Monsoon coasts are characterized by active convection, but the features of associated convection initiation (CI) are not well understood. An analysis of kilometer‐resolution radar observations in South China reveals a distinct dependence of CI on monsoon activity, land‐sea contrast, and coastal orography. The CI occurrence becomes highly active after the monsoon onset and extends northward in summer. Several CI hotspots appear on tropical islands, peninsulas, and windward slopes of coastal mountains. Daytime CI occurrences are concentrated on land within 100 km of the coastline. The nocturnal CI maxima are located on coastlines and in the offshore areas with adjacent mountains. CI shows an offshore propagation after midnight prior to rainfall, probably due to the convergence of land breezes or downslope winds with diurnally enhanced onshore monsoon flows. The improved understanding of CI may provide additional benefit for forecasting severe weather because CI foretells the development of full‐fledged deep convection.

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.

This study explores the convection initiation (CI) of a high‐impact squall line that occurred in central eastern China on 3 June 2009 based on observations and numerical modeling. The CI occurred in a scenario in which a set of intersecting gust fronts, organized in a distinct scalloped pattern, propagated toward an area of enhanced moisture produced by a near‐surface convergence line. This convergence line developed in a quasi‐stationary dryline zone. The dryline primed the preconvective environment by deepening the moist layer prior to the arrival of the intersecting gust fronts. The onset of CI occurred approximately 30 min after these intersecting gust fronts passed through the CI location, which was on the dry side of the dryline. Although these gust fronts acted as a strong signal for CI potential, CI did not occur along the entire length of the scalloped pattern of the intersecting gust fronts. The exact locations of the initiated convective cells were at the vertices of the scalloped pattern. An idealized simulation using a cloud model was conducted, demonstrating that the vertex regions were characterized by more favorable dynamical conditions for CI compared to the nonvertex regions along the scalloped outflow boundary. The greater CI probability over the vertex region was attributed to the greater magnitudes and larger vertical and horizontal extents of updrafts.

The online version of this article (doi:10.1007/s11434-016-1005-2) contains supplementary material, which is available to authorized users.

An EF4 supercellular tornado hit Funing County, Yancheng, Jiangsu Province, China, from about 1410 to 1500 local standard time 23 June 2016, causing 98 fatalities and 846 injuries. It was the deadliest tornado in the past 40 years in China. This paper documents the storm environment, evolution of the radar signatures, real-time operational tornado warning services, and the damage distribution during this event. The tornado was spawned from a supercell that developed ahead of an upper-level trough extending southwestward from a low pressure vortex in northeast China and dissipated following the occlusion of the tornado vortex. The radar-based rotational velocity of the mesocyclone peaked at 42.2 m s−1. The strength of the tornado vortex signature (gate-to-gate azimuthal radial velocity difference) peaked at 84.5 m s−1. Surface observations at 1-min intervals from a mesoscale network of in situ surface weather stations revealed the surface wind pattern associated with the mesocyclone, such as convergent and rotational flows. The tornado formed after the peak updraft strength of the supercell, producing a damage swath that was 34.5 km long and with a maximum width of 4.1 km. The review of the tornado warning process for this event reveals that there is much work to be done to develop operational tornado forecast and warning services for China.