A power spectral analysis is made for the data of upper winds at Canton Island (S 02°46", W 171°43') from June 1957 to May 1960. For the whole data of three years, the power spectrum of the meridional wind component gives a peak at a period of about four days. To examine the relation between the disturbances with this period and the mean flow which exhibits quasi-biennial periodicity, the mean flow and the power spectra of the disturbances with periods less than ten days are computed for every 60-day time series data. For the meridional wind component, the time sequence of the power spectra shows that disturbances with a period of about four days are prevailing in the lower stratosphere during almost all the time. The regions of relatively strong power spectral density in this period range descend with time as the alternation of the mean flow does. It is shown that these regions are observed where the mean flow weakens with height. The vertical extent of the disturbances is roughly 4km on the average. The phase of the disturbances at lower levels lags behind that at upper levels. The phase lines of the disturbances tilt westward with height, in consideration of their westward propagation supported by additional synoptic analyses.
The heat and water budgets over the Japan Sea and the Japan Islands are analysed with special emphasis on the physical links among the energy supply from the sea surface to the atmosphere, the convective transfer of heat and moisture and the heavy snowfall for various synoptic situations in the winters of 1963, 1964 and 1965 by using aerological data with the aid of the marine, radar, dropsonde and aerophotographic cloud observations. When a cold vortex aloft passes over, the increase of evaporation from the Japan Sea and the convergence of water vapor over the Japan Islands are not so remarkable as the increase of snowfall and the sensible heat supply from the sea. The large amount of snowfall is explained by the high efficiency in condensation process and the convergence of the condensed water substance over the coastal area. The terms concerning the convective transfer of heat and moisture are introduced in the large scale budget equation and the magnitudes of the transfer across various levels are estimated. The amount of transfer as well as that of the supplied energy are found to increase when the stratification becomes unstable under the influence of the cold air aloft. The magnitude of the convective transfer obtained in large scale analysis coincides almost with the estimation based on the observation data about cumulus convection such as the convective upward speed or excess temperature in clouds.
Statistical analyses on the activity of the cumulus clouds over the Japan Sea in wintertime are made in relation to the water vapor convergence in the subcloud layer and the evaporation from the sea surface by using radar and aerophotographic cloud observations. The convective transfer of water vapor across the base of cloud layer is defined in the water vapor budget in the subcloud layer. High correlation between the magnitude of convective transfer and the connective activity is verified. Over the zone of convergence of large-scale or mesoscale disturbance, thehorizontally converged water vapor can not be compensated by the upward transfer due to the mean vertical motion. Thus the upward convective transfer is required to balance the water vapor budget. The convective transfer of 10mm, day-1 is expected only over the zone of convergence whose order is of 10-4sec-1, a characteristic value of the convergence field of mesoscale systems. This is a reason why active cumulus groups exist only over the mesocsale convergence zone. When the distribution of mean coverage of convective cells observed on PPI radar scope is obtained by a time average for longer time interval, the effect of evaporation from the sea surface upon the cumulus activity becomes observable. It is shown that only a few mm. day-1 difference of the amount of evaporation remarkably control the activity of cumulus coluds. Some statistical features of cumulus clouds such as the height-spacing relation and the area ratio of the core of the upward motion are also discussed.
The validitiy of finite difference approximations in solving the dynamic instability problem of non-divergent barotropic currents is examined numerically. We first study the instability of a sine-curve symmetric zonal current confined between two rigid walls. The relation between the solutions of the finite difference equations and those of the original differential equation is discussed. Then the accuracy of the finite difference method in describing the instability is examined by varying the number of subdivisions. For a sufficiently accurate description of the instability, a large number of subdivisions, at least 20, are required for this velocity profile. We then study the instability of a symmetric and an antisymmertic zonal currents extending to infinity. The effect of rigid boundaries placed at a finite distance from the central shearing wind belt on the critical wavelengths is examined by varying the position of the boundaries. For both velocity profiles, it is shown that the exact boundary conditions at infinity can be replaced by rigid boundary conditions at a distance equal to the half width of the shearing wind belt.
The vertical transport of zonal momentum due to the large-scale westward moving disturbances with a period of about four days in the westerlies of the equatorial lower stratosphere is studies by spectral analyses for the data of special observations over the tropical Pacific during the periods from March to July 1958 and from April to July 1962. By neglecting the effects of heat source and horizontal advection in the thermodynamic equation, the quadrature-spectral estimates between the zonal wind component and temperature are shown to give a measure of the vertical transport of zonal mementum. An analysis at the 50-mb level shows that the vertical transport of westerly momentum due to the disturbances in the period range of three to five days is upward all over the equatiorial area within the latitudes of ten degrees. The maximum upward transport of westerly momentum at this level is roughly 2×10 (m.sec1)2 at about 6° latitude, and rather less at the equator. The disturbances are shown to carry the sensible heat poleward, and upward transport of wave energy due to the disturbances is implied.
Analyses of the structure and behavior of planetary waves in the upper stratosphere are made by the use of the meteorological rocket observations together with the daily stratospheric synoptic charts during the period of 1966 wintertime. The vertical time-sections of temperature for Fort Greely, Fort Churchill and other stations indicate that large-scale disturbances exist in the higher latitudes extending vertically from the lower to the upper stratosphere. The vertical cross sections of temperature along 60°N also indicate the evidence of planetary-scale wavelike disturbances characterized by the westward tilting of isotherms, as well as the trough axis, with increasing height. The separation of traveling planetary waves from the standing wave is made by taking time derivatives of temperature along 60°N. The results show the existence of eastward moving waves of wave number two and westward moving waves of wave number one. The downward propagation of temperature phase as indicated by the vertical time-section at a given station is related to the westward movement of wave disturbances. Finally discussion is extended to the relationship between the behavior of transient waves and the time variation of zonal wind in terms of stability properties of polar night westerlies.