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.
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.
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.
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.
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.
In order to get a deeper understanding of the water vapor characteristics and sources of rainstorms in semi-arid areas in Northwest China, and improve the ability of rainstorm forecasting in this area, based on upper-air and surface observation data and European Centre for Medium-Range Weather Forecasts (ECMWF) fifth-generation global atmospheric reanalysis (ERA5) (0.25 × 0.25),the characteristics of water vapor transport and budget of two large-scale rainstorm processes with different intensities occurring in northern Shaanxi under different circulation on July 11 and August 9, 2022 were analyzed. Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model was used to quantitatively analyze the source and contribution rate of water vapor. The results show that the high level trough, low level shear line and vortex and low level jet were the main influence systems of the rainstorm occurring on July 11. 700 hPa cyclonic convergence and 850 hPa low vortex strengthened and moved slowly, causing regional rainstorms. The short-wave trough and low-level shear line were the main influence systems of the rainstorm process on August 9, and the secondary circulation on both sides of the shear line lifted the warm and humid air flow outside the West Pacific Subtropical High (referred to as "West Pacific Sub-high") triggering the release of unstable energy and forming a large-scale convective rainstorm weather. On July 11, the vertically integrated water vapor fluxes from ground to 300 hPa was stronger, 700 hPa southwest jet and the 850 hPa southeast jet formed two obvious water vapor transport belts. The strong convergence lasted longer, the wet layer was deep and the weather process was dominated by stable precipitation. On August 9, under the control of subtropical high, the atmosphere over northern Shaanxi had high temperature and humidity, and the total amount of precipitable water was large. The water vapor transport was weak and the strong convergence maintenance time was short, the wet layer was thinner, but the energy was sufficient, the weather process was dominated by convective precipitation. On July 11, the water vapor net income mainly came from the ground to 500 hPa, of which accounted for 52% during 800-500 hPa. The water vapor income of the eastern boundary below 800 hPa increased rapidly during the precipitation intensification stage and the combination of increased zonal income and strong meridional income keeps the regional net income at a high value, resulting in regional heavy rain. On August 9, almost all of the water vapor net income came from meridional income, and the water vapor net income mainly came from the ground to 800 hPa (accounting for 88%). The 700 hPa shear line moving southward and the convergence in the north of Yulin increased, reduced the outflow of the southerly wind, significantly increased the meridional income of water vapor, and strengthens the rainstorm. Water vapor backward trajectories with HYSPLIT model showed that on July 11, water vapor mainly came from tropical oceans, and the South China Sea contributed the most, the local and surrounding near surface atmosphere with high specific humidity also contributed significantly. On August 9, the water vapor mainly came from the high specific humidity atmosphere in the near-surface layer of the inland, followed by the South China Sea.
