The magnitude of dual polarization observations is much smaller than the reflectivity factor (Z_H), and the accuracy of the observations is the basis for the application of dual polarization radar data. Anomalies in dual polarization observations caused by ground have largely been overlooked in the past. For the first time, utilizing the observation data from 93 CINRAD radars upgraded with dual polarization, the characteristics and scope of ground influence are determined by the anomalous areas after long-term data accumulation. Statistical analysis is then conducted to reveal the overall proportion of the influence in CINRAD radars, the main sources of the influence, the relationship between the influence and radar types and observation modes, the temporal variations of the influence, the nature of the influence and the distribution characteristics of dual polarization observations. Results are as follows: (1) The ground can cause significant deviations of the accumulated values of dual polarization observations from the precipitation interval, the differential reflectivity factor (Z_DR) is decreased by more than 1 dB, the correlation coefficient (ρ_HV) falls below 0.92, and the standard deviation of differential phase (Φ_DP) is greater than 10°. This characteristic served as the main basis for identifying ground influence. (2) The ground influence is not limited to the near-ground region. As the elevation angle increases, the sidelobe reception of ground signals can also produce similar effects. The proportion of influence in the near-ground region can reach 14% to 32%, while at high elevation angles it reaches 4% to 7%, far more severe than the beam blockage. (3) The ground influence is mainly caused by mountains and buildings with strong scattering capabilities. When the site is below the terrain height, surrounding mountains are the main contributors; when the site is close to the terrain height, buildings within urban areas are the main contributors. (4) The ground influence can be quantitatively described by the superposition of random errors and systematic errors, leading to increased uncertainty in the dual polarization observations within the affected area at a specific time. Moreover, the influence is more severe in the mainlobe region than in the sidelobe region.

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.

North China experienced devastating rainfall from 29 July to 1 August 2023, which caused substantial flooding and damage. This study analyzed observations from surface rain gauges and S-band dual-polarization radars to reveal the following unique features of the precipitation evolution from the plain to the mountains during this event. (1) The total rainfall was found concentrated along the Taihang Mountains at elevations generally > 200 m, and its spatiotemporal evolution was closely associated with northward-moving low-level jets. (2) Storms propagated northwestward with southeasterly steering winds, producing continuous rainfall along the eastern slopes of the Taihang Mountains owing to mountain blocking, which resulted in the formation of local centers of precipitation maxima. However, most rainfall episodes with an extreme hourly rainfall rate (HRR), corresponding to large horizontal wind shear at low levels, actively occurred in the plain area to the east of the Taihang Mountains. (3) The western portion of the extreme heavy rain belt in the north was mainly caused by long-lasting cumulus–stratus mixed precipitation with HRR < 20 mm h⁻¹; the eastern portion was dominated by short-duration convective precipitation with HRR > 20 mm h⁻¹. The contributions of convective precipitation and cumulus–stratus mixed precipitation to the total rainfall of the southern and middle rain belts were broadly equivalent. (4) The local HRR maxima located at the transition zone from the plain to the mountains were induced by moderate storm-scale convective cells with active warm-rain processes and large number of small-sized rain droplets. (5) During the devastating rainfall event, it was observed that the rainfall peaked at around 1800 local time (LT) every day over the upstream plain area (no diurnal cycle of rainfall was observed in relation to the accumulated rainfall centers over mountain areas). This was attributable to convective activities along the storm propagation path, which was a result of the more unstable stratification with a suitable steering mechanism that was related to afternoon solar heating and enhanced water vapor. The findings of this study improve our understanding and knowledge of the extreme precipitation that can develop from the plain to the mountains in North China.