From 7 to 9 August 2023, heavy precipitation occurred in multiple areas of Northern Xinjiang. According to the cumulative process precipitation, 69 stations recorded rainfall reaching the heavy rainfall threshold, and 5 stations reached the torrential rainfall threshold, primarily distributed in the Ili, Bortala Prefecture, and Altay region (Figure 1). The primary and secondary precipitation centers were both located in the Altay region (Qibeiling Reservoir and Kala Wengger Village, abbreviation of Qibeiling and Kala stations respectively, hereinafter), with total accumulated precipitation amounts correspond to 83.0 mm and 51.8 mm (Figure 2b).

In terms of daily precipitation distribution, 27 stations within the study area recorded daily rainfall reached the heavy rainfall criteria, mainly concentrated in the Altay Region (16 stations) and Ili Prefecture (7 stations). The Qibeiling Station recorded 57.3 mm, classified as torrential rainfall. Additionally, short-duration intense rainfall occurred at multiple stations, predominantly in the Altay region (31 stations) and Ili Prefecture (10 stations). Stronger short-duration rainfall (≥20.0 mm) was primarily observed in the Altay Region (10 stations), with the maximum intensity recorded at Qibeiling Station (44.5 mm rainfall in 2 h) between 11:00 a.m. and 13:00 p.m. on 8 August (Figure 2c). Consequently, the Altay Region, where heavy rainfall and short-term heavy rainfall were most concentrated, was selected as the key research area.

The heavy rainfall area was mainly concentrated in the Altay region, which has only one sounding station (Altay Station). Therefore, the atmospheric stratification conditions at Qibeiling and Kala stations were analyzed using ECWMF model sounding grid data. As shown in Figure 6, at 06:00 GMT on the 8th, two representative stations show a clear warm advection below 500 hPa as the wind direction turns clockwise with height, while above 500 hPa, there is a cold advection with wind direction turns counterclockwise with height. This lower-level warm and upper-level cold stratification created conditions conducive to convective initiation and development. The representative stations have a high Freezing Level (FL) and equilibrium levels (EL), but low Lifting Condensation Level (LCL) indicating low cloud base heights, vertically extensive clouds, and a thick warm cloud layer. These characteristics are favorable for enhancing precipitation efficiency.

According to the stratification parameters, both stations exhibited K-index values exceeding 35°C, with Showalter Index (SI) and Lifted Index (LI), indicating a highly unstable atmosphere; strong Vertical Wind Shear (VWS) shows the presence of intense updrafts within the cloud cluster; the Convective Available Potential Energy (CAPE) at both stations exceeded 1000 J·kg⁻¹, while the Convective Inhibition (CIN) remained below 50 J·kg⁻¹. These metrics showed if sufficient triggering mechanisms were activated, this would lead to the release of unstable energy and the development of severe convective weather. Specific convective parameters for both stations are detailed in Table 1.

From the pseudo-equivalent potential temperature (Θ _se) at 850 hPa, it can be seen that on the 8th, the northern part of Altay Region and the Ili and Bortala areas were all located in the high-energy zone with Θ _se > 336 K and Θ _se500-850 <-3 K during the daytime. By 08:00, Qibeiling was situated in the high-energy tongue zone, with Θ _se500-850<−8 K. This configuration was maintained for about 2 h, leading to intense precipitation in the region (Sun, 2014) (Figure 10a). On the 9th, the high-energy zones in northern Xinjiang were distributed in a dispersed, patchy manner, corresponding to Θ _se500-850<−4 K all day. The overlap of the high-energy zones or tongues of Θ _se with Θ _se500-850 < 0 is one of the main reasons for the instability of the atmospheric stratification, which triggers short-duration heavy precipitation in multiple locations.

Analysis of the Time-Height cross sections of Θ _se and temperature advection at the heavy rainfall center reveals that there is a high-energy tongue zone with Θse > 336 K below 800 hPa before and during the heavy rainfall. Meanwhile, around 700–500 hPa, a low Θse zone between 700 and 500 hPa indicated convective instability in the atmosphere. On the temperature advection cross-section, there is strong warm advection spatially correlated with the high-energy tongue below 800 hPa before and during the heavy rainfall, contrasted by pronounced cold advection above 500 hPa. This warm-over-cold vertical configuration provided favorable thermodynamic conditions for convective development, consistent with vertical wind field variations captured in the model sounding at this station (Figure 10b).

