Abstract: Black day phenomenon and sudden hard rain occur in Tianjin on 16 June 2009. Based on several monitoring data such as FY-2 satellites data, multi-radar composite and intensive automatic stations data, combing with VDRAS data, the origins of black day and the rainstorm are analyzed. The thermal and dynamical structure which leads to the occurrence and development of meso-β-scale, meso-γ-scale convective systems in circular meso-α-scale convective system are also studied. 31 circle-shape MCSs which lead to severe weather on western coast of Bohai Bay in 2004—2009 are preliminarily sorted and summed up in size and life-circle.Less than 16% circular MαCS on the western coast of Bohai Bay develop into MCC with no more than 15×10⁴ km² large (where the TBB is equal to or below-52 ℃). The MCCs generally last no more than 8 hours and always happen in night. But in the South China, it's common to see MCC larger than 20×10⁴ km² which last more than 10 hours."6.16" MαCS has special characteristics in range, time of occurrence and precipitation echoes on the thickness, which are the main causes for black day phenomenon. New MβCS and MγCS develop constantly in the west of MαCS move eastward into the high-energy region with warm-moisture intensively and maintain, leading to rainstorm in Tianjin.Cold air intrudes into MαCS from its back at the height of 1.3—2.4 km, flows out from rain echoes convergence line or close-gradually line, and triggers the development of MβCS with southwestern warm-moisture flow.In the ascending center of MαCS, the vertical velocity is 0.7 Pa·s^-1 at height of 500 hPa. Below the height of 700 hPa (about 3 km), ascending vertical velocity reaches 1.8 m·s^-1, and each of MβCS1—3 has independent vertical circle. Under the height of 1 km, there is corresponding boundary layer circulation for MβCS1—3. With evolution of MβCS1—3, cold pool (the areas of negative perturbation temperature) appears under the height of 2 km and the area of positive perturbation temperature appears above it, so the vertical structure is stable.