Based on hourly precipitation data at 146 automatic weather stations of Lanzhou from 2012 to 2019, the refined characteristics of precipitation in Lanzhou were analyzed from different time scales. The results are as follows: (1) The average annual precipitation was less in the north and more in the south of Lanzhou, and that was more in the edge and less in the interior from 2012 to 2019. The inter-annual change of precipitation was obvious from 2012 to 2019, the precipitation in 2018 was abnormally more by 46%, while that in 2015 and 2017 was abnormally less, especially in 2015 it was less by 30%. (2) The precipitation mainly concentrated in July and August in Lanzhou, and it in the south was obviously more than that in the north due to the influence of atmospheric circulation situations, while the spatial difference of precipitation wasn’t obvious in other months. (3) The diurnal variation of rainfall was obvious in Lanzhou, the precipitation was less in morning and more at night, and the range of rainfall was larger at night and smaller in the daytime. The precipitation in urban areas was generally less than that in mountain areas under the influence of altitude, and it mostly concentrated from afternoon to evening due to heat island effect, the convective rainfall was more, while the diurnal distribution of rainfall was more uniform in mountain areas, the fluctuation was smaller as a whole. (4) Overall, the frequency of short-time heavy rainfall in Anning district of Lanzhou was the highest, but the short-time heavy rainfall at Liuhe station of Gaolan county and Xujiamo station of Yongdeng county was the most frequent, and that in Yongdeng county was a critical concern.

Based on the hourly precipitation data at 81 national meteorological observation stations of Gansu Province from 1981 to 2018 and NCEP reanalysis data, the climate and circulation characteristics of extreme rainstorms were emphatically analyzed in different falling areas of Gansu Province. The results are as follows: (1) The extreme rainstorm weathers occurred mainly in Longnan, Tianshui, Pingliang and Qingyang of eastern Gansu, and the heavy rainfall centers concentrated in Kangxian and Huixian of Longnan. The extreme rainstorms were classified into four types including eastern Gansu, southern Gansu, southeastern Gansu and dispersion patterns, according to the falling areas of rainstorms. (2) The extreme rainstorms were easily to occur in July and August in Gansu, especially in mid-August. The extreme rainstorms in southern Gansu were earlier than in eastern Gansu. The precipitation of extreme rainstorms at night was more than in the daytime as a whole, the night rain characteristic was remarkable in Gansu, especially in southern Gansu and southeastern Gansu. In additional, the convective characteristic was significant in Gansu. (3) There were 2.5, 5 and 10 years period of extreme rainstorms in Gansu during 1981-2018, and the 2.5-year periodic oscillation was obvious. (4) The extreme rainstorms in Gansu were correlated with the subtropical high, and the falling area of rainstorm was significantly related to the location of subtropical high. Moreover, the extreme rainstorms in eastern Gansu were also related to the easterly airflow at the bottom of northern high ridge, the extreme rainstorms with dispersion pattern were related to the tropical low pressure in South China Sea, while the extreme rainstorms in southern and southeastern Gansu depended on the intensity and location of short-wave trough in Tibet Plateau.

Abstract: The two regional shorttime heavy precipitation processes occurred in southeastern Gansu on April 19-20, 2019 (hereinafter using abbreviation “4·19”) and on April 26-27, 2019 (hereinafter using abbreviation “4·26”). Based on the conventional meteorological observations and ERA5 reanalysis data, the dynamic characteristics of two processes were analyzed through the diagnosis of physical quantities including frontogenesis function, temperature advection and vertical wind shear. The results are as follows: (1) The two processes belonged to baroclinicfrontogenesis severe convection, in which cold advection existed in the upper air, cold and warm air converged in low level, and obvious cold fronts and cold shear lines existed on the ground. (2) Under the same significant baroclinic atmospheric conditions, the dynamic characteristics of the two processes were significantly different. “4·19” process presented the characteristics of frontal precipitation, in which the cold air was relatively deep, strong and cold advection in the middle and low layers drove the lowlevel frontogenesis, and convection occurred under the dynamic unstable conditions forced by uplift of the front. However, “4·26” process had obvious characteristics of convective precipitation, in which cold air was diffused in the lower and near ground layers, and convective activity occurred under the combined action of near ground layer frontogenesis and lowlevel jets, and it was dominated by thermal instability. (3) The frontogenesis functions on 700 hPa and 850 hPa could quantitatively describe the temporal and spatial distribution and evolution characteristics of affecting systems in the lowlevel and near ground layer. Since the near ground layer trigger system was difficult to define due to the influence of terrain, especially in “4·19” process the pathes of cold air in the lower layer and near ground layer were complicated, the frontogenesis function could be used as a physical quantity index for the near ground layer trigger system.

Based on conventional meteorological observation data, FY4A satellite data, vegetation cover data and reanalysis data, the formation mechanism, transmission characteristics and influencing factors of a largescale severe dust storm occurring from March 14 to 18, 2021 in northern China were analyzed. The results are as follows: (1) Drought, high  temperature, less rainfall and low vegetation coverage in the earlier period in most parts of northern China provided a good sand source condition for occurrence of the strong sand dust storm weather. (2) The dust weather was caused by the strong development of Mongolia cyclone and the combined action of cold high pressure behind it. (3) The dust weather process was divided into two stages. At the first stage it was caused by the strong winds passing behind the ground cold front, which mainly occurred in North China and Northeast China. At the second stage it was caused by the southward diffusion of highaltitude dust that did not dissipate at the early stage and the backflow of east wind, which mainly occurred in Northwestern China. (4) The analysis of physical quantities such as horizontal helicity, the mixing layer height and vertical velocity showed that the dust emission conditions were insufficient in most areas of China, so the sand dust weather was mainly affected by the upstream sand dust transportation. For the eastern region of China, the contribution of dust storm was mainly from the southern part of Mongolia. While in Northwest China it was mainly contributed by Mongolia and its own dust.