Characteristics of Regional Short-Time Heavy Rainfall in Eastern Gansu During Summer

KONG Xiangwei¹²³, LI Chenrui², DI Wenjing², YANG Yi¹, FU Zhengxu², YANG Xiumei²³

¹Earth System Model Development Research Center, College of Atmospheric Sciences, Lanzhou University, Lanzhou 730000, Gansu, China
²Lanzhou Central Meteorological Observatory, Lanzhou 730020, Gansu, China
³Lanzhou Institute of Arid Meteorology, Key Laboratory of Arid Climatic Changing and Reducing Disaster of Gansu Province, Key Laboratory of Arid Climatic Changing and Reducing Disaster of China Meteorological Administration, Lanzhou 730020, Gansu, China

Abstract

Using hourly precipitation observations from automatic weather stations in eastern Gansu Province during summer months from 2010 to 2021, 50 regional short-time heavy rainfall events were identified and analyzed from a weather process perspective to characterize their intensity, temporal, and spatial distribution. The results show that: (1) The intensity of short-time heavy rainfall in these events primarily ranged between 20–30 mm·h⁻¹, accounting for over 60% of all occurrences, while intensities exceeding 50 mm·h⁻¹ represented less than 10% of events. However, precipitation exceeding 40 mm·h⁻¹ occurred in 74% of the regional short-time heavy rainfall events, indicating a notable probability of high-intensity precipitation. (2) Regional short-time heavy rainfall mainly occurred from mid-June to late August, with the highest frequency period in late July to mid-August. These events occurred every year, but with substantial interannual variability closely related to the position of the Western Pacific subtropical high, anomalous warm-moist water vapor transport from the South China Sea or East China Sea, and significant baroclinic features resulting from interactions between mid- and low-latitude weather systems. (3) The average affected area of regional short-time heavy rainfall accounted for only 3.17% of the total area of eastern Gansu, with notable regional characteristics in spatial distribution. High-frequency zones were concentrated near the Taizishan Mountains, western extension of the Qinling Mountains, Liupanshan Mountains, and Ziwuling Mountains, with high-frequency centers generally accompanied by short-time heavy rainfall exceeding 40 mm·h⁻¹. (4) The distribution of station occurrences varied significantly under different circulation patterns. The eastward-moving plateau trough type showed a relatively dispersed distribution with the highest overall number of short-time heavy rainfall stations. The subtropical high-edge southwestern flow type had the lowest median among the four types. The two-high shear type exhibited concentrated station occurrences, while the northwest flow type had the fewest stations. Differences in intensity distribution were not pronounced, though precipitation of 30–50 mm·h⁻¹ occurred more frequently in the two-high shear type.

Keywords: eastern Gansu; short-time heavy rainfall; weather process; spatiotemporal distribution

1. Data and Methods

The precipitation data used in this study consisted of hourly observations from automatic weather stations (AWS) in eastern Gansu Province during summer (June–August) from 2010 to 2021. After quality control procedures including climate boundary value tests and spatial consistency checks, 1,075 stations with good continuity and stability were selected [FIGURE:1]. The study adopted the short-time heavy rainfall standard specified in the Gansu Provincial Standard (DB 62/T), which defines short-time heavy rainfall as hourly precipitation ≥20 mm·h⁻¹. Regional short-time heavy rainfall events were identified when the total number of stations experiencing short-time heavy rainfall on a given day (using 08:00 Beijing Time as the daily cutoff) reached or exceeded a threshold. A total of 50 regional short-time heavy rainfall events were identified during the 12-year period.

Following the classification method of Kong et al. [reference], these 50 events were categorized into four synoptic circulation types: eastward-moving plateau trough type, subtropical high-edge southwestern flow type, two-high shear type, and northwest flow type. This study further analyzed the distribution characteristics of short-time heavy rainfall under these different circulation patterns.

2. Distribution Characteristics of Regional Short-Time Heavy Rainfall

2.1 Intensity Distribution

Analysis of the 50 regional short-time heavy rainfall events revealed that the frequency of occurrence decreased rapidly with increasing precipitation intensity. Precipitation intensities of 20–30 mm·h⁻¹ were most common, accounting for over 60% of all short-time heavy rainfall stations. Intensities of 30–40 mm·h⁻¹ represented the next most frequent category, while intensities exceeding 50 mm·h⁻¹ accounted for less than 10% of occurrences [FIGURE:2]. However, precipitation exceeding 40 mm·h⁻¹ occurred in 74% of the regional events, indicating that although individual high-intensity events were rare, the probability of encountering such intensities during regional short-time heavy rainfall processes was not negligible.

The maximum hourly precipitation in each event showed some correlation with the total number of stations experiencing short-time heavy rainfall [FIGURE:3]. Generally, when the number of affected stations was large, the maximum hourly precipitation was also substantial. For events with fewer than 50 stations, the average maximum hourly precipitation was 54.2 mm·h⁻¹, while for events with more than 50 stations, the average reached 65.7 mm·h⁻¹. This pattern aligns with the understanding that stronger weather processes tend to produce both more widespread short-time heavy rainfall and higher precipitation extremes.

