Based on 5 years of operational Doppler radar data, the influences of atmospheric synoptic patterns and low‐level prevailing wind speed on the intensity and diurnal variations of summer orographic convection occurrence frequencies over Pearl River Delta, South China, have been investigated. Results show that the inland orographic convection and rainfall generally occurs under synoptic pattern characterized with the prevailing southwesterly wind within the lower troposphere. The summer orographic convection over the mountains in northeastern Pearl River Delta is not only controlled by the orographic thermal conditions but also the dynamic forcing with the increase of wind speed. Owing to the strong windward mechanical lifting and moisture transport associated with the strong ambient onshore winds, the number of convection occurrences characterized by a dominant diurnal afternoon peak occurs much more frequently in the high‐wind speed days. While due to the weak orographic mechanical lifting and moisture supply in the low‐wind speed days, the number of convection occurrences in the afternoon decreases considerably and two comparable peaks occur in the afternoon and early morning. The nighttime peak in the low‐wind speed days is mainly attributed to the nocturnal acceleration of the low‐level southwesterly wind associated with the inertial oscillation and the corresponded enhanced windward lifting effects.

South China coast suffers frequent heavy rainfall every warm-season. Based on the objective classification method of principle components analysis, the key role of synoptic pattern in determining the heavy rainfall processes occurred over the South China coast in warm season during 2008-2018 is examined in this study. We found heavy rainfall occurs most frequently under three typical synoptic patterns (P1-P3 hereafter) characterized by strong low-level onshore winds. P1 and P3 are featured by a prevailing southwesterly monsoonal flow in the lower troposphere, with heavy rainfall frequently occurring over the inland windward region in the afternoon associated with the orographic lifting and solar heating. The onshore wind of P3 is stronger than P1 as the western Pacific subtropical high extends more westward to 122°E, which induces stronger low-level convergence along the coastline than P1 when the ageostrophic wind veers from offshore to onshore direction in the early morning. Hence, a secondary early morning rainfall peak can be found along the coastline. P2 is characterized by a low-level vortex located over the southwest of south China. Heavy rainfall under P2 usually initiate over the western part of the coastal region in the morning and then propagate towards inland in the afternoon. Overall, the synoptic patterns strongly determine the spatial distribution and diurnal cycle of heavy rainfall over the South China coast. It is closely related to the diurnally varying low-level onshore winds rather than the low-level jets, as well as the different interactions between the low-level onshore winds and the local orography, coastline and land-sea breeze circulations under different synoptic patterns.

The initiation of deep moist convection, which we usually refer to as convection initiation (CI), dramatically affects the subsequent convective storm development and occurrence of severe weather (Markowski & Richardson, 2010). It has long been recognized that convection results from the multi-scale coupling effects of synoptic and local circulations and underlying surfaces, such as coastlines, mountains, or urban agglomeration (

A hailstorm with an inclined structure occurred in the western part of the South China coast on 27 March 2020. This study investigates the detailed evolution characteristics of this inclined structure using the Doppler radar data assimilation system (VDRAS) and the improved fuzzy logic hydrometeor classification algorithm (HCA). Obvious differential reflectivity (often referred to as ZDR) arc characteristics, ZDR column characteristics, and the specific differential phase (often referred to as KDP) of the column are observed using dual-polarization radar prior to hailfall. Both the ZDR column and KDP column reached their strongest intensities during the hailfall phase, with their heights exceeding the height of the −20 °C layer (7.997 km above ground level), displaying a cross-correlation coefficient (CC) valley during this phase. Meanwhile, two centers of strong reflectivity were found, with one (C1) being located at 2–4 km, and the other (C2) being located at 6–8 km. The maximum horizontal distance between the two centers is 8 km, suggesting a strongly inclined structure. This inclined structure was closely related to the interaction between upper-level divergent outflows and ambient horizontal winds. The updraft on the front edge of the hailstorm continued to increase, keeping C2 at the upper level. At the same time, large raindrops at the lower part of C2 are continuously lifted, leading to ice formation. These ice particles then fell obliquely from their high altitude, merging with C1.