A Study of Atmospheric Circulation Characteristics of Hourly Heavy Rainfall During Warm Seasons in Shaanxi

Based on the data on hourly precipitation in the Shaanxi province in China and ERA5 atmospheric reanalysis data, the principal component triangular transformation (PTT) method is employed to subjectively classify hourly heavy rainfall (HHR) events. Four primary synoptic patterns identified are west wind trough (WWL), northwest flow (NWF), low vortex (LV), and inverted trough (IT) with a frequency of 65.1, 29.4, 3.8, and 1.6%, respectively. The WWL pattern is primarily influenced by a west wind trough extending from the Hexi Corridor to Shaanxi. The NWF pattern is characterized by a forward-tilted trough and differential advection. The LV pattern refers to a low vortex situated between two high-pressure systems. Lastly, the IT pattern represents an eastward inverted trough on the southern side of an anomalous strong high-pressure system. Additionally, the box-and-whisker plots of physical quantities reveal that the median of K-index during HHR events is approximately 30°C for all four patterns, with the NWF pattern mainly influenced by the temperature difference between 850 and 500 hPa, while other patterns are primarily impacted by the humidity in the mid-lower layer.