| Seasonal and diurnal variations of extreme hourly precipitation over China(east of 96oE) are investigated using rain gauge data during 1981-2013. The 99.9th percentile is used as the threshold to define the extreme hourly rainfall for each station. The extreme hourly rainfall occurs as early as in March in South China; the occurrence frequency increases substantially and expand northward into the Sichuan Basin and North China Plain during April and May; about 77% of the extreme rainfall records happen in summer(June-August) with a peak in July(30.4%) over the whole domain; after the end of August, the occurrence frequency decreases rapidly and retreat southward.Using weather maps and composite radar reflectivity data, nearly 3800 extreme hourly rainfall records in 2011-2013 are classified intofour types according to the synoptic situations under which they occur: the tropical cyclone(TC), frontal, vortex/shear line, and weak-synoptic types. They contribute 9.0%, 14.9%, 37.5%, and 38.6%, respectively, to the total occurrence and present distinctive characteristics in regional distribution and seasonal or diurnal variations. The TC type occurs most frequently along the coasts and decreases progressively toward inland China; the frontal type is distributed relatively evenly east of 104oE; the vortex/shear line type shows a prominent center over the Sichuan Basin with two high frequency bands extending from the center eastward to southeast China and northeastward to north China, respectively; the weak-synoptic type occurs more frequently in southeast China, southwest China, the North China Plain, and east portion of the northeast China. As for the seasonal variations, the TC type occurs most frequently during July to September and peaks in August, while the other three types concentrate in summer and peak in July. The frontal type contributes significantly(about one third) to the monthly occurrence in March and April; the vortex/shear line type tends to shift toward late spring and early summer(May-July) with contributions over 40% to the monthly total;the weak-synoptic type contributes substantially(38%-47%) for the monthly total during April to September.Moreover, a long-lived, quasi-stationary mesoscale convective system(MCS) producing extreme rainfall(maximum of 542 mm) over the eastern coastal area of Guangdong Province on 20 May 2015 is analyzed using high-resolution surface observations, sounding data, and radar measurements. New convective cells are continuously initiated along a mesoscale boundary at the surface, leading to formation and maintenance of the quasi-linear-shaped MCS from about 2000 BST 19 May to 1200 BST 20 May. The boundary is originally formed between a cold dome generated by previous convection and southwesterly flow from the ocean carrying higher equivalent potential temperature(θe) air. The boundary is subsequently maintained and reinforced by the contrast between the MCS-generated cold outflow and the oceanic higher-θe air.The low speed(≤5 m s-1) of the cold outflow, probably owing to the large humidity in the lower troposphere and the weak horizontal wind in the middle and lower troposphere, is comparable to that of the near surface southerly flow from the ocean, resulting in very slow southward movement of the boundary. The boundary features temperature contrasts of 2o-3oC and is roughly 250-500 m deep. Despite its shallowness, the boundary appears to exert a profound influence on continuous convection initiation because of the very low level of free convection and small convection inhibition of the near surface oceanic air, building several parallel rainbands(of about 50 km length) that move slowly eastward along the MCS and producing about 80% of the total rainfall amount. Another MCS moves into the area from the northwest and merges with the local MCS at about 1200 BST. The cold outflow subsequently strengthens and the boundary moves more rapidly toward the southeast, leading to end of the event in 3 hours. |
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