Typhoon precipitation characteristics and diagnostic indicators of heavy rainstorms affecting Shandong under four types of circulation patterns

In recent years, the northward movement of typhoons has become increasingly active. Shandong is a major area in northern China vulnerable to typhoon-induced rainstorms. However, few targeted studies have been carried out on relevant diagnostic indicators, which cannot effectively guide local disaster prevention and mitigation work. Using Typhoon Yearbookand ERA5 reanalysis from 1979 to 2020, this study adopted the K-Means clustering method to classify the 500 hPa geopotential height fields of 90 typhoon events when their precipitation began to affect Shandong. The characteristics of typhoon precipitation and the configurations of meteorological elements are systematically analyzed for each pattern. Quantitative diagnostic indicators for typhoon rainstorms under each circulation pattern are subsequently developed. The results are as follows. (1) Ranked by frequency, typhoons affecting Shandong with precipitation can be categorized into four types based on 500 hPa circulation: ridge-trough confrontation pattern, straight westerly flow pattern, trough pattern, and high-pressure dam pattern. (2) Typhoon precipitation under the first three patterns concentrates in central-eastern Shandong, while the high-pressure dam pattern exhibits concentrated precipitation west of 120°E. The accumulated precipitation volume varies significantly between heavy rainfall cases and other non-heavy rainfall cases. (3) Regarding the atmospheric element configuration, typhoon-induced rainstorms under the ridge-trough confrontation, trough, and straight westerly flow patterns are predominantly driven by warm advection within 24 hours post-impact, with significant correlation to low-level southerly wind convergence at 850 hPa near the typhoon's periphery. In contrast, the high-pressure dam pattern maintains warm advection dominance for up to 72 hours. Subsequently, frontal genesis triggered by cold air intrusion enables heavy rainfall on the typhoon's western flank even under weak humidity conditions. (4) The 850 hPa specific humidity (q), 850 hPa relative humidity (R_H), precipitable water (P_WAT), and 850 hPa pseudo-equivalent potential temperature (θ_se) constitute reliable diagnostic indicators for typhoon rainstorms in Shandong. Typhoon rainstorms under ridge-trough confrontation and high-pressure dam patterns predominantly occur in energy-moisture enriched environments characterized by q>11 g·kg⁻¹, R_H>75%, P_WAT>49 kg·m⁻², and θ_se>338 K, followed by the trough pattern, while the straight westerly flow type demonstrates the minimum indicator requirements.