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Under the influence of the stable snowstorm circulation situation of the typical double-resistance type warm area in northern Xinjiang, an extremely heavy snowfall event occurred in Altay, from January 6 to 12, 2024. Several meteorological stations broke the historical maximum records for the same period, resulting in avalanches in several places of the Altay Mountains. This event can be divided into three stages according to the evolution of circulation situation and snowfall characteristics. In this study, the snowfall characteristics, circulation situation, extreme causes, and model forecasting performance of the event were analyzed using the hourly observation data of national and regional meteorological stations, ERA5 reanalysis data, and CMA-GFS, CMA-MESO, CMA-TYM, ECMWF model forecast data. The results are as follows: (1) The event was characterized by several extreme features, such as long duration, large accumulated precipitation, and large hourly precipitation. The precipitation distribution was increasing from the plain areas to the mountain areas. The strong snowfall process mainly concentrated in the second and third stages. In these two stages, the increase in snow accumulation, large hourly snowfall, and high snow-to-water ratio in the mountainous areas along the northern Altay provided key meteorological conditions for the avalanches. (2) During this event, a strong polar front at 500 hPa continuously split short-wave systems, which successively moved eastward to affect Altay. The warm shear line at 850-700 hPa was maintained for a long time, accompanied by strong warm front frontogenesis. Heavy snowfall occurred in the divergence area at the exit of the polar front jet at 300 hPa, from the north of the shear line at 850 hPa to the regions near the shear line at 700 hPa. (3) In the second and third stages, the water vapor conditions in the north of Altay, especially in the northern mountains, were abnormally strong. With the effect of terrain amplification, the low-level dynamic uplift was exceptionally strong and maintained for a long time. These were the important reasons for the extremely heavy snowfall. (4) The regional model performed better than the global model in predicting heavy snowfall, but the capability to forecast extreme snowfall was still limited. ECMWF Extreme Forecast Index (EFI) forecast product and the Ensemble Average Abnormal Weather Forecast (EMAF) product were better at identifying extreme weather signals at medium-range lead time. It is suggested to strengthen the application of related products and make adjustments according to the quantitative forecast results of the model to further improve the precision service effect of forecasting and warning of extreme weather events.

Understanding the relationship between hydrological and sediment connectivity under extreme rainfall conditions is of great significance for water and sediment regulation and high-quality development of soil and water conservation in the basin under the new situation of vegetation restoration in the Loess Plateau. In this study, the influence of extreme rainfall and vegetation restoration on water-sediment connectivity and the characterization ability of sediment transport potential was analyzed based on the index of connectivity, the structural sediment connectivity (SCst) improved by topographic location index and vegetation cover factor, the functional hydrological connectivity (HCfn) improved by the cumulative surface runoff depth factor. The results showed that the rainfall and vegetation were the main functional and structural factors affecting hydrological and sediment connectivity, respectively. The mean value of normalized difference vegetation index (NDVI) increased by 0.41 from 2000 to 2020, while the mean value of sediment connectivity decreased by 3.54, with a negative correlation coefficient of −0.93. The spatio-temporal distribution of rainfall was in good agreement with the spatio-temporal distribution of hydrological connectivity, both of which were highly correlated on the monthly scale. When extreme rainfall occurred, rainfall runoff factors played a dominant role and determine hydrological and sediment connectivity. The improved water-sediment connectivity index was well applied to the study of large-scale watershed connectivity in the Loess Plateau. Pearson correlation analysis showed that the functional hydrolodical connectivity had a good correlation with runoff and sediment transport capacity in the basin under the inter-annual and seasonal variation, and could better reflect the phenomenon of runoff and sediment increase in the basin under extreme rainfall. Compared with previous studies in which functional connectivity was calculated by using factors such as rainfall erosivity under annual changes, which could not well characterize the actual sediment transport capacity of the basin, the functional connectivity used in this study had obvious advantages, which would provide a more direct and effective tool for water and sediment management in large basins on the Loess Plateau in the future. The interaction between basin structure and functional connectivity changed significantly with seasons. In dry season, vegetation cover was low, rainfall was low, structural sediment connectivity was high and HCfn was increasing. At this time, SCst had a weak influence on HCfn, and the two showed a high negative correlation. In the wet season, with sufficient rainfall and lush vegetation, the correlation between structure and work connectivity changed with the trend of rainfall. At this time, the SCst was low, which limited the function of HCfn to a certain extent. When encountering extreme rainfall, high-intensity rainfall runoff played a dominant role, and SCst couldn’t significantly affect HCfn, while HCfn changed SCst in turn, and both increased at the same time, showing a strong positive correlation. In the normal water season, vegetation gradually withered due to the weakening of rainfall intensity, SCst increased while HCfn decreased, and the negative correlation between the two was enhanced. In conclusion, the improved water-sediment connectivity index has certain advantages and limitations, which can provide guidance for water and sediment management and ecological construction of large watershed in the Loess Plateau under the changing environment in the new era. [ABSTRACT FROM AUTHOR]

Based on daily temperature data of 86 weather stations during winter producing season from 1961 to 2017 in Guangdong Province, the temporal and spatial distribution of extreme chilling was analyzed. The return periods of extreme chilling at county level were calculated with the theory of generalized extreme value distribution to provide technical support for extreme chilling risk assessment and early warning together with real-time meteorological disaster prevention and mitigation in winter planting areas. Results showed that there was a general downward trend of the extreme values of chilling accumulation in main winter planting zones. Zhanjiang and Meizhou had declined significantly since the mid-1980s, while Maoming, Shaoguan and Guangzhou had declined significantly since the 21st century. The extreme minimum temperature followed a curve trending generally downward first and upward later on, with a significant rising in Shaoguan from later 1980s while a descending in Guangzhou in the last five years. The order of the number of extreme value stations of chilling accumulation was 1970s ＞ the 21st century ＞ 1990s ＞ 1960s ＞ 1980s. The order of the number of stations of extreme minimum temperature was 1960s ＞ 1990s ＞ the 21st century ＞ 1970s ＞ 1980s. The theoretical forecast values of cold disaster of 2, 5, 10, 20, 50, and 100 year return period showed obvious zonal distribution characteristics. The chilling accumulation diminished from the north to the south and the extreme minimum temperature was higher in the south and lower in the north, which was consistent with the distribution trend in the actual occurrence of cold damage. Typical cases showed more extended chilling return periods in the south than in the north. In Zhanjiang and Maoming, the main producing areas of winter crops in Guangdong Province, the return period of cold damage was long, the probability of extreme cold damage was small but the damage was heavy, and the area of winter crops was in the front rank, which should be paid attention. Our results could provide scientific reference for local government and relevant departments in Guangdong Province to guide winter planting and develop countermeasures against climate change.