Summer High-Level Cyclones in the Region of Japan

In the present study, the development, path and thermal structure of the high-level cyclone which appears in summer and traces a very erratic path in the upper troposphere over the region of Japan are investigated from the synoptic point of view. Generally speaking, this cyclone is formed by the extreme meandering of the westerlies, but it is not an intense cyclone and has few immediate influence upon the pressure pattern at sea level. In §1, the author describes briefly the movement of the high-level cyclone in the region of Japan throughout a year. In §2, the characteristics, frequency and path of the summer high-level cyclone are explored and the cyclone is classified into 6 types, including the cyclone in the Bai-u season, according to their paths. The frequency of the cyclone has increased in recent years and its maximum was presented in the year 1958, while the sunspot maximum was reached in the year 1957. But it is not clear for lack of data that there is an intimate relationship between the extreme meandering of the westerlies in summer and the increase of sunspots. The path is very erratic compared with that of the high-level cyclones in other seasons and the movement is steered by the wind in the upper troposphere near the level of the jet stream, or, in other words, by the distribution of anticyclones in that level. Among these cyclones is included the one which moved perfectly westward from South Japan toward South China in the year 1959. The movement of this cyclone was steered by the easterlies south of the subtropical anticyclone which increased in intensity after the cyclone development. In §3, thermal structure of the cyclone is studied using vertical cross sections. The figure of vertical cross sections is not so noticeable as that of the high-level cyclones in other seasons, but both cyclones belong to the same type except for the existing level. And the structure differs far from that of typhoons in the same season. In §4, the deepening of the trough in which the cyclone is formed is discussed applying the equation of the vorticity change of a travelling air particle. The term including the vertical velocity is mainly explored and the order of magnitude of it is compared with others. Though the descending velocity is estimated at several cm per sec., the order of magnitude of this term is not large, because the wind shear is not large in this season. The trajectory of a descending air parcel is determined using the potential temperature. Descending air parcels can not penetrate into lower layers on account of the high stability in these layers ; the cyclone, therefore, exists only in the upper level. Finally in §5, typical examples of the cyclone are examined to ascertain the theoretical process carried in this study