Drought disaster is one of the most frequent meteorological disasters in the Gannan Plateau, which seriously affects agricultural and animal husbandry production and ecological environment security in this region. Monthly precipitation and air temperature data from 31 meteorological stations in the Gannan Plateau and its surrounding areas from 1973 to 2022 are used to characterize meteorological drought employing the Standardized Precipitation Evapotranspiration Index (SPEI), and the temporal and spatial distribution of drought and its variations on annual and seasonal scales in the Gannan Plateau are analyzed by using Mann-Kendall test and Sen’s slope estimation methods. Results show that the annual SPEI in the Gannan Plateau presented significant downward trend with an obvious turning point in 1986, and the whole Gannan Plateau tended to be dry in the past 50 years. There were seasonal differences in the variation trend of drought, and the trend of drought intensified in summer and autumn, but in spring and winter it mitigated. In addition, there were spatial differences in the trend of annual and seasonal SPEI. In summer, it presented drought intensification trend in the middle and eastern regions of the Gannan Plateau, and in spring it showed similar to that in summer, but the area and degree of drought intensification were obviously smaller than that in summer. While in winter, it showed drought decreasing trend in the whole region. There were obvious spatial differences in the frequency of drought with different levels in the Gannan Plateau at the annual and seasonal scales. Light drought occurred frequently in the central and eastern parts of the Gannan Plateau, while medium and severe drought occurred frequently in the southern part of the Gannan Plateau, and the frequency of serious drought was less across the whole regions. Overall, the frequency of drought in the western was less than that in the central and eastern parts of the Gannan Plateau.
Based on the hourly precipitation data from April to September during 1960-2019 at 9 national meteorological stations and 400 regional meteorological stations built year by year in Longnan of Gansu Province from 2008 to 2019, NCEP FNL 1°×1° reanalysis data and MICAPS data, the spatial-temporal distribution and mesoscale characteristics of short-time heavy precipitation in Longnan of Gansu Province were analyzed. The results are as follows: (1) The occurrence frequency of short-time heavy precipitation in Longnan became more from the northwest to the southeast, with two relatively concentrated areas. The short-time heavy precipitation with rainfall intensity greater than 50 mm·h-1 occurred in Cheng county, Hui county and Kang county in the east of Longnan. The occurrence frequencies of short-time heavy precipitation and rainstorm were more in the southeast of Longnan, and for short-time heavy precipitation it was also relatively high in the mountainous areas in the northwest. (2) Since 1960, the stations occurring short-time heavy precipitation in Longnan increased slowly.The monthly variation showed a single-peak type, with the maximum in August and accounting for 37.5% of the total stations occurring short-time heavy precipitation. The ten-day variation presented a double-peak type, with two peaks in early July and early August, respectively. The stations occurring short-time heavy precipitation was the most from late July to mid-August, accounting for 47.2% of the total stations. Diurnal variation showed that there was more short-time heavy precipitation at night than in the day, there were multiple peaks. The stations occurring short-time heavy precipitation increased significantly since 15:00 BST. The diurnal peaks of short-time heavy precipitation occurred at 23:00 BST, accounting for 9.4% of the total stations. (3) The short-time heavy precipitation in Longnan was closely linked to rainstorms. The mesoscale concept modes of short-time heavy precipitation in Longnan mainly showed three types, including low vortex shear, northwest air flow following the trough moving eastward and the southwest air flow beside the sub-tropical high.
The climatological features of the winter snow depth over the Tibetan Plateau and the summerprecipitation in China are diagnosed by use of the datasets obtained from the 78 snow observation stations and 160 rainfall stations during 1957 to 1998. Climatic effects of snow anomaly over the Tibetan Plateau on the regional summer monsoon climate in China are diagnosed and numerically simulated by use of a regional climate model (RegCM2).The SVD technique is adopted to diagnose the relationships between the prior winter and spring Plateau snow depth anomalies and the firing and summer regional precipitation in China. It is found that the snow depth anomaly especially in winter is one of factors influencing precipitation in China;however.it isperhaps not the unique one and even not the most important one. Nevertheless .it is proved that the winter snow anomaly over the Tibetan Plateau is relatively more important than the spring one for the regional precipitation in China. Results of numerical simulations show that the snow anomaly over the Plateau has evident effects on China's summer monsoon climate. The increase of both snow cover and snow dq}th can delay the onset and weaken the intensity of summer monsoon obviously .resulting in the decrease of precipitation in south China and the increase in the Yangtze and Huaihe River basins. The influence of the winter snow dq}th is more substantial than that of both the winter snow cover and the firing snow depth. The mechanism of how the Plateau snow anomaly influences the regional monsoon climate is briefly analyzed. It isfound that snow anomalies over the Tibetan Plateau change the soil moisture and the surface temperatune through melting process of snow at first .alter heat .moisture and radiation fluxes from the surface to the atmo}here at the meantime. Abnormal circulation conditions induced饰changes of surface fluxes may affect the underlying surface properties in turnSuch a long time interaction between the wetland and the atmo}here is the key process resulting in later climate changes.
