Research On The Relationship Between Tropical Disturbance And Remote Rainstorm By Statistical And Diagnostic Analysis And Numerical Simulation

A study of the relationship between tropical disturbance and remote rainstorm events in the region of 0°-50·N,90°-150°during May to September in the period of 2006-2011 was performed. The results are as follows:Under favorable atmospheric circulation background, a tropical disturbance, although the intensity of which is lower than a tropical depression, can also cause remote heavy rainfall combining with the effect of mid-latitude systems.The statistics suggest 21 events can be divided into three categories according to the type of moisture channel on 850hPa:"S" pattern moisture channel, double moisture channel and northwestward moisture channel. The remote heavy rainfall events with "S" pattern moisture channel occur most frequently and the percentage is 52%, the remote heavy rainfall events occur most frequently in July and the percentage is 38%. The composite analysis shows that the formation of the remote heavy rainfall is closely related to the atmospheric circulation formed by the mid-latitude systems, tropical disturbance and subtropical high. The mid-latitude systems provide a favorable atmospheric circulation background and the remote heavy rainfall occurs in the area with high moisture flux, which indicates the importance of moisture transportation. The composite analysis of low-level jet of having and not having remote heavy rainfall shows that the southerly low-level jet located in the east of the tropical disturbance plays an important role in the formation of the remote heavy rainfall and is the key to make a connection between the low-latitude and mid-latitude systems no matter the tropical disturbance is strong or weak.The diagnosis of a case with "S" pattern moisture channel indicates that the combined action of the tropical disturbance, mid-latitude systems and subtropical high causes the remote rainstorm. The rainstorm area lies in front of the westerly trough and the right of the high-level jet, which provide a favorable large-scale background for the rainstorm, and the development of the shear line and low vortex supplies beneficial conditions for the convergence and ascending motion of air. During the remote rainstorm, the upper trough moving eastward couples the shear line or low vortex in low layer and the genesis, evolution, movement and extinction of the mesoscale convective systems with different scale and intensity are influenced by the formation, development, movement and extinction of the shear line and low vortex, which leads to the different periods of the remote rainstorm. The diagnosis also indicates that the low-level jet between the tropical disturbance and subtropical high is the main moisture and energy channel that transport abundant moisture and energy to the remote rainstorm area.The numerical study of a case with "S" pattern moisture channel further indicates that the southerly low-level jet is the main channel to transport moisture to the mid-latitude rainstorm area and its intensity is one of the main factors affecting the intensity of the remote rainstorm. Numerical sensitivity experiments suggest that the tropical disturbance not only contributes to the northward moisture transport but also the moisture flux convergence in the remote rainstorm area. Sensitivity experiments show that the tropical disturbance can also induce the propagation of Rossby wave energy towards the northeast, the intensity of which is proportional to the intensity of the tropical disturbance, affecting the remote rainfall distribution. After removing the tropical disturbance, wave trains can not form which is not beneficial for the dispersion of wave energy and the development of the remote rainfall.