Heavy Rainfall in the Kwantô District in September, 1935

[摘要] Heavy rainfall was experienced in Kwantô district from 21st to 26th September, 1935, when the precipitation amounted more than 600mm in mountainous region.1) Heavy rainfall of the first period: A line of discontinuity laid in the offing of Kwantô and Tôkaidô from 21st to 23rd, which had the structure of warm front, but scarcely changed its location throughout the period. In this case the rainfall limited on the northern side of the line of discontinuity and the amount of precipitation decreased according the distance from the line2) Heavy rainfall of the second period: The second line of discontinuity, which extended from northeast to south-west and swept over Kwantoô toward north-westwards, accompanied by heavy squall going ahead it.The first squall (A group) broke out near Suruga Bay and advanced east-north-eastwards with velocity 47km per hour. This direction seemed to run rather parallel to the line of discontinuity and did not coincide with the upper air current.The second squall (B1 group) broke out in the south-eastern part of Kwantô district along the line, parallel to and ahead the line of discontinuity. This squall advanced north-westwards.3) Heavy rainfall of the third period: After the passage of second line of discontinuity, heavy rainfall accompanied by thunder storm occurred in the mountainous region of the western and north-western parts of Kwantô. This heavy rain seemed to occur excited by the ascending current, due to the orographic effect of the mountain on the south eastern tropical maritime air mass.4) The inclination of the first surface of discontinuity was 15', which had been determined by the observation on the earth's surface and Mt Ibukiyama. On the other hand the inclination of the second one was about 1°, determined by the vertical distribution of equivalent potential temperature.5) A relation is calculated between the intensity of rainfall ds/dt, horizontal wind velocity in the upper warm air of the surface of discontinuity υ, inclination of the surface of discontinuity ε1, the height of upper warm air /H, and the density of air ρ1, mixing ratio w1, and temperature of /T1 at the lowest part of warm air respectively. The relation is as follows: where β is the lapse rate, R is the gas constant, g is the acceralation of gravity and log10 b is 2308. The numerical values within the blacket of the equation were calculated in a table, for every values H and T1.6) The vertical velocities of the warm air of the second surface of discontinuity, calculated from the maximum intensity of rainfall observed at twelve stations by the method of above mentioned, does not exceed 1 meter per second, with the assumption that the upper limit of the warm air is 8km from the sea level.7) The mechanism of the first surface of discontinuity is the common one which appeared in many text books of meteorology. But the mechanism of the second surface of discontinuity, which is also a kind of warm front, is very complicated, compared with the former, for the sake of the steep inclination of the surface of discontinuity, and heavy squall advancing parallel and perpendicular to the line of discontinuity, which suggests the existence of violent vertical current within the warm air.8) The vertical structure of the cold air is very stable while the warm air is conditionally unstable and has abundant humidity. Therefore it should be able to cause the heavy rainfall.9) The mechanism of the advancement of the heavy squall seems to be, that the breakage of instability occurred at some place, propagates to the direction of more u stable structure, and never propagates according with direction of the upper air current.Explanation of the Illustrations.

[学科分类] 大气科学