An analytical and modeling study of binary typhoon interaction

• Main Authors: Ling-Feng Hsiao, 蕭玲鳳
• Other Authors: Koung-Ying Liu
• Format: Others
• Language: zh-TW
• Published: 2002
• Online Access: http://ndltd.ncl.edu.tw/handle/01452719227048205952

博士 === 中國文化大學 === 地學研究所 === 91 === Many factors result in deviations in forecasting tracks of tropical cyclones over the West Northern Pacific. The complexity of prediction increased when two or more tropical cyclones coexisted in a certain region. In this work, statistical study and MM5 simulations are performed to investigate interactions of binary typhoons for better understanding of the relative motion between them. The binary typhoon systems in the last fifty-six years (1945-2000), according to the alterations of their relative rotation and the separation distance between the typhoon centers are divided into four classes. In order to catch those cases with large separation but pronounced interaction in a week synoptic situation, subjective analysis was adopted in the work of case selection. With this method, the case with large separation and in weak environment flow with or without sharp interaction can be easily identified. The Statistical result indicates that in the past fifty-six years the binary systems with counterclockwise rotation, and with the centers approaching to each other are dominant. Their number ratio is up to 43.9%. In addition, principal component analysis is used to characterize the synoptic situation of the selected cases(one for each class) for the four categories. As a result, for the clockwise rotating cases, the relative motion is mainly controlled by the outer circulation of the Pacific High. Therefore, the relative locations of the two typhoons and their center position relative to the Pacific high are important factors which affecting their relative movement. The motions of clockwise rotation are clearly presented on the first principal component. In regards to the counterclockwise cases, the environment flow is generally very weak, therefore, the second principal component is dominated. In general, in all of the four classes the intensity of binary typhoons, the relative location with respect to the Pacific High, and the relative location of the two typhoons are the important factors on the synoptic environment distribution. Typhoons Bopha and Saomai of 2000 composed a very good case in the period of investigation. They are categorized into the counterclockwise rotation and approaching group. In associate with their sharp interaction and the most pronounced relative motion, this case is selected to analyze and simulate with MM5. The evidence of synoptic analysis exhibits that weak environment provides this binary system a well-developed background condition for interaction. During the whole process of counterclockwise rotation, typhoon Saomai is sustained entirely by a suitable synoptic condition. But, on the other hand, for typhoon Bopha, which is weaker and located on the west of typhoon Saomai, typhoon Saomai’s influence is dominate. The northerly flow from the west of typhoon Saomai not only induces the southern deflection of typhoon Bopha but also, due to the intrusion of the cold air, increased the dissipation of typhoon Bopha. Meanwhile, in order to understand the interaction, model simulations included the experiments of removing and intensifying of typhoon Saomai are carried out. Results show that the track of typhoon Bopha varies with different conditions. Simulating tests show that the model has better capability for the 72-hour track simulation especially for the southern deflection of typhoon Bopha although the simulated center pressure is not so good, which may result from the deviation of the predicted location of the Pacific High. Results are, also, indicating the influence of steering flow on the interaction between binary typhoons. Removing the circulation of typhoon Saomai reveals that typhoon Bopha appears to have slow translation and smaller angle of southern deflection. It makes the track of typhoon Bopha more northwestward than without removing it. This result represents the interaction between binary typhoons has significant impacts on the relative tracks. On the other hand, the simulation also indicated that as typhoon Saomai was removed the central pressure of Bopha decreased. It represents the intensity of typhoon Bopha was also influenced by typhoon Saomai. In addition, the experiment also detected the environment change after removing typhoon Saomai and its different effects on the motion and intensity of binary typhoons. Intensity experiments exhibited that to change the intensity of typhoon Bopha has much significant impact on the binary typhoon movement than that due to the changes of typhoon Saomai. Based on the results of the intensity experiment, and the vertical PV (potential vorticity) analysis we found that the tendency of PV increase is concentrated around the typhoon center. Furthermore, the PV budget analysis reveals that the tendency in the middle troposphere depended mostly upon vertical advection, but in the lower part, the horizontal advection and the condensational heating contributed more. However, in the lower level, the contribution of vertical advection and that of the condensational heating are opposite in sign so the horizontal advection played the major role in this level. On the other hand, resulting from the weaker intensity of typhoon Saomai and Bopha, the height of condensational heating is lower than other strong typhoons as shown in previous studies. The condensational heating generated in the lower part has small contribution on the local change of PV. Therefore, the PV change was mainly due to the lower level vertical advection and horizontal advection. In the other hand, the contribution of dynamic term is larger than thermal one on this binary case. The weighted vertical PV analysis proved that typhoon Saomai increased her PV flux via the southwest flow from South China Sea in the one way, and through the eastern flow on the north of typhoon Saomai in the other way. It is also shown that change intensity of typhoon Saomai could increase PV in its vicinity but not affected the environment, but as we changed intensity of typhoon Bopha, the PV flux transported to typhoon Saomai was increased. Finally, this case study indicated that, in a binary system the typhoon which located more northwestward than another have the larger influence on the related typhoon. The reversed influence is much weaker.