最敏感扰动的演变与夏季乌拉尔地区的持续性异常环流

THE EVOLUTION OF OPTIMAL SENSITIVITY PER-TURBATION AND THE PERSISTENT ANOMALOUS CIRCULATION AT URAL IN SUMMER

中文关键词:  最敏感扰动  正压不稳定  持续异常环流

英文关键词:Optimal sensitivity pertur bation  Barotropic instability  Persistent anomalous circulation

中文摘要:

      通过数值实验对初始扰动不稳定发展建立异常环流型的过程进行了分析.选择了东亚夏季风异常的1991年和1985年两个典型年份的平均环流作为基流,以共轭敏感性分析得到的最敏感扰动作为初始扰动,分析其在不同的环流背景下的发展情况.结果说明,1991年欧亚地区的基流不稳定性较强,且初始扰动结构有利扰动发展,因而通过能量频散在乌拉尔地区激发扰动并强烈发展,建立起阻塞形势.而1985年的扰动在乌拉尔地区形成正涡度距平,不利于阻高的建立.从而导致1991年和1985年夏季乌拉尔地区几乎相反的异常环流.一系列对比实验说明,异常环流型的建立,不仅依靠能量的频散,更依靠扰动通过正压不稳定过程从基流吸收能量而发展.它既有赖于基流自身的不稳定,又取决于初始扰动的结构以及相对于基流的位置.基流选择了具有特定结构的初始扰动型,只要在有利位置上给予扰动,就能够激发出扰动波列,建立强的持续异常环流型.不论初始波列的位相在一定范围内如何改变,扰动总是倾向于在基流的特定不稳定区域发展.

英文摘要:

      In this work,the process of the set up of anomalous circulation patterns through unstable development of initial disturbances was investigated by numerical experiments.We chose as basic flow the mean circulation of two typical anomalo us East A sian summer monsoon years of 1991 and 1985,and as initial disturbances the optimal sensitivity perturbations obtained from adjoint sensitivity anal yses.The development of these disturbances in different circulation background has been analyzed.The result shows that,the instability of the 1991 basic flow in Eurasia region is stronger,and the initial perturbation possesses the structure of preferred growing perturbation.Therefore the disturbances are excited near U ralarea through energy dispersion,then grow very rapidly and develop the blocking circulation.Whereas the disturbances in 1985 developed into a positive vorticity anomaly and is not favorable for the set up of blocking high.This lead to the almost opposite anom alous circulation patterns in Uralarea in 1991 and 1985.A series of experiments indicate that the development of anomalous circulation is due to the growth of disturbances by extracting energy from basic flow through barotropic instability as well as energy dispersion.This depends not only on the instability of the basic flow it self but also the structure of the initial disturbance and its position with respect to the basic flow.The basic flow selects the initial disturbance with specific structure.When there is a disturbance at certain favorable position,it can excite the whole wave train and develop into a strong anomalous pattern.No matter how the phase distribution of the initial wave train changes within certain range,the disturbances tends to develop in the same unst able area of the basic flow.