Abstract
The characteristics and formation of a synoptic situation that causes a sudden turning motion of warm-sector mesoscale convective systems (MCSs) over South China are described, based on the collection and investigation of associated cases during April–June 2011–2017 using high-resolution observational data and ERA (ECMWF Re-Analysis)-Interim data. The results show that the blocking of a marked low-level high over eastern China (eastern high) on a strengthening low-level trough over southwestern China (southwestern trough) results in significant enhancement of southerly winds ahead of the trough, which produces a strong southeastward vertical wind shear at low levels near western Guangdong province. This low-level vertical wind shear results in sudden southeastward turning motion for the warm-sector MCSs entering into Guangdong province from Guangxi province. The formation of the eastern high is mainly attributable to the strong cyclonic wind anomaly over the northwestern Pacific Ocean, which continuously brings cold air from higher latitudes to eastern China, where high synoptic-scale transient anomaly of geopotential height (SSTA-GH) forms. This cyclonic wind anomaly is induced by a low SSTA-GH, which travels from the north and south sides of the Tibetan Plateau to the northwestern Pacific Ocean and develops significantly as a result of a strong upper-level low SSTA-GH coupling with it or approaching it. On the other hand, the high SSTA-GH over eastern China blocks the eastward extension of the low SSTA-GH originating from the Tibetan Plateau. Consequently, this low SSTA-GH turns to extend or move southeastward/southward to southwestern China, leading to intensification of the southwestern trough.
Highlights
The rainfall of the pre-flood rainy season (April–June) contributes more than 40% of the annual rainfall for South China, and warm-sector mesoscale convective systems (MCSs) have been indicated as the principal systems causing heavy or extreme rainstorms in this rainy season [1,2,3,4]
Numerous studies have emphasized the important role of low-level jets (LLJs) in the initiation of MCSs over South China [6], because of their significant effect on the enhancement of upward motion (e.g., References [7,8,9]), moisture flux (e.g., References [10,11,12]) and atmospheric instability (e.g., References [13,14,15,16])
The L-band radiosonde data are utilized to estimate the motion vectors of the MCSs influenced by the synoptic situation of the studied cases, based on the vector-based method described in Corfidi et al [34]
Summary
The rainfall of the pre-flood rainy season (April–June) contributes more than 40% of the annual rainfall for South China, and warm-sector mesoscale convective systems (MCSs) have been indicated as the principal systems causing heavy or extreme rainstorms in this rainy season [1,2,3,4]. In addition to the 850–300 hPa mean wind and a diffluent thickness, the key ingredients shown in these synoptic conditions include maximum convective available potential energy, a frontal boundary, an 850-hPa theta-e (equivalent potential temperature) ridge axis, a 300-hPa upper-level jet, and a southerly LLJ These ingredients help in identifying the nature of MCS motion based on the relationship between the area favorable for new cell development and the MCS location, as MCSs tend to move to places of new cell generation.
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