Some Problems concering the Storm Prediction for the Oosaka District (The 1st Paper)

[摘要] It is well-known that the following two kinds of wind prevail near the surface in the Oosaka district. The one is a NE-erly light breeze blowing parallel to the River Yodo which has an intimate relation to the generation of fog and rain in this district_??_ The other is a SW-erly swift wind blowing along the major axis of the Oosaka Bay, which is closely connected with the storm of this district.It is important to know for the storm warning from when and where and with what strength a stormy wind blows. We first investigated in this paper statistically the time of beginning of a storm from the positions of centers of tropical cyclones, and in the second place the strength of wind from the pressure gradients there. The results thus obtained are as follows.(1) The frequency of storm hour at Oosaka is investigated for respective direction of wind during 1895-1936 (inclusively). In this district most part of storm hours is occupied by the SW-erly wind and when the wind velocity exceeds 15m/s, SW-erly gale becomes to prevail conspicuously.(2) When a storm begins to occur in the Oosaka district, the cyclone centers are found at the eastern side of the longitude of the Oosaka (135°E), i.e. the storm at Oosaka begins when the cyclone center comes to the northern side of Oosaka or its NE-ern quadrant; this fact may contradict to the general idea that the tropical cyclone gives the strongest storm in its SE-ern quadrant.(3) We may sometimes experience NE-erly gales in the case of a developed cyclone approaching from the offing of Tosa coast, SW to Oosaka, but it must be mentioned that the SW-erly storm bursts out as soon as the cyclone center has arrived at the longitude-135°E.(4) There will be the risk of the storm at Oosaka if the mean value of pressure gradient between the storm center and Oosaka becomes greater than 1 mmHg/deg. The max_??_mum wind velocity at Oosaka is apt to be greatly influenced by the steepness of gradient at the time of storm begining.(5) For the forcasting of storm in the cold season due to the powerful anticyclone on the continent, the pressure differences between Saisyû Island and Oosaka, Oosaka and Niigata are useful. When these differences attain 6mm. and 4mm. respectively, the storms come in almost all cases.(6) The empirical formulae of the relation between the wind velocity and the pressure gradient are obtained for the Oosaka district. These are classified into two. according to the direction of wind and applicable when the velocity is less than 15m/s.VWSW=1.0+5.6G_??_for SW-erly wind, VNE=0.6+2.0G_??_for NE-erly wind.where V is the velocity in m/s and G the gradient in mm/deg.(7) A simple formula is used for the evaluation of the pressure gradient at a place by the pressure change observed there when a typhoon is approaching, which is where _??_p being the pressure change per hour, V the moving vel. (km/h) of the typhoon, and θ the angle between the direction of movement of typhoon and the line connecting the typhoon center to that place. Comparing the gradient calculated by the above method with that obtained from weather chart, the growth or decay of the typhoon can be examined.(8) By three typhoons 1911 VI 19, 1912 IX 23, and 1934 IX 21, the relation between pressure gradient and the wind velocity is examined_??_ In this case also, the relation is quite different with each other for the two wind direction, NE-erly and S-erly. In the latter case the relation is expressed in a parabolic form in spite of linear in the former case.(9) The distribution of wind velocity at 10m. height in the Oosaka city is approximately expressed as where V0 being the wind velocity at the mouth of the Oosaka harbour and V that of station apart by D km. from the above place.

[学科分类] 大气科学