2.2 Spatial Distribution

With an average station spacing of approximately 10 km in eastern Gansu, each meteorological station represents roughly 100 km². To investigate the spatial characteristics of regional short-time heavy rainfall, the study area was divided into a uniform 10 km grid. A grid cell was considered affected if it contained at least one station with short-time heavy rainfall. Using this approach, the average affected area across all 50 events was calculated to be 5,941.08 km², representing only 3.17% of the total area of eastern Gansu [TABLE:1]. Even for the two-high shear type, which had the largest average affected area (7,430.00 km²), the proportion was merely 3.97%. This demonstrates that the affected areas of regional short-time heavy rainfall are both small and relatively dispersed, consistent with the semi-arid to semi-humid climate background of the region.

The spatial distribution exhibited pronounced regional characteristics [FIGURE:4], with high-frequency zones concentrated near mountainous terrain including the Taizishan Mountains, western extension of the Qinling Mountains, Liupanshan Mountains, and Ziwuling Mountains [FIGURE:5]. Along the Taizishan range, the occurrence frequency ranged from 0.1 to 0.2, slightly higher than surrounding areas. The western extension of the Qinling Mountains showed a more significant topographic influence, with frequencies generally between 0.2 and 0.3 and some stations reaching 0.4 to 0.5. The area between Liupanshan and Ziwuling Mountains also exhibited elevated frequencies of 0.2 to 0.3.

In terms of precipitation intensity [FIGURE:5], areas receiving rainfall exceeding 40 mm·h⁻¹ showed a spatial pattern closely related to topography, with concentration zones corresponding well to high-frequency areas. The distribution of extreme precipitation exceeding 50 mm·h⁻¹ was more limited, primarily occurring near the western extension of the Qinling Mountains and west of Ziwuling Mountain, further illustrating the important role of terrain in modulating the intensity and location of short-time heavy rainfall.

2.3 Temporal Distribution

2.3.1 Ten-Day Period Distribution

Regional short-time heavy rainfall events in eastern Gansu occurred primarily from mid-June to late August, with the highest frequency in late July to mid-August [FIGURE:5]. Notably, mid-August was particularly active, with the frequency of events exceeding 20% and maximum hourly precipitation in all events during this period surpassing 60 mm·h⁻¹. This temporal pattern aligns with the main occurrence period for regional heavy rainstorms in eastern Northwest China, which also peaks in late July to late August, indicating that short-time heavy rainfall is a primary contributor to heavy rainstorm events in this region.

The concentration of events in mid-August may be related to thermal and moisture conditions [FIGURE:6]. Although moisture conditions in mid-August did not differ substantially from those in early August, thermal conditions were significantly weaker, with the Convective Available Potential Energy (CAPE) contour retreating from central to eastern and southern boundaries of the study area. The reduction in thermal conditions decreased the persistence and areal extent of short-time heavy rainfall, but the increased frequency and intensity of cold air disturbances during this period created favorable conditions for triggering convective systems. The anomaly fields of specific humidity and pseudo-equivalent temperature at 700 hPa were positive, with the mid-August positive anomalies greater than those in early August, corresponding well with the higher frequency of regional short-time heavy rainfall events.

2.3.2 Interannual Variation

Although regional short-time heavy rainfall events occurred every year from 2010 to 2021, substantial interannual variability was evident [FIGURE:5]. The highest frequency occurred in 2018 with nine events, followed by 2020 with eight events, while 2014 and 2015 had only two events each. The interannual variation in maximum hourly precipitation was relatively small, primarily ranging from 60 to 90 mm·h⁻¹.

The interannual differences in event frequency were closely related to the position and intensity of the Western Pacific subtropical high (WPSH), baroclinic characteristics of interacting weather systems, and anomalous warm-moist water vapor transport. In high-frequency years (2018 and 2020), the 500 hPa geopotential height anomaly field over eastern Northwest China showed positive anomalies, with the WPSH positioned more westward and northward than normal [FIGURE:7]. Correspondingly, the 700 hPa pseudo-equivalent temperature anomaly field also exhibited positive anomalies. In contrast, low-frequency years (2014 and 2015) featured negative anomalies in both the 500 hPa geopotential height field and 700 hPa pseudo-equivalent temperature field.

