关键词: 青藏高原, 降雹日数, 集中度和集中期, 气候变暖

Abstract:

The study of hail days over the Qinghai-Xizang Plateau plays a crucial role in regional hail disaster mitigation and response strategy formulation. Based on observational data from 89 meteorological stations over the Qinghai-Xizang Plateau and ERA5 reanalysis datasets from the European Centre for Medium-Range Weather Forecasts (ECMWF) in the warm season (May-September) from 1969 to 2024, this study examined the spatiotemporal variation of hail days and the causes of the reduction of hail days using methods such as linear trend estimation, concentration degree and concentration period analysis, and correlation analysis. The results show that the number of hail days has decreased significantly at a rate of 1.7 d·(10 a)^-1 in the warm season over the Qinghai-Xizang Plateau from 1969 to 2024, and has remained consistently below average, especially since 2008. Hail events are frequent in the high-altitude regions of the central Qinghai-Xizang Plateau, while low-frequency areas are scattered across the Qaidam Basin, the Hehuang Valley, and the plateau margins. Moreover, the rate of decrease of hail days accelerates with increasing altitude. Over the past 56 years, hail concentration degree has increased, and the concentration period has been delayed over the Qinghai-Xizang Plateau. Especially since the beginning of the 21st century, the concentration period has been significantly delayed further, and interannual variability has increased. Hail events have become more concentrated in low-altitude areas, whereas the concentration period occurs later in high-altitude regions. Under climate warming, enhanced mid-to-upper tropospheric warming has significantly elevated the 0 °C and -20 °C isotherms. With the -20 °C isotherm rising more rapidly, the upward shift of these critical levels has compressed the vertical growth space of hail embryos and has intensified hail melting during their descent. Concurrently, the marked rise in near-surface minimum temperature and the reduction in diurnal temperature ranges have impaired the atmospheric energy accumulation process. The number of days with moderate dew point depression (5-15 °C) has decreased, thereby further suppressing the favorable surface conditions required for hail occurrence. Since the 21st century, the atmospheric profile over the Qinghai-Xizang Plateau has shown strong low-level moistening but weak upper-level moistening. In the mid-lower troposphere, the pseudo-equivalent potential temperature has increased significantly, with the greatest amplification in the lower layers near the hail-freezing layer (600-500 hPa). In contrast, the atmospheric stability in the hail growth layer (400-300 hPa) has tended to increase. These combined factors have suppressed hailstorm development. Overall, the reduction in warm-season hail over the Qinghai-Xizang Plateau results from the coordinated changes in atmospheric thermal structure, stability, and moisture conditions under climate warming.

Key words: Qinghai-Xizang Plateau, hail days, concentration degree and concentration period, climate warming