Causes of Frequent Severe Convective Weather in Shandong Province, China, in May 2020 and Analysis of the Typical Case

A total of 13 severe convective weather events occurred in Shandong Province in May 2020, including 8 hailstorm events. Hail was observed at 81 stations (65.9%) in 15 cities (93.8%), with a maximum of 4 station-times of hail occurrence. Ten events produced thunderstorm gales above level 10, and 5 events brought short-duration heavy rainfall. The frequency, coverage, intensity, and associated disasters of severe convection were rarely seen in the past decade. Among them, the severe convective event on 17 May was the most intense, with an extremely large hail-affected area. The maximum hail diameter reached 4.5 cm, the maximum wind speed was 36.6 m·s⁻¹ (level 12), and the maximum hourly rainfall intensity was 56.9 mm.

Based on ERA5 reanalysis data, encrypted automatic weather station observations, Doppler weather radar data, and lightning location data, this study analyzes the characteristics and causes of frequent severe convective weather in Shandong in May 2020. Taking the extreme severe convective event on 17 May as a typical case, the radar echo characteristics, vertical motions within storms, and maintenance mechanism of the long-lived squall line are investigated. The results show that:(1) The subtropical high was stronger than normal. On the one hand, it favored the transport of warm and moist southwesterly flows to Shandong; on the other hand, it blocked westerly systems, resulting in a strong forward-tilting trough that persisted over Shandong for a long time. Cold air carried by northwesterly flows ahead of an anomalously strong warm ridge on 500 hPa overlaid a strong warm temperature ridge on 850 hPa, maintaining persistent convective instability over Shandong. (2) Under the above favorable synoptic background, abundant moisture and large CAPE existed over Shandong. Strong cyclonic convergence occurred at the junction of Hebei, Shandong, and Henan provinces, and a persistent convergence line was maintained over central Shandong, which readily triggered severe convection and led to frequent severe convective weather in May. (3) Highly organized convective storms, regional supercell clusters, and a strong squall line exceeding 500 km in length were the direct causes of the 17 May extreme severe convection. The updraft velocity inside convective storms reached up to 28 m·s⁻¹. (4) Sustained low-level warm and moist advection transported warm and moist air into Shandong, which acted as the mechanism for CAPE reconstruction and the main energy source supporting the long-term maintenance of supercell clusters and the long-lived squall line.