Relationship Between the Development of Inertia-gravity Wave and the Movement of Rain-band During Meiyu and Storm Rainfall Period

Storm rainfall in Meiyu season in 1991 causes great damages to mid-lower reaches of the Yangtze River, and many researchers have done a lot of studies on it. But there are only a few researches on the relationship between the movement of Meiyu rain-band and inertia-gravity wave. NCEP/NCAR reanalysis data and MM5 nonhydrostatic model are used to simulate the precipitation procedure during the period of 00:00 (UTC), July 5, 1991 to 12:00, July 6, 1991, and the result is diagnosed to reveal the inner relationship between the propagation of inertia-gravity wave and the movement of rain-band in this process.The analysis on the meso-scale perturbation field of the storm rainfall in the Meiyu season indicates that the development and propagation of inertia-gravity wave is closely related to the development and propagation of rain-band, and the propagation form of upper inertia-gravity wave is different from the lower one. At the early stage of the rainfall, positive and negative vorticity band lies from east to west in the upper and lower layer of mid latitudes. The positive and negative vorticity in the southern part of rainfall area distributes alternately from northeast to southwest. Typical inertia-gravity character and southward-propagation tendency is also revealed in divergence diagram. The upper inertia-gravity wave propagates southwards later than the lower one, and vorticity and divergence bands distribute like the shape of "8". North-east vorticity and divergence bands propagate southwards, but south-west ones propagate northwards. In the mid-period of precipitation, upper inertia-gravity wave shows the tendency of counter clock-wise rotation around the lower vortex center, at this time, rainfall begins to increase and moves eastwards. Inertia-gravity wave has the character of distributing in the out-of-phase form. The lower layer does not shows the rotation around vortex feature similar to the upper layer. The allocation of upper and lower layers provides beneficial dynamical condition to the long-term maintaining of this precipitation.The main resaosn of the propagating inertia-gravity wave in the upper and the non-propagating inertia-gravity wave in the lower layer is that there are more convective stability and symmetric instability areas in the upper layer, and there are more convective instability areas in the lower layer. And the reason of the easy development of inertia-gravity when it propagates southwards in lower layer but the hard development vice versa is that the southern part of rain area is convective relative stability area, but the northern part is convective instability area. These reasons are revealed in the aspect of equivalent potential vorticity in the study. The convective instability in the lower layer triggers precipitation and inertia-gravity wave. The non-propagating inertia-gravity is firstly produced, then, the convective instability and symmetric instability in the upper layer motivate the production of propagating inertia-gravity. These should be the cause of the production of inertia-gravity rotating around vortex in the upper layer. The inertia-gravity wave propagating northwards in the upper layer is beneficial to the rain band's and vortex's movement northwards. The inertia-gravity wave propagating southwards in the upper and lower layer is beneficial to the formation of many rain-bands.