥状、盘状冰雹粒子和过冷雨滴组成。ZDR 和 KDP高值与大雨滴和冰雹融化过程有关。另外，最大垂直积分液态含水量（VILmax）值与云顶高度和地面降雹尺度也存在较好关系。本研究结果表明，青藏高原大气环流和过冷雨滴冻结过程在冰雹形成中具有重要作用。

英文摘要:

      On basis of synoptic, dual-pol radar and Disdrometer data, the synoptic and microphysical properties for 16 hail events during 2022 ~ 2023 in Honghe, southern Yunnan are investigated. Results indicate that the synoptic circulation for the formation of hail events in winter and spring had apparent differences in the south Yunnan. The synoptic pattern for hail events in winter and spring were closely associated with wave perturbations of southern breach of the westerly wind trough induced by the Tibet Plateau and the South Asia High, while those in summer were closely related to the upper-level anticyclonic circulation over the Tibet Plateau and the South Asia monsoon circulation, which were beneficial to the strength for unstable stratification and increase of water vapor required for the occurrence of hailstorms in this region. Hailfall primarily occurred in the afternoon with dominant sizes below 10 mm and maximum size of 20-30 mm, indicating that the strong solar radiation heating and inhomogeneous heating under complex orographic conditions in the afternoon might play a critical role in convection initiation. All hail clouds had a warm base and the temperature at cloud base was ranging from 10 to 20 ℃. The thickness of warm layers was 1.8-3 km. The maximum cloud top was of 15 km and reflectivity of 65 dBZ. The dominant surface hail size was less than 10 mm with a maximum value of 20-30 mm. There were some apparent properties in polarization parameters in different seasons. In the hail formation area with reflectivity larger than 50 dBZ, the differential reflectivity (ZDR) and specific differential phase (KDP) were similar with general values of -2 ~ + 0.2 dB and -0.8 ~ +0.5 o km-1, respectively, in winter and spring seasons. However, correlation coefficient (CC) was higher in winter than that in spring, with values of 0.95 ~ 0.98 and about 0.93, respectively, indicating a decrease in CC. These polarization parameters indicate that the smaller-sized conical, lump, discoidal and spherical hail, as well as supercooled raindrops should be dominant in hail formation area in winter and spring seasons, and the composition of liquid-phase and ice-phase particles could become complex as weather became warmer. In contrast, the ZDR and KDP increased significantly in summer with values of -2 ~ +5 dB and -0.4 ~ +2.4 o km-1, respectively, but CC became smaller with the value of about 0.85, indicating that the composition of liquid-phase and ice-phase particles became more complicated and the larger-sized horizontal-oriented lump and discoidal hail, and supercooled raindrops were dominant in the hail formation area. The high values in ZDR and KDP are related to the large-sized raindrops and strong melting process of hail. In addition, the maximum vertical integrated liquid water content (VILmax) is closely related to the cloud-top height and the hail size at the surface. This study indicates that the freezing process of supercooled rainwater play a critical role in the hail formation.