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 rainfall station observations and the products of Multi-source Merged Precipitation Analysis System of China Meteorological Administration (CMPAS), eight kinds of satellite-based precipitation products (FY-4A, CMOPRH-RT, IMERG-Early, IMERG-Late, GSMaP-Now, GSMaP-Gauge, PERSIANN-Now, PERSIANN-CCS) are comprehensively evaluated during the record-breaking extremely heavy precipitation process in East Gansu on July 15, 2022 by using quantitative analysis, classification and structural similarity methods. The results show that eight kinds of satellite-based precipitation products basically reflect the spatial distribution characteristics of precipitation with more in the central and eastern regions and less in the northwest. Except for the GSMaP-Now product, the other seven satellite-based precipitation products all underestimate the precipitation at the center of the rainstorm. The eight kinds of satellite-based precipitation products have a good ability to describe the peak value of heavy precipitation, and both peak stages of the heavy precipitation process are reflected, but all of them seriously underestimate the magnitude of heavy rainfall and above. For precipitation of different magnitudes, the GSMaP-Gauge is the best for estimating precipitation of magnitude below torrential rain, while the CMOPRH-RT is the best for heavy rain and above, and all products cannot correctly hit the precipitation of torrential heavy rainfall. In terms of the structural similarity index, the CMOPRH-RT product can best represent the structural distribution of the precipitation process from three aspects of total precipitation, precipitation magnitude, and precipitation morphological distribution. In summary, for this precipitation event, the CMOPRH-RT precipitation product had the best performance in all aspects.

Precipitation is closely related to the occurrence and development of clouds. The study for the characteristics of convective cloud is of great significance to the precipitation monitoring and prediction, as well as researches of precipitation mechanism. A torrential rain struck Qingyang on15 July 2022, which is located in semi-arid region of Northwest China, causing the daily rainfall and hourly rainfall at several stations to exceed the historical extreme values. Based on products of the advanced geostationary radiation imager (AGRI) from FY-4A and FY-4B geostationary satellite, fusion products of micro-wave humidity sounder (MWHS) and micro-wave temperature sounder (MWTS) from FY-3D polar orbit satellite, the cloudsytem evolution, macro and micro characteristics of cloud, and the atmospheric environmental conditions during this torrential rainstorm event were analyzed. The results are as follows: (1) The rainstorm cloud top types formed from supercooled water clouds, mixed clouds, opaque ice clouds and multi-layer clouds. The cloud top types of heavy precipitation are mainly opaque ice clouds, and the height of cloud top is more than 14 km. The rainstorm cloud system is deep and mainly composed of small ice particles, and accompanied by strong updraft. (2) In this rainstorm event, there exist a process of convective cloud formation, merging and strengthening, and the continuous influence of convective clouds leds to the occurrence of extremely heavy rain. Precipitation was closely related to the black body temperature (TBB) of cloud top and its variation. Low TBB corresponded to heavy precipitation, and the TBB drops rapidly before the heavy precipitation. (3) Before the occurrence of heavy precipitation, atmospheric stratification is shallow convective instability. The strong humidification in the middle and lower layers is the main reason for the development of convective instability. The difference of absolute humidity in the lower layers leads to the difference of instable conditions and precipitation intensity.

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.

An extremely heavy rainstorm occurred on 14-15 July 2022 in Longdong, Gansu Province, which is located in semi-arid region, the daily rainfall and hourly rainfall intensity at several stations broke the historical extreme values. Based on the minutely and hourly precipitation observation data from 20:00 BST 14 to 20:00 BST 15 July 2022, the daily and hourly precipitation data from 192 regional meteorological stations since the establishment of them in Qingyang of Gansu Province, and the CMPAS (CMA Multi-source Merged Precipitation Analysis System) 10-minute and hourly precipitation data, the precipitation characteristics and extremity of the torrential rain event are analyzed. The results show that the extremely torrential rain event is characterized by large cumulative precipitation, concentrated rainfall area of rainstorm and above, strong convective precipitation, stable and less movement of the rainfall centers, and long duration of short-term strong precipitation, among which the extremities of cumulative precipitation, hourly precipitation intensity and short-term heavy precipitation duration are obvious. The heavy rainstorm center, Zhaijiahe station, is located in the middle and north part of Qingcheng County, the maximum accumulated rainfall is 373.2 mm, which is the maximum since the station establishment, and is nearly twice of daily precipitation extreme values at all national stations in Gansu Province. The maximum hourly rainfall is 84.9 mm, ranking the third in historical records at all stations in Qingyang, and the longest duration of short-term strong rainfall is as long as 6 hours, which is the maximum in historical records at all stations in Qingyang since establishment of them. The evolution of minutely precipitation shows that the precipitation intensity in the heavy precipitation center has pulsating change, and the precipitation location has swing in the east, west, south and north, but the swing amplitude is small.

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.