台風Francisco (T0118) の初期渦形成過程

Abstract

台風の発生過程における初期渦形成過程について, Top-Down仮説とBottom-Up仮説のどちらが実際の過程をよく説明するのか検証した。そのためにメソ気象モデルを用いて2001年18号台風Franciscoを再現した。再現されたFranciscoは現実より約3度北で8時間早く発生した。発生前には中層に約100kmスケールのメソ対流渦, 下層に複数の約50kmスケールの渦が存在した。これらの渦が接近したあとに, 多数の対流スケールの渦が発生し, 併合することで初期渦が形成された。メソ対流渦と下層渦の領域で渦度の鉛直プロファイルを調べると, メソ対流渦の領域では時間経過とともに渦度が中層から下層へ広がり, 逆に下層渦領域では渦度が下層から上層へ広がった。従って再現された初期渦形成過程では, 注目するスケールを変えることでTop-DownとBottom-Upの両方のプロセスが見られた。

The initial vortex organization (IVO) in the early phase of the tropical cyclogenesis is a matter of interest in current study. Two hypotheses for the process of IVO associated with MCVs are proposed. One is Top-Down, and the other is Bottom-Up. These hypotheses, however, have not been confirmed enough. Therefore, we investigate the IVO process in the Typhoon Francisco (2001) case by using numerical simulation, and which process is seen in two hypotheses. The simulated Francisco is organized at nearly the same time and location. In simulated TCG process, one MCV, several low-level cyclonic vortices (LLVs), and small-scale vortices (SSVs) are seen. SSVs are being merged gradually from 00 UTC 16 to 00 UTC 17. The merged vortex has monolithic PV area with many convective clouds. The vortex got intensity, and becomes tropical storm later. To investigate development of MCV and LLVs, we investigate vertical profile of PV at the MCV and LLVs areas. PV in MCV area extends from mid-level to low-level. On the other hand, PV in LLVs extends from low-level to mid-level. Therefore, the temporal evolution of vorticity in MCV area seems Top-Down, but that in LLVs area seems Bottom-Up.