Further analysis of circulation patterns revealed that in high-frequency years, the summer atmospheric circulation over Northwest China exhibited meridional characteristics, with active upper-level trough systems in the westerlies creating a "west-low-east-high" pattern. Eastern Gansu displayed significant baroclinic features from northwest to southeast. The 700 hPa wind field anomaly showed anomalous southerly flow in 2018 and anomalous easterly flow in 2020 [FIGURE:8]. The water vapor transport in 2018 primarily originated from the South China Sea, while in 2020 it came from the East China Sea along the Yangtze River valley to the Sichuan Basin before turning northward to reach eastern Gansu—an uncommon but important pathway. The water vapor flux divergence anomaly field showed the most significant negative anomalies in 2018, particularly in southern Gansu and along the plateau slope, while 2020 had a smaller negative anomaly area. Correspondingly, the specific humidity anomaly in 2018 was 0.6–1.2 g·kg⁻¹ with the maximum center in western Gansu, whereas in 2020 it was 0.2–0.4 g·kg⁻¹ with the center shifted eastward. In low-frequency years, the specific humidity anomaly was negative (-0.6 to -0.2 g·kg⁻¹), indicating significantly less moisture.

2.4 Distribution Characteristics Under Different Circulation Patterns

The spatial distribution of short-time heavy rainfall varied among the four circulation types [FIGURE:9]. The eastward-moving plateau trough type showed the widest distribution, with an average affected area of 6,277.95 km² (3.35% of the total area) and the highest number of affected counties (34.33% of total counties). High-frequency zones were concentrated along the Taizishan Mountains (frequency 0.5–0.6) and the western extension of the Qinling Mountains (frequency 0.4–0.5), with precipitation intensity exceeding 40 mm·h⁻¹ distributed across a broad area.

The subtropical high-edge southwestern flow type had an average affected area of 6,514.29 km² (3.48% of total area), slightly larger than the plateau trough type but shifted southward. High-frequency zones were concentrated along the western extension of the Qinling Mountains (frequency 0.4–0.5), with precipitation intensity distribution similar to the plateau trough type but with more dispersed areas exceeding 40 mm·h⁻¹.

The two-high shear type exhibited the largest average affected area at 7,430.00 km² (3.97% of total area), with high-frequency zones concentrated on the southern and eastern sides of Liupanshan Mountain and west of Ziwuling Mountain. Precipitation intensity was similar to the subtropical high type, but with more frequent occurrences of 30–50 mm·h⁻¹ rainfall.

The northwest flow type had the smallest affected area (4,500.00 km², 2.45% of total area) and the fewest affected counties. Short-time heavy rainfall was extremely rare under this pattern, with almost no occurrences exceeding 50 mm·h⁻¹.

The distribution of station occurrences differed significantly among circulation types [FIGURE:10]. The eastward-moving plateau trough type showed a dispersed distribution with the highest number of stations (median 60–180 occurrences). The subtropical high-edge southwestern flow type had the lowest median among the four types. The two-high shear type showed concentrated station occurrences with a median similar to the plateau trough type, while the northwest flow type had the most concentrated distribution with the fewest stations (median between 60–90 occurrences). Differences in intensity distribution were not pronounced, though the two-high shear type had the highest frequency of 30–50 mm·h⁻¹ precipitation and the northwest flow type had the most concentrated intensity distribution around 24–26 mm·h⁻¹.

3. Conclusions

Based on hourly precipitation data from 2010 to 2021, this study analyzed 50 regional short-time heavy rainfall events in eastern Gansu Province from a weather process perspective. The main conclusions are as follows:

(1) The intensity of regional short-time heavy rainfall in eastern Gansu primarily ranged from 20–30 mm·h⁻¹, accounting for over 60% of occurrences. Although high-intensity precipitation exceeding 50 mm·h⁻¹ was relatively rare at individual stations, such intensities occurred in 74% of regional events, indicating a notable probability of extreme precipitation during these weather processes. The maximum hourly precipitation in each event correlated with the total number of affected stations, with larger events typically producing higher precipitation extremes.

(2) Regional short-time heavy rainfall occurred primarily from mid-June to late August, peaking in late July to mid-August. While events occurred every year, substantial interannual variability existed, closely related to the position and intensity of the Western Pacific subtropical high, significant baroclinic features from interactions between mid- and low-latitude weather systems, and anomalous warm-moist water vapor transport from the South China Sea or East China Sea.

(3) The average affected area of regional short-time heavy rainfall accounted for only 3.17% of the total area of eastern Gansu, with relatively dispersed distribution patterns. Terrain exerted a significant influence, with high-frequency zones concentrated near the Taizishan Mountains, western extension of the Qinling Mountains, Liupanshan Mountains, and Ziwuling Mountains. Areas receiving precipitation exceeding 40 mm·h⁻¹ corresponded well with these high-frequency centers.

(4) The distribution of station occurrences varied significantly among different circulation patterns. The eastward-moving plateau trough type had the most dispersed distribution and the highest overall number of stations. The subtropical high-edge southwestern flow type had the lowest median station count. The two-high shear type showed concentrated station occurrences, while the northwest flow type had the fewest stations. Intensity distribution differences were less pronounced, though precipitation of 30–50 mm·h⁻¹ occurred more frequently in the two-high shear type.

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