Characteristics of synoptic background and microphysics for hail events in southern Yunnan

Based on synoptic analysis and dual-polar radar and disdrometer observations, synoptic background and microphysical characteristics of 16 hail events during 2022—2023 in Honghe, southern Yunnan are investigated. Results indicate that synoptic circulations for the formation of hail events in winter and spring are apparently different to that in summer in southern Yunnan. The synoptic patterns for hail events in winter and spring are closely associated with wave perturbations in the southern breach of the westerly trough induced by the Qing-zang plateau and the South Asia high, while those in summer are closely related to the upper-level anticyclonic circulation over the Qing-zang plateau and the South Asian monsoon circulation, which are favorable for the strengthening of unstable stratification and increase of water vapor required for the occurrence of hailstorms in this region. Hailfall primarily occurs in the afternoon with dominant sizes below 10 mm and the maximum size of 20—30 mm, indicating that strong solar radiative heating and inhomogeneous heating under complex orographic conditions in the afternoon might play a critical role in convection initiation. All hail clouds have a warm base and the temperature at cloud base ranges within 10℃ to 20℃. The thickness of warm layers is 1.8—3 km. The maximum cloud top is 15 km and the maximum reflectivity is 65 dBz. The dominant surface hail size is smaller than 10 mm with a maximum value of 20—30 mm. Polarization parameters show obvious seasonal characteristics. In the hail formation area with reflectivity larger than 50 dBz, the differential reflectivity (Z_DR) and specific differential phase (K_DP) are similar with general values of –2—0.2 dB and –0.8—0.5 °/km, respectively in winter and spring. However, correlation coefficient (CC) is higher in winter than in spring with values of 0.95—0.98 and about 0.93 respectively, indicating a decrease in CC in spring. These polarization parameters indicate that smaller-size 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 becomes more complicated as the weather becomes warmer. In contrast, the Z_DR and K_DP increase significantly in summer with values of –2—5 dB and –0.4— +2.4 °/km, respectively, but CC becomes smaller with the value of about 0.85, indicating that the composition of liquid-phase and ice-phase particles becomes more complicated and larger-size horizontal-oriented lump and discoidal hail and supercooled raindrops are dominant in the hail formation area. The high values of Z_DR and K_DP are related to large-size raindrops and strong melting process of hail. In addition, the maximum vertically 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 atmospheric circulations over the Tibetan Plateau and the freezing process of supercooled rainwater play a critical role in hail formation in southern Yunnan.