Convection Initiation in Monsoon Coastal Areas (South China)

• Published in: Geophysical research letters Vol. 47; no. 11
• Main Authors: Bai, Lanqiang, Chen, Guixing, Huang, Ling
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 16.06.2020
• Subjects: Breezes; Climatology; coast; Coastal zone; Coasts; Convection; convection initiation; Convective activity; Daytime; Diurnal; diurnal cycle; Downslope winds; Global climate; Global precipitation; Hydrologic studies; Hydrology; Land breezes; monsoon; Monsoon onset; Monsoon winds; Monsoons; Mountains; Offshore; Orography; Peninsulas; Propagation; Radar; Rain; Rainfall; Regional climates; Severe weather; Summer; Summer monsoon; Tropical climate; Weather forecasting; Wind; Winds; China
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2020GL087035

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. Plain Language Summary A large fraction of global rainfall occurs in the coastal areas of summer monsoon which are highly populated. Characterizing convective activity along monsoon coasts has been an important issue in regional/global climate and hydrological studies. For the first time, this study presents a long‐term radar climatology of convection initiation (CI) on the monsoon coasts of South China which have dense observation networks. The results show that CI becomes highly active after the summer monsoon onset. Most CI occurrences are concentrated within 100 km of the coastline, and the local maxima are strongly affected by topography and land‐sea contrast. The triggering of coastal convection also shows strong diurnal cycles with a daytime maximum on land and an offshore propagation after midnight. The findings provide key information to improve our understanding of how convective activity emerges in monsoon coastal areas. Key Points We present the first long‐term climatology of convection initiation on the coasts where monsoon flows interact with local circulations Convection initiation is highly active within 100 km of the coastline, with maxima near local mountains during onshore southwesterlies The triggering of convection is dominant on land in the afternoon and off the coast after midnight, particularly in the monsoon season