Comparing Synoptic Conditions and Environmental Characteristics for Fronts with and Without Prefrontal Convection Initiation and Heavy Rain over Coastal South China

To understand why convection initiation and heavy rain sometimes occur ahead of fronts over South China in the presummer rainy season but sometimes do not, a climatology of 137 fronts is constructed, in which 34% of the fronts exhibit no prefrontal convection initiation (NoPCI), 31% of the fronts exhibit prefrontal convection initiation (PCI), and 35% of the fronts exhibit prefrontal convection initiation and heavy rain (PCI+HR). An anticyclonically curved upper -level jet streak and midtropospheric QG forcing produce synoptic-scale descent for the prefrontal region in NoPCI events, whereas the right-entrance region of a straight upper-level jet streak and forcing for ascent dominate the prefrontal region in PCI and PCI+HR events. Whether prefrontal convection and heavy rain occur is also related to the character of low-level flows. NoPCI features anticyclonic southerly winds, with an environment having low dewpoint throughout the tropo-sphere, unfavorable for convection initiation. However, synoptic circulation of PCI and PCI+HR events favors a broad prefrontal surface low, which determines the greater cyclonic character of airflows in PCI+HR events, in contrast with that of the PCI events. Convective available potential energy is useful in distinguishing NoPCI and PCI events, and the three events have statistically significant differences in precipitable water. Moreover, larger magnitudes of precipitable water and bulk wind shear in PCI+HR events are conducive for prefrontal convection to produce heavy rain compared to those of PCI events. These results indicate the importance of the upper-level forcing on the prefrontal convection initiation, and heavy rain is sensitive to the changes in prefrontal airflow and moisture.