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.

Boundary layer convergence lines (boundaries), an important trigger of convective storms, can be produced by land surface contrasts. This study explored a five‐year summertime radar climatology of boundaries and their associated convection in response to vegetation contrast around the bend of the Yellow River in North China. A total of 323 boundaries were identified with 44% being convection‐associated. The boundaries especially the convective boundaries were more frequent over the arid area than those over the vegetated area and tended to have an orientation parallel to the vegetation contrast line. The boundary activities collocated well with the diurnal variation in surface temperature difference across the vegetation contrast. Compared with the nonconvective boundaries, the convective boundaries formed earlier and moved faster into the inner arid area, obtained maximum length around midday, and then initiated convection. Vegetation contrast might also affect the high‐frequency location and magnitude of boundary‐associated convective precipitation.

Impacts of sudden stratospheric warming (SSW) on three severe cold events (C1–C3) over Northern Hemisphere in early 2021 were investigated using subseasonal‐to‐seasonal ensemble forecasts. The SSW was characterized by successive displacement and split of the stratospheric polar vortex (SPV). Ensemble‐based sensitivity analyses showed that C1 over Siberia and C2 over western Canada were more related to SPV variations at a lead time of 1–2 weeks than C3 over central U.S. Within ensemble forecasts, a more elongated SPV with higher geopotential height over northern Eurasia were conducive to C1, whereas a SPV with the more poleward retreat from displacement contributed to C2. The forecast accuracy of stratospheric heights over the sensitive region at 2 weeks lead was significantly correlated with the forecast skill of the tropospheric circulation pattern preceding C1, implying that prediction of specific cold weather events may be improved by a better forecast of key features in SPV variations.

Sichuan Province, the most industrialized and populated region in southwestern China, has been experiencing severe ozone pollution in the boreal warm season (April–September). With a surface ozone monitoring network and reanalysis dataset, we find that nearly all cities in Sichuan Province showed positive increasing trends in the warm-season ozone levels. The warm-season daily maximum 8-h average (MDA8) ozone levels increased by 2.0 ppb (4.8%) year⁻¹ as a whole, with slightly larger trends in some sites such as a site in Zigong (5.2 ppb year⁻¹). Seasonally, the monthly ozone level in Sichuan peaks from May to August (varies with year). The predominant warm-season synoptic patterns were objectively identified based on concurrent hourly meteorological fields from ERA5. High-pressure systems promote ozone production and result in high ozone concentrations, due to strong solar radiation as well as hot and dry atmospheric conditions. The increased occurrence of high-pressure patterns probably drives the ozone increase in Sichuan. When ozone pollution is relatively weak (with MDA8 ozone around 170 μg m⁻³), the air quality standard could be achieved in the short term by a 25% reduction of NO_x and VOCs emissions. Strengthened emission control is needed when ozone pollution is more severe. Our study provides implications for effective emission control of ozone pollution in Sichuan.