In August 2017, there were torrential rains in Longnan, Gansu Province. The 24-hour precipitation at the Li County and Wudu weather stations broke through the historical extreme values, with obvious extremes and localities. Based on ERA5 reanalysis data of the European Centre for Medium-Range Weather Forecasts, radar data and ground observation data, a comparative analysis of two heavy rain cases that occurred in Longnan, Gansu Province from 6 to 7 and from 19 to 20 August 2017 are carried out. The circulation background and the radar reflectivity factor, radial velocity and physical quantity characteristics of the heavy rainfall processes of the two cases are discussed emphatically. The results show that the two rainstorms all occurred at the intersection between the northerly airflow in the westerly trough and the southerly warm and humid airflow in the middle-lower layer, but the main impact systems and triggering conditions are different. The radar echo shows from August 6 to 7, the convective system caused by the cold shear line is stronger, with higher reflectivity factor, lower central height, higher precipitation rate and shorter duration. The reflectivity factor of precipitation in warm region from August 19 to 20 is lower, and its central height is higher, and the precipitation rate is smaller, and the precipitation process maintained for longer time.

Based on the hourly precipitation data from 302 regional automatic weather stations in the Loess Plateau arid region of eastern Gansu from 2013 to 2020, digital elevation model data from shuttle radar topography mission and ERA5 reanalysis data of European Centre for Medium-Range Weather Forecasts, etc., the spatio-temporal distribution characteristics of short-term heavy rainfall are analyzed, and on this basis that the relationship between it and topography, geography factors is discussed. Then combined with an extremely short-term heavy rainfall event in 2021, the influence mechanism of terrain is summarized. The results are as follows: (1) The short-term heavy rainfall mainly occurs in summer in the Loess Plateau arid region of eastern Gansu, and in July the proportion of short-term heavy rainfall days （35.9%） is the most and the extremity is the strongest, while in August the proportion of short-term heavy rainfall times (46.9%) is the most and the rainfall intensity is the strongest. The rainfall intensity mainly ranges from 22.0 to 31.0 mm·h^-1, and it shows a multi-modal diurnal distribution with the most active, strongest and most extreme from 17:00 BST to next 00:00 BST, whose proportion of times is 56.8%. (2) The spatial distribution of the occurring times and hourly precipitation extremum of short-term heavy rainfall events is extremely uneven. The occurring times is less in the northwest and more in the southeast, it decreases sharply with the increase of rainfall intensity, and the short-term heavy rainfall occurs frequently in the area with valley bell-mouth topography, moreover, the palm landform is also the high incidence area of heavy precipitation above 30.0 mm·h^-1. The extremum is small in the middle and large in the northeast and southwest, the large value mostly distributes in eastern Qingcheng and western Heshui. (3) The influence of geographical and topographic factors on occurring times of short-term heavy rainfall is significant, and its contribution comes from geographical location, while their influence on precipitation extremum isn’t obvious. In general, the topographic forced uplift isn’t main influence mechanism on short-term heavy rainfall in the Loess Plateau arid region of eastern Gansu. (4) The mountain-valley wind circulation and surface mesoscale convergence line induced by it are important inducement for the formation of short-term heavy rainfall in valley bell-mouth topography area in the Loess Plateau arid region of eastern Gansu.

A torrential rain struck Qingyang on July 15, 2022, which is located in semi-arid region of eastern Northwest China, causing the daily rainfall and hourly rainfall at several stations to exceed the historical extreme values. The formation mechanism of the torrential rain in semi-arid region of Northwest China is analyzed based on multi-source observation data and ERA5 reanalysis data, so as to provide some useful reference for rainstorm forecast in arid and semi-arid areas. The results show that the torrential rain process occurred under the background of weak synoptic scale baroclinic forcing, weak unstable energy and deep wet layer in the complex terrain of the Loess Plateau, with characteristics of strong locality and long duration of heavy precipitation, which is a warm-sector torrential rain. The special circulation configuration of South Asian high, western Pacific subtropical high and pressure system at the lower level is conducive to the occurrence and development of mesoscale convective system. Convective initiation and development were triggered by surface wind convergence line and low-level southerly jet. Development and long-time maintenance of the low-level jet intensified surface convergence line continuously. The left side of low-level jet (rainstorm area) formed two stable secondary circulations with the right side of the exit and entrance of it, respectively, which is the key factor for the maintenance of the convective system. The release of condensation latent heat caused local frontogenesis and low-level positive vorticity development, which is another important factor for development and maintenance of convective systems, and it is also an important reason for maintenance of atmospheric instability. The mesoscale convective system exhibited deep, low center of mass and quasi-stationary characteristics under the combined effects of the above mentioned factors, the radar echoes were characterized by backward propagation and train effect.

Extreme precipitation events in arid areas often lead to huge casualties and economic losses, the study on its evolution characteristics and formation mechanism can provide an important support for improving the accuracy of weather forecast. A rainstorm process occurred on 13 August 2022 in Jinta County of Gansu Province, which was located in arid region of Northwest China. Both daily precipitation and hourly precipitation broke through the historical extreme value at national meteorological station Hexi Corridor, and their extreme and local characteristics were significant. European Centre for Medium-Range Weather Forecasts (ECMWF) fifth-generation global atmospheric reanalysis (ERA5) and observation data were used in this paper to analyze the causes of the rainstorm. The results show that the rainstorm occurred in the north side of the stable South Asia high, and the dynamic forcing in the upper and middle level of troposphere was weak. The baroclinic system, the lower level shear line and surface cold front in front of the 500 hPa short-wave trough, was mainly located in the lower level. The continuous transport of low-level water vapor around the thermal over the Qinghai-Tibet Plateau provided the extreme water vapor condition and the moderate intensity stratification instability for the rainstorm area. In front of the formation of surface cold front, the regional difference of low-level water vapor transport in central and eastern parts of Jiuquan City formed an obvious wet frontal and dryline. The meso-γ-scale convective system which caused extreme short-term heavy precipitation was triggered by the dryline, and developed into deep moist convection leading to extreme heavy rain at the intersection point of the cold front and the dryline. The local characteristics were significant during the development of the dryline convective cells to deep moist convection.

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.