From the temporal evolution of horizontal divergence in the vertical direction over the rainfall center, significant convergence zones associated with convective instability area were observed before and during the heavy rainfall. A strong convergence center (−7 × 10⁻⁵ s⁻¹) was located near 900 hPa, while divergence dominated above 850 hPa, peaking at 6 × 10⁻⁵ s⁻¹. In the vertical velocity field, weak upward motion began in the lower troposphere over the representative station at 04:00 GMT on the 8th. By 10:00–12:00 GMT, this upward motion extended to the mid-upper levels, forming a primary upward velocity center (−1.0 Pa·s⁻¹) near 825 hPa and a secondary center near 450 hPa. Both centers were situated within regions of sharp gradients between strong convergence and divergence. The vertical coupling of these ascending centers, combined with alternating horizontal divergence patterns, enhanced local atmospheric “siphoning effects.” Additionally, the tilted structure of vertical velocity and divergence fields favored the development of intense convection (Figure 10c).

The heavy rainfall occurred primarily on the southwestern slopes of the Altai Mountains’ foothills, highlighting the critical role of topography. The Altai Mountains slope downward from northwest to southeast. Low-level southerly/southwesterly flows (including a low-level jet) intensified convergence and stagnation near the foothills, promoting water vapor accumulation and uplift. At the surface, cyclonic convergence and strong easterly winds further enhanced convergence as airflow ascended northwestward along the slopes. At Qibeiling Station, located in an area with higher terrain to the northeast and lower terrain to the southwest, local topography amplified vertical ascent and created favorable convergence-divergence patterns. This configuration led to rapid intensification of precipitation. The terrain-driven divergence field near Qibeiling Station exhibited a distinct low-level convergence and upper-level divergence structure, coinciding with maximized upward vertical motion during peak rainfall periods.

Topographically, the heavy rainfall occurred primarily on the southwestern slopes of the Altai Mountains’ foothills, highlighting the critical role of topography. The Altai Mountains slope downward from northwest to southeast. Low-level southerly, southwesterly flows (including a low-level jet) intensified convergence and stagnation near the foothills, promoting water vapor accumulation and uplift. At the surface, cyclonic convergence and strong easterly winds further enhanced convergence as airflow ascended northwestward along the slopes. At Qibeiling Station, located in an area with higher terrain to the northeast and lower terrain to the southwest, local topography amplified vertical ascent and created favorable convergence-divergence patterns. This configuration led to rapid intensification of precipitation. The terrain-driven divergence field near Qibeiling Station exhibited a distinct low-level convergence and upper-level divergence structure, coinciding with maximized upward vertical motion during peak rainfall periods.

Based on the in-depth analysis of atmospheric circulation, dynamic-thermodynamic conditions, water vapor supply, mesoscale features, and topographic influences, a conceptual model of upper- and lower-level configurations for this heavy rainfall event is illustrated (Figure 11). Specifically, the key components are as follows: above the middle-level atmosphere, establishment of the eastern mode of the South Asian High at 100 hPa; intensification of the upper-level southwesterly jet at 200 hPa; Vigorous development of the upstream European blocking high and downstream East Siberian Ridge, accompanied by the southward and westward displacement of the West Siberian Low Vortex, which continuously split short-wave troughs that merged with the Central Asian trough at 500 hPa. Below the lower level, a southwesterly low-level jet provided abundant water vapor and significant unstable energy to the rainfall area at 700 hPa; high-energy frontal zone and convergence of southwesterly winds with topography enhanced water vapor pooling below 800 hPa.

Vertically, the vertical configuration of alternating convergence (−) and divergence (+) zones from lower to upper levels reached peak intensity simultaneously with upward motion, exhibiting a slightly forward-tilted structure, promoting convective intensification and water vapor uplift.

This study conducted an in-depth analysis of an extreme heavy rainfall event in northern Xinjiang from 7 to 9 August 2023, utilizing data from automatic weather stations, FY-4A satellite products, NCEP/FNL reanalysis, ERA5, and ECMWF model soundings. The main conclusions are as follows:

(1) The rainfall was caused by the eastern mode of the South Asian High. The West Siberian trough, influenced by the upstream European blocking high and downstream East Siberian Ridge, moved southward and westward, merging with the Central Asian trough. Short-wave troughs ahead of this system moved eastward into northern Xinjiang. A persistent southwesterly low-level jet (LLJ) and its frontal convergence zone, along with shear lines and mesoscale convergence over the rainfall center, supported the development and maintenance of M_βCS, creating a favorable environment for extreme rainfall.

