The Evolution of a MeiYu Rain Band in an Idealized Model

The formation and development process of a Mei-Yu rain band is investigated through idealized numerical simulations. This Mei-Yu rain band shows a core-gap structure and a gradual propagation toward the south. The entire rainfall process experiences a distinct reinvigoration and so it is split into two main phases. In the first phase, the coexistence of two slantwise circulations is found and deep right-hand and shallow left-hand circulation are respectively attributed to the release of conditional symmetric instability (CSI) and Delta-M adjustment. The intrusion of cold and dry northerly flow is responsible for the onset of secondary enhanced rainfall phase and also the primary force for the rain band southward movement. Stronger inertial adjustment collocates with more release of CSI and this positive feedback confirms the Delta-M adjustment can precondition the atmosphere for the CSI formation. The differences between the polar front and Mei-Yu front is also shown by considering the weight coefficient associated with the cross-front water vapor gradient in the moist thermal wind relation. The performance of rainfall pattern is very sensitive to microphysical schemes and the contribution of sublimation cooling is also presented.