Numerical study on the periodic bahavior of mesoscale convective systems

摘要

The observed or simulated MCSs often shows pulsation with a period of a few hours. Two-dimensional experiment in this study suggest that such periodicity can be interpreted in terms of interaction between gravity waves and convective system. In a medium range of low-level environmental wind shear, simulated convective clouds are periodically organized into a larger convective clouds with the time period of a few hours and pulsative strong rainfall occured. The analysis of model output demonstrates rearward propagating meso-β cloud and assosicated updrafts/downdrafts have characteristics of 'vertically propagating' gravity waves. A conceptual model of a evolution of MCS in one cycle is proposed in Fig.6. In response to the tilted heating in the meso-β cloud, internal gravity waves with reaward tilting phase-lines are generated and these disturbances propagates rearward as wave train. Propagation of downdraft phase over the low-level convective clouds supresses the development of each clouds and weakning of the system. As the downward phase propagates further rearward and an updraft phase of the waves arrives over the cloud area, it contribute to the development of next meso-β cloud and reintensification of the system. The periodic formations of highly tilted clouds in the medium shear range, are not usually reported in other numerical studies of midlatitude squall lines (e.g. Fovell and Ogura, 1989) . The difference of the storm behavior can be attributed to the thermody-namic profiles used in our study. We used input ther-modynamic conditions which are conductive to producing weak cold pool (～ -6K), while many other simulated midlatitude storms possesed rather strong cold pool (～ -10K). When the cold pool is strong and the gust front propagates fast, the clouds formed at the gust front are readily broken away from the top of the gust front and rapidly decay as it propagates reai-ward. On the othehand, when the cold pool is weak and gust front is slow moving, the cloud can be connected to the gust front long time and the formation of highly sloped cloud is possible.