Cold Air Influence on the Tibetan Plateau Vortex Moving out of the Plateau

Storm rainfall and torrential storm rainfall and flood to the east of the Tibetan Plateau and even over a large area of China can be led to by some vortices that move out of the main Plateau region. Much attention is paid by meteorologists to the research on Plateau vortex moving eastwards, but the study of cold air influence on the Tibetan Plateau vortex moving out of the Plateau is few. Based on analyzing the different vortex activities which lead to the severe flood in large area of China, the middle troposphere level cold air influence on the Plateau vortex moving out of the Plateau and by using the synoptic diagnose, numerical simulation and experiment, the cold air influence on the Plateau vortex moving out of the Plateau is discussed.By investigating and analyzing the different east moving processes of Plateau vortices during 1998—2004, typical Plateau vortex processes leading to the severe flood disasters over different region of China are selected. By synoptic analysis and using 4 times daily 1°×1°NCEP data and the same time upper air observational data, the potential vorticity of Tuole Plateau vortex moving out of the Plateau during August 12—14, 2002 is analyzed. By using the MM5 version 3.4 PSU/NCAR high resolution non-static mesoscale model, a series of numerical experiments are conducted. The following results are obtained. There is cold air influence both before and after the Plateau vortex moving out of the Plateau which has long active time (more than 36 hours). The Tuole Plateau vortex moves with shear line and moves out of the Plateau by influence of the cold air flow invasion from the northeast. By potential vorticity analysis on the Tuole vortex moving out of the Plateau, it is found that the Plateau vortices is influenced by cold air flow of northeast region of China and moves out of the Plateau when there is high level potential energy air moving into low level potential energy air and the cold air flow is forced close to wet and warm air flow. It is demonstrated through the numerical simulation of cold air influence on Tuole Plateau vortex that when there is no cold in Plateau vortex area or no cold temperature trough, the northeast main cold air is made lean to east and weak and less influence to Tuole vortex of northeast China cold air flow. The baroclinic instability is made small.The speed of the Plateau vortex moving out of the Plateau will also be slowed down, and the intensity of the vortex will be weaker and the maximal wind speed will be lowered. The influence of the China northeast cold temperature trough is especially much obvious and the Plateau vortex region will be small when there exists a China northeast cold temperature trough, and the Plateau vortex will draw back to the west and float around the Plateau edge.