Organizational Modes of Mesoscale Convective Systems associated with Warm-Sector Heavy Rainfalls

The organization of mesoscale convective system (MCS) has been an important factor that affects the distribution of the rainfall they produce. Heavy rainfall that happens in the relatively uniform southwesterly or southeasterly winds far from the westerly trough or even without a westerly trough in south China in pre-summer rainy season, which is usually referred to as warm-sector rainfall, has been a big forecast challenge. This work examines the organizational modes and their environmental features of the MCSs associated with the warm-sector heavy rainfalls in south China during May–June of 2007–2014, in comparison to their counterparts in mid-latitude China.

Results showed that the MCSs associated with the warm-sector heavy rainfall in south China have mainly eight organizational modes including one nonlinear mode (NL) and seven linear modes: trailing stratiform precipitation (TS), no stratiform precipitation (NS), leading stratiform precipitation (LS), parallel stratiform precipitation (PS), embedded lines (EL), bow echoes (BE), and training lines (TL). NL mode accounted for 39% of the total and linear modes accounted for 61%, basically agreeing with the statistical results of middle latitude MCSs in China. The most frequently observed modes were NL, TS and NS. The TS mode occurred the most frequently among the linear modes, which was different from mid-latitude China where EL mode was the dominant linear modes. On average, the lifespans of linear modes were longer than that of the NL mode with the TS mode having the longest average lifespan. The MCSs had a major peak in the early morning and a minor peak in the early afternoon. The major peak was contributed by 45% of linear modes, and the minor peak was contributed by 40% of linear modes. The larger number of linear modes in the early morning peak probably resulted in its longer lifespan.

Different formation features were found for these top three organization modes. About 70% of NL systems formed via broken areal mainly near the coastline. About 67% of NS systems formed via broken line more inland. TS systems formed via merging (about 57%) and broken areal (about 43%) in the coastal region. The mature locations of NL systems were similar to the initial locations, suggesting a slow movement. The mature locations of TS and NS systems were obviously east of the initial locations. Analysis on the environmental characteristics of NL, TS, and NS modes showed that 0–1 km vertical wind shear was the key factor to distinguish different organizational modes. There was no statistically significant difference in other environmental parameters among the top three modes, such as CAPE, CIN, LCL, Lifting Index, precipitable water, and Bulk Richardson Number.

In comparison to the middle latitude MCSs in China, the MCSs in south China had a longer averaged lifespan. Compared with the most dominant peak of initial time in the afternoon in mid-latitude China, the most dominant peak of initial time of MCSs in south China was in the early morning. Generally speaking, the top three modes in south China formed in an environment with smaller CAPE, CIN, LI, and higher precipitable water, compared with middle latitude MCSs in China. How these differences are related with the organizational modes of MCSs will also be examined.