Characteristics of mesoscale convective systems in central East China and their reliance on atmospheric circulation patterns

Based on mosaics of composite radar reflectivity for a 4-year period from July 2007 to June 2011, a total of 354 mesoscale convective systems (MCSs) over central East China were identified and their characteristics were investigated. Using the objective classification method of obliquely rotated principal analysis in T mode, the atmospheric circulation patterns over central East China were classified into nine typical types based on the geopotential height fields at 850 hPa. It was found that 62.2% of daytime MCSs and 67.7% of nocturnal MCSs occurred in meridional atmospheric circulations, which were associated with the Western North Pacific Subtropical High (WNPSH) to the east and a low-pressure system to the west. The relationship between atmospheric circulation types and characteristics of MCSs was also investigated. Composite analyses revealed two different atmospheric circulation patterns at 500 hPa in meridional atmospheric circulations, the pre-short-trough pattern and the cold vortex pattern. The former featured relatively stronger vertical wind shear, warm advection, and convergence at the exit region of low-level jets (LLJs), while the latter was associated with relatively stronger frontal zones and frontogenesis process. Numbers of MCSs decreased significantly from 2008 to 2010, which might be attributed to changes in the location of WNPSH, the trough to the west and LLJs between the two systems.