Case study of TAMEX IOP13 frontal rainband: the characteristic of the convective behavior

• Main Authors: Hong, JingShan, 洪景山
• Other Authors: Ben Jong-Dao Jou
• Format: Others
• Language: zh-TW
• Published: 1998
• Online Access: http://ndltd.ncl.edu.tw/handle/90855152826288610849

博士 === 國立臺灣大學 === 大氣科學系 === 86 === The paper investigate the convective behavior by using the dual Doppler radar observations and the two dimensional nonhydrostatic cloud model. There are three major part in the paper. The Part I describes the the frontal rainband by using the observation data and addresses the interested questions for further studies. The momentum budget analysis have been studied by using the dual-Doppler radar wind data and the retrieved perturbation pressure field in part II. The results show that the pressure gradient force in mid-troposphere, which results from the interaction of the convective updraft and the vertical wind shear, plays important roles in the evolution of the long-lifed convective cell and leads to the tilting of the convection toward the downshear side. The convection transports the along-line component of momentum down the mean gradient and the cross-line component upgradient. The downshear tilt of the convective system is a necessary condition which transports the cross-line momentum upgradient. Finally, we try to interpret what the interaction of the convective updraft and the vertical shear is by the momentum transport thinking. The horizontal acceleration induced by the vertical momentum flux convergence tends to generate a high pressure at downstream side and low pressure at upstream side. The pressure dipole can be considered as the inertia of the flow field and the associated pressure gradient force tends to offset the external forcing which is due to the momentum transport. The inertia depends on the aspect ratio of the external forcing, e.g. , the pressure gradient force will be larger when the scale along the forcing is larger than the scale cross the forcing. In the case, the net force of the momentum flux convergence and the pressure gradient force will be smaller and tends to keep the initial status of the flow field. The thinking can be also extended to the interpretation of the buoyancy induced pressure gradient force. Part III systematically analyze the charactristics of the downshear tilt convection by the idealized two dimensional nohydrostatic cloud model and compare with the observations. By the way, the simulation results also show that using the CAPE (Convective Available Potential Energy) to present the convective instability of the atmosphere may casuse improper interpretation. The vertical profile of the CAPE is a more realistic idea. By the way, the eimulataion results also suggest the low level jet will moisterize the boundary layer and then modify the vertical profile of CAPE, thus favor the development of the convection.