This study conducts numerical experiments on tropical cyclone Saola (2023) using the Typhoon Rapid Refresh Analysis and Nowcasting System developed by the Chinese Academy of Meteorological Sciences, assimilating radial wind velocity data from S-band Doppler Radar and X-band Phased Array Radar with the Ensemble Kalman Filter method. The assimilation effects on Saola's track and intensity are evaluated, with particular focus on the assimilation sensitivity of X-band Phased Array Radar data at different altitudes. Results indicate that compared with solely assimilating S-band Doppler Data assimilating X-band Phased-Array Radar data in addition to S-band Doppler Radar data further reduces track error and intensity error by 13.7% and 58.0%, respectively. X-band Phased Array Radar data above 4 km primarily influences the geopotential height, horizontal wind, and warm-core intensity above 600 hPa at the initial time though dynamic and thermodynamic processes, although this effect diminishes progressively during forecast integration. X-band phased array radar data below 4 km mainly affect geopotential height and horizontal wind below 700 hPa through dynamic processes, directly strengthening the mid-to-lower-level pressure and wind fields. Moreover, after 3 hours of the deterministic forecast initiation, the entire wind and pressure fields are influenced. This study finds that assimilating X-band Phased Array Radar data below 4 km provides equivalent forecast skill of tropical cyclone's track, intensity, and structure compared to assimilating X-band Phased Array Radar data at all altitudes. Furthermore, it verifies the effectiveness of X-band Phased-Array Radar in complementing the low-level coverage of S-band Doppler Radar in tropical cyclone data assimilation and forecasting, offering actionable insights for refining operational tropical cyclone forecasting.

This study examined the rainfall characteristics and related synoptic processes of two extreme rainfall events that affected North China during 29 July–1 August 2023 (“23·7” rainstorm) and 3–5 August 1996 (“96·8” rainstorm), respectively. A stable dual-typhoon circulation pattern was observed in both rainstorm events. The surviving vortex of a landed typhoon, slowly approaching the rainstorm region, was blocked by a high-pressure system as it moved northwestward. Meanwhile, the second typhoon over the western Pacific Ocean facilitated remote northward transport of moisture. The low-level jet between the surviving vortex and the western Pacific subtropical high relayed moist warm air from the area of the South China Sea and western Pacific into the rainstorm region. Although the circulation patterns are similar, the stratification conditions, driving factors, and moisture budget of the two rainstorms differed during the main period of rainfall. The “23·7” rainstorm was categorized as warm-sector rainfall, as a result of the lifting of warm moist air over the eastern foothills of Taihang Mountains. In comparison with the situation of the “96·8” rainstorm, the surviving vortex of the “23·7” rainstorm traveled further northeastward and directly impacted the occurrence and progression of the rainfall, leading to relative northward displacement of the rainfall center, while the stronger net inward moisture flux caused greater regional average rainfall. The “96·8” rainstorm was broadly analogous to precipitation of a cold front, and the rainfall center was observed in the convergence area of warm and cold air masses before the mountains; the surviving vortex did not exert direct impact on the rainfall; and the more unstable stratification led to stronger hourly rainfall. The results derived through comparison of the two rainstorms could serve as valuable scientific reference for operational forecasting of heavy rainfall under similar environmental conditions over North China.