Convection Initiation at a Coastal Rainfall Hotspot in South China: Synoptic Patterns and Orographic Effects

This study documents the detailed features and processes of convection initiation (CI) at a coastal rainfall hotspot in South China, with emphasis on the typical meteorological conditions and effects of small-scale orography. The CI occurrences are identified using high-quality radar mosaics during 7-year warm seasons. They tend to maximize in low-lying areas adjacent to coastal small-scale mountains rather than on the windward slopes of major mountains as previously expected. The genesis of the coastal convection exhibits a remarkable diurnal peak in the early afternoon and mostly occurs on the days of low-level southwesterlies with a jet-nose structure in offshore region. These onshore flows enhance the low-level moistening and convergence on coasts. The characteristic synoptic patterns become common after the onset of summer monsoon, suggesting the key role of monsoonal flows in triggering coastal convection. Driven by the atmospheric conditions of the dominant synoptic pattern, quasi-idealized numerical simulations demonstrate that the coastal small-scale mountains disturb the low-level onshore flow and produce multiple enhanced convergent zones downstream of the mountains. The induced local variations in upward motion and moisture pooling explain the CI occurrences that are located close to these small-scale mountains. When these mountains are artificially removed in the simulation, convection preferentially initiates on the windward slopes of major mountains as expected. The findings suggest that the small-scale orography can strongly regulate the local patterns of CI hotspot through disturbing low-level onshore flow. An explicit resolving of the small-scale coastal orography is warranted to improve the modeling of local severe weather.

Automated summary

This study explores the characteristics and processes of convection initiation at a coastal rainfall hotspot in South China, revealing that the occurrences are concentrated in low-lying areas near small-scale mountains. The findings also suggest the crucial role of summer monsoonal winds in triggering coastal convection, while the presence of small-scale topography disrupts low-level onshore flow and facilitates the initiation of convection events.