To reveal the environmental conditions, triggering mechanisms, and predictability of extreme short-term heavy rainfall in urban areas under weak synoptic-scale forcing, a comprehensive analysis of an abrupt heavy precipitation event that occurred in the main urban area of Zhengzhou on 22 July 2024 is conducted. Data from dense automatic weather stations, ERA5 reanalysis data, short-term forecast products from various numerical models, and multi-source novel observational data including FY-4B geostationary satellite imagery, dual-polarization radar, wind profile radar, cloud radar, and GNSS/PWV are used. The results indicate that this event occurred at the edge of the western Pacific subtropical high, where the synoptic forcing was weak. Westerlies prevailed at 500 hPa, while warm and moist southerlies prevailed in the lower levels. The surface was controlled by a warm low-pressure system. Prior to the heavy rainfall, the environment in Zhengzhou was characterized by extreme high-temperature and high-humidity, with the Convective Available Potential Energy (CAPE) exceeding 5000 J/kg and the Precipitable Water Vapor (PWV) above 80 mm, indicating persistent enhancement of atmospheric instability. FY-4B infrared imagery revealed that the heavy precipitation was caused by the development and southward movement of a Mesoscale Convective System (MCS) originating from northern Henan. During its mature stage, the MCS exhibited a significant expansion of cold cloud, with the heavy rainfall concentrated in the region of large brightness temperature gradient at the leading edge of the MCS. Further analysis suggests the rainfall was triggered by boundary-layer processes in a phased manner, i.e., the urban heat island effect initially induced a meso-γ-scale surface low pressure over the main urban area, triggering the initial convection. The outflow from the cold pool of this convective system converged with the ambient warm, moist flow, triggering new convections accompanied by persistent small-scale vortices. The thunderstorm high pressure from northern Henan moved southward, forcing the rise of the leading-edge warm, moist air and triggering a widespread convective outbreak. Short-term forecasts from several numerical models failed to predict this event, reflecting their limitations in simulating the evolution of meso- and micro-scale systems under weak synoptic-scale forcing. In contrast, high spatiotemporal resolution observations demonstrate significant value for nowcasting. Wind profile radar detected the establishment of a westerly jet stream at 4—5 km altitude 2—3 h before the precipitation, and the 0—6 km vertical wind shear increased from weak to moderate, favoring convective organization in the high-CAPE environment. GNSS/PWV data showed a sharp increase of over 4 mm about 1 h before the rainfall, indicating rapid low-level moisture convergence and uplift. Cloud radar observations captured a sharp decrease in cloud base height, signaling the critical phase of microphysical processes transitioning into heavy precipitation formation. In conclusion, under weak synoptic-scale forcing, the multi-stage triggering mechanism that involves urban heat island, cold pool outflow, and thunderstorm high pressure was the key to the occurrence of this extreme short-duration heavy rainfall. The continuous evolutionary signals revealed by multi-source novel observations provide crucial information for improving the nowcasting capability of such kinds of abrupt local heavy precipitation events.

Boundary layer convergence lines (BLCLs), an important type of weather system that can trigger deep moist convection, correspond to quasi-linearly extending airflow confluence zones in the boundary layer. Multiple types of BLCLs exist, and mechanisms for their triggering of convection are complicated. This paper summarizes the existing studies on the climatological statistics of BLCLs (excluding cold fronts) and mechanisms for their triggering of convection, and the effects of local temperature and moisture disturbances on the convection triggering. These studies have presented that the triggering probability, the storm location and time correlation are all affected by different synoptic systems and BLCL types in different regions. Under certain favorable conditions, local temperature and moisture can not only affect BLCL intensity, but also affect the distribution of atmospheric stratification near BLCLs, and thus convection initiation can occur. Moreover, the dynamical and thermal interactions between local temperature and moisture disturbances and the environment make the convection triggering mechanisms more complicated. The paper also discusses some convection triggering issues that need to be further studied. Based on fine observations and numerical simulation, it is also suggested to carry out more systematic studies on the characteristics and mechanisms of convection triggering for different regions and different types of BLCLs in the future.