日本附近に於ける温帯性低氣壓の發達に關する研究

第二報 不連續線の走向と低氣壓の發達

抄録

The present paper treats in detail the difference in mechanical structure between the two lines of discontinuity which appeared one after another, one extending from north to south and the other from west to east, with special care to point out the contrast between the weather conditions accompanying them and to make clear their effects upon the development of cyclones.
The principal results thus obtained are as follows:
1. In general, depressions on a line of discontinuity from north to south develop remarkably, and the bad weather accompanying them passes quickly.
2. When there exists a trough of low pressure from north to south, a frontal zone with the temperature gradient from southeast to northwest is produced and gives abundant vorticity to depressions along the trough. As these depressions grow vigorous, warm and cold air-masses are drawn towards the center of depression from remote places. Thus the temperature gradient of the frontal zone where the cyclonic vorticity is prevailing becomes greater and greater, until the frontal zone soon converts into a remarkable front exhibiting the so-called “animate growth” of depression by taking foods from the surrounding field.
3. In Japan and her neighbourhood, the warm front whose intensity increases with the deepening of the center of cyclone is also much affected and sharpened by the topography of Japan proper. The cyclone does not move with the air of the warm sector as in Europe, but rather along the warm front (it was formerly called a steering line) which in general extends to eastnortheast or northeast from the center. At the warm front, southerly (warm) and northerly (cold) currents converge and heavy rain falls there.
4. As the depressions develop, the cold front becomes also more remarkable.
5. Depressions on a line of discontinuity extending from west to east do not develop in general, and owing to its slow motion gloomy weather continues for several days.
6. In the above case, the high pressure on the northern side of the line of discontinuity enlarges its area of influence eastwards, pushing away the warm front southwards, therefore the warm front turns out to be a cold one and the cyclonic vorticity decreases.
7. When warm air becomes fresh in the frontal zone, a depression is liable to form there. In an unstable field where the pressure gradient is small, a depression is easily formed even in a single air-mass by a slight discontinuity of the wind direction or air temperature which can be produced by a small difference between the paths of air-masses branched off from the same origin.
8. When a line of discontinuity extends from west to east, the field is nearly barotropic. But when it extends from south to north, the field is baroclinic and cyclogenetic, the direction of pressure gradient being from west to east and that of temperature gradient from southeast to northwest. In the latter case a depression necessarily develops.
9. In front of a depre sion on the line of discontinuity extending from south to north, the direction of isothermal lines is divergent, but in the rear convergent. On the other hand, in front of a depression on the line lying from east to west the cold northeasterly air currents prevail, hence the result is completely opposite to that of the former case. Consequently the present argument agrees partly with Scherhag's divergent theory, and partly with Sutcliffe's theory of geostrophic departure.