The stability of medium scale disturbances is discussed in connection with the Rossby andRichardson numbers. In case where the Richardson number is of the order of magnitude unity, we may find the unstable disturbances whose wave lengths are about 1, 000km-2, 000km in middle latitudes. Numerical experiments are also performed in order to simulate the rapid cyclogenesis of medium scale disturbances, which is frequently observed in the lower atmosphere near the southern part of the jet stream over Japan throughout all seasons except summer.
The upper wind data of the tropical Pacific stations during the period April through July 1962, which were previously studied by Yanai et al. (1968), are re-examined with respect to both the zonal and meridional wind components covering a wider spectral range from 2 to 25 day periods. A few new samples are added to study the wave disturbances in the western Pacific south of the equator. The results of the power spectrum analysis are presented in height frequency diagrams. Besides the previously known 4- to 5-day periodicity of the meridional wind component, a pronounced zonal wind oscillation with periods longer than 10 days is found throughout the entire troposphere and the lower stratosphere at most of the stations. A periodicity near 7 days in the zonal component is isolated at some stations. All of the major spectral peaks show a maximum concentration of power density in the upper troposphere. Regional differences in the spectral density distribution in the height-frequency diagrams are examined. A preliminary consideration of the interpretation of the time series analysis is also made.
Basing on the continuous radar observations during the four-day period from July 8 to July 12, 1968, a time lapse composite echo pattern is constructed. A remarkable wave shaped configuration appears on the composite echo distribution. There is a good correspondence between this echo pattern and a family of small cyclones generated on the Baiu front. The composite echoes are also compared with the time sections of precipitation and upper atmospheric situations.
A linearized stability analysis is made for convection arising in a horizontal layer of fluid with constant unstable temperature gradient and penetrating into a layer of fluid with constant stable temperature gradient lying above the unstable layer. With the Boussinesq approximation, solutions for the entire fluid are obtained by matching solutions for the unstable and stable layers at the interface. The critical Rayleigh number is calculated as a function of the depth and the static stability of the stable layer. Eigenfunctions for the vertical distributions of vertical velocity and temperature are also calculated. It is found that, for a fixed depth of the stable layer, the horizontal scale of convective cells at the marginal state is increased as the stability in the stable layer is decreased. However, with the infinitely deep stable layer, the vertical scale of convective cells is also increased and cells look more slender as the stability becomes smaller. For a fixed stability, as the epth of the stable layer is increased, both the critical Rayleigh number and the corresponding horizontal wavenumber are decreased first, attain minimums and then show oscillatory variations about asymptotic values. This oscillatory behavior of the critical Rayleigh number appears to be associated with the transition of the number of secondary cells occurring in the stable layer.
The critical Rayleigh number of a fluid layer confined in a box which is very thin in one horizontal direction and has lateral walls of finite thermal conductivity is studied theoretically. The lateral walls suppress the onset of convective motions not only mechanically but also thermally. The critical Rayleigh number, therefore, is far larger than that of a fluid layer, infinite in horizontal extent, 1708. The result coincides well with the experimental value obtained by Ukaji and Sawada (1969).
The behavior of cloud patterns portrayed on the ESSA 2 photographs after typhoons Doris (September 1966) and Winnie (August 1966) encountered the pre-existing stationary fronts in the vicinity of Japan is discussed in relation to the results of the analysis based on the previously proposed hypothesis on the transformation of typhoons (Sekioka, 1956a) : that a typhoon induces the formation of a new extratropical cyclone on the pre-existing front within the typhoon area producing a complex system of the typhoon and the cyclone. Some evidence is given in support of the validity of the hypothesis. The field of 500mb relative vorticity is also investigated in regard to the movement of typhoon Doris and the formation of the new extratropical cyclone.
In stereoscopic photogrammetry of the APT satellite pictures, the APT pictures with distortion were measured stereoscopically without optical rectification. The absolute heights of cloud tops were determined by deducting the errors caused by the distortion of the photographs, which were previously calculated. Thus the distribution of clouds between the earth surface and a 12.5km level over the Hokkaido Island and its surroundings was analyzed at every 2.5km height which was the maximum accuracy of the resolution of the photograph. By use of this method, it was shown that the analysis of the three dimensional structure of cloud patterns were possible and moreover, the orographic clouds over the land, which were difficult to distinguish in individual photographs were readily distinguished.
A series of observations on the shapes of solid precipitation was carried out directly using microscopic photography at Syowa Station, Antarctica from the beginning of February 1968 to the end of January 1969. Besides, the replica solution method was used at the same time and sometimes, snow crystal sonde method by captive balloon was used jointly. As a result, unknown and peculiar shapes of solid precipitation as shown in Photos. 1-12 were discovered. It is considered that the majority of the peculiar shapes has some connection with bullet and the side plane shape snow crystals known hitherto. Especially, it seems worthy of attention that the minimum temperatures in the clouds in which the peculiar crystals grew as estimated from sounding curves were higher than -35° in almost all cases of the crystals.
Observations of pre-activated ice nuclei were carried out at Barrow, Alaska, and at the summit of Mt. Fuji, 3776m above sea level, in winter. Outside airat subzero temperature was sampled simultaneously by two millipore filters of pore size 1.2&. One of the filters was developed by the usual method of using supercooled sugar solution for counting the number of ice nuclei, without warming it above 0°C. The other one of them was developed after warming above +20°C. Difference of number of ice nuclei between the cold filter and warmed one indicates the number of ice nuclei pre-activated by cooling. It was concluded from these observations that the pre-activation process by cooling occurs in polar air masses and in the upper atmosphere.