On the Movement of Rainfall-_??_roup (IV)

[摘要] In this paper, the author discusses the movement of rainfall-groups by considering the typhoons “Kezia” and “Jane” which landed at south Kyushu and at Kansai District in the second and first decades of Sepember, 1950. These typhoons are both known as a typhoon having no frontal character, same as “Kitty” which landed at Kanto District in 1949. “Kezia” had pretty circular isobars while crossing Kyushu District, but “Jane” had no such circular isobars. So the former is adequate for discussing the characteristic rainfall-groups due to a typhoon having circular isobars which have been considered as a characteristic property of a typhoon before its invasion into middle latitudes.The obtained results are as follows:(1) In the former case three rainfall-groups, which had no direct relation with the structure of typhoon and were moving to the direction of winds aloft, appeared almost before the approach of the typhoon center. The times of occurrence of the groups at south Kyushu are respectively 10h, 20h of the 12th and 1h of the 13th, and the amount of rainfall of each group is large at the concave part of isochrone, especially at its west part. It would be due to the convergence of air current by irregularity of the field current which is due to the existence of tropopause wave. The speed of propagation of isochrone is larger at the convex part of isochrone than at the concave one.(2) Three chracteristic rainfall-groups, the first of which winds round the second one logarithmic-spirally, the second is accompanied by the central region of the typhoon and has two circular forms or a long elleiptic form and the third moves northwards after the second on winding round it, appeared at south Kyushu at 5h, 8h and 14h of the 13th. The third has a curvature reverse to that of the first and accompanies strong winds. The speed of propagation is equal to the speed of typhoon.(3) In the latter case, similar rainfall-groups appeared as shown in Fig. 5.(4) The form of isochrone at some hours is shown in Fig. 6. It shows the similar mark of typhoon _??_. The autographic records of the group winding round the central region of typhoon are shown in Fig. 7.(5) The relations of rainfall amount per unit time with the time elapsed after the beginning of each group is shown in Fig. 8. And it shows that the rainfall amount takes its maximum value when some time has elapsed after landing and decreases rapidly. Then the magnitude of maximum rainfall amount increases with time. This observed fact will be due to the dispersive medium in which the disturbance propagates. Some theoretical considerations were made in the case of water wave by using the equations (1)_??_(6), where y denotes the displacement, x the cordinate showing the direction of motion, t the time, k0 the predominent value of k at x, t and U the group velocity. The main feature of Fig. 8 will be explained by the displacement like the wave as shown in the equation (5) or (6), where _??_ is the proportional term to t when x is constant.(6) Assuming that the wind distribution by a typhoon is composed of the logarithmic spiral stream lines having a circular ring with maximum wind speed and the wind due to the tropopause wave which the author has pointed already, we can get the similar wind speed distribution as shown in actual cases. Calculationg the vorticity by this calculated wind distribution, we get Fig. 9. This shows the main feature of the above mentioned property of characteristic rainfall group at a typhoon, that is, a circular or an elliptic region of positive vorticity round the central region and the two long positive regions winding round the former logarithmic-spirally from the frontal ad rear sides of central region.

[学科分类] 大气科学