Abstract:
The extremely heavy rainfall that occurred in Baoding, Hebei Province from 29 Jul to 2 Aug 2023 was examined by using conventional and hourly observation data, wind profiles, and dual polarization radar detection data. The results showed the following: 1) Under the combined influence of a northern high-pressure dam by continental high pressure and subtropical high pressure, the inverted trough of weakening typhoon Doksuri, a warm shear line on the east side of Doksuri, and the southerly airflow of the subtropical high, Baoding experience three stages of precipitation, with the longest stage lasting 70 h. From 0700 LST 30 Jul to 0300 LST 1 Aug, continuous short-term heavy precipitation occurred owing to the “train effect” in the shallow mountainous area upstream of Zhuozhou, with altitudes between 0.1–1 km, leading to severe local flooding. 2) The rain belt evolution on 29 Jul showed a northeast–southwest orientation, moving northwest over time, with precipitation intensity below 40 mm/h. The subtropical high shifted westward, increasing the potential gradient on the east side of the inverted trough on 30 Jul. Concurrently, the southeast airflow on the north side of typhoon Khanun merged with the low-level southeast jet in the Hebei and Shandong region, strengthening and extending northward. The rain belt formed a herringbone shape, maintaining its area while the frequency and intensity of short-term heavy rainfall within the area increased, reaching an intensity of 82.2 mm/h. 3) During the shear line precipitation stage, the convective instability layer thickened, with precipitation primarily concentrating between the SW–SE and S–E shear lines. This stage saw numerous instances of short-term heavy rainfalls locally, with a maximum rainfall intensity of 76.3 mm/h. 4) After the impact of Doksuri, significant convergence of wind speed in the low-level southerly airflow west of the subtropical high led to local precipitation in the warm area. The rain belt exhibited a quasi north–south direction and was divided into multiple rounds, each moving eastward and decreasing in intensity. The maximum precipitation intensity during the stage was 54.2 mm/h. 5) Dual polarization radar images during the inverted through precipitation stage initially showed warm cloud precipitation with low convective development height and large raindrop particles owing to warm cloud collision. As the southeast flow from Khanun merged, the 0°C layer decreased, while the −10°C layer rose, increasing the convective height. This supported the growth of graupel particles, polymerization, melting of ice particles, and increased raindrop concentration. During the shear line precipitation stage, the 0°C–10°C layer thickness decreased while the 10°C–20°C layer thickness increased, favoring higher convective development and graupel particle growth. 6) The terrain significantly influenced precipitation. During the inverted trough precipitation stage, the southeast airflow strengthened and converged with the easterly wind in the central and northern parts of Baoding as it extended northward, triggering flash floods in the shallow mountainous areas of Baoding. During the inverted trough precipitation stage with the southeast flow of Khanun and the shear line precipitation stage, the southeast wind was almost orthogonal to the northern section of the Taihang Mountains. This alignment contributed to a 24-h precipitation peak in the mountainous area of Baoding from 0800 LST on 30 Jul to 0800 LST on 31 Jul.