The occurrence of this heavy rainfall event in northern Xinjiang resulted from the synergistic effects of multi-scale systems under an anomalous Central Asian circulation background. From the perspective of atmospheric circulation configuration, heavy rainfall in Xinjiang is often associated with a bimodal SAH at 100 hPa (Zhang et al., 1986). However, during this event, the South Asian High exhibited an eastern mode, which facilitated the establishment, maintenance, and southward extension of the pre-trough southwesterly jet. The rainfall primarily occurred in Altay and Ili in northern Xinjiang, consistent with findings from previous studies (Zhang et al., 2015). Notably, the intensity of the European blocking high during this event remained between 588 and 584 dagpm, comparable to historical extreme rainfall events on the northern slopes of the Tianshan Mountains (Chen et al., 2012).

The persistent eastward movement of short-wave troughs split from the Central Asian trough—driven by coordinated upstream and downstream high-pressure systems—served as the primary synoptic background. However, the widespread heavy rainfall across multiple stations suggests that the atmospheric environment in the study area was characterized by widespread instability and abundant water vapor, both of which were validated through detailed analysis of atmospheric stratification and water vapor conditions.

Comparative analysis of atmospheric stratification over the rainfall center using EC model soundings, ERA5 reanalysis, and FY-4A/TBB products revealed vertical wind shear and thermal advection confirmed a warm lower-level and cold upper-level structure. Cloud-top brightness temperatures (TBB ≤ −40°C) and equilibrium heights indicated deep convective cloud clusters with high vertical extension. High CAPE values and sustained near-surface updrafts suggested subsequent energy release, leading to localized high-energy tongues and intense short-duration rainfall. Statistical analysis of convective parameters at representative stations prior to the rainfall event confirmed thresholds for short-duration heavy precipitation (Zhuang et al., 2018).

While water vapor for Xinjiang rainfall typically originates from westerly flows and local evaporation, this event exhibited a dual-channel convergence of upstream and downstream water vapor sources. Combined with the unique topography of the Altai Mountains, abundant water vapor converged and ascended along the southern foothills, resulting in water vapor flux and convergence intensities exceeding those of other foothill rainfall events. Although water vapor transport was dominated by upstream systems, the prolonged contribution from the downstream channel played a critical role in sustaining precipitation.

By comparing with previous research on extreme precipitation processes in the region, it was found that this precipitation process has the novel characteristics as follows. Firstly, this process has a wide impact range and high precipitation intensity. Unlike the previous weather pattern of a southward high-pressure system, this process is mainly of the eastern type of the southward high-pressure system. The convergence of mountain front airflow triggers the mesoscale convective system, resulting in local rainstorm weather. Most areas in northern Xinjiang have an unstable atmospheric layer with abundant water vapor present.

Since there was no weather radar in Altay where the rainstorm center was located at that time, this paper uses FY-4A products to discuss the changes of mesoscale cloud clusters and water vapor transport and convergence, which demonstrated strong alignment with ground observations, highlighting their potential for monitoring extreme weather in radar-blind zones. However, the 15-min temporal resolution of FY-4A full-disk observation mode limits the detection of finer mesoscale features and rapid water vapor variations. Additionally, Xinjiang’s location at the edge of the satellite’s coverage hindered effective monitoring of water vapor transport west of 50°E.

Conclusions drawn from this single case study are inherently limited. Recent advancements in Xinjiang’s meteorological infrastructure—including over 3,000 automatic weather stations, more than 30 radars, and vertical profiling systems—provide a robust foundation for future real-time monitoring and research. Next, we will use multi-source observation data fusion and numerical simulation to deepen the understanding of extreme precipitation mechanisms in arid areas and improve the predictability of heavy rainfall system over complex terrain.

The shortwave trough is the main influencing system of extreme summer precipitation weather in northern Xinjiang. When analyzing, forecasters often focus on the location and intensity of low troughs (vortex) but tend to overlook the heavy precipitation weather caused by the rapid eastward movement of shortwave troughs. In addition, the distribution of such rainstorm centers is related to the local terrain height, and it is not uncommon to see cases of short-term heavy precipitation caused by the local uplift and convergence of warm and humid air flow. Therefore, emergency disaster reduction departments need to timely grasp the direction of local flash floods and prevent the harm caused by heavy rainfall floods to downstream areas.