The influence of eastward-moving, mid-latitudinal planetary waves on the tropics is investigated, using a numerical model of primitive equations in spherical coordinates. Under the existence of a horizontal shear between the middle latitude westerly and the equatorial easterly, these waves encounter critical latitude where their phase speed is equal to the mean flow in the course of propagation. The preliminary consideration using linearized equations shows that mid-latitudinal planetarywave with zonal wavenumber 1 under the mean flow of 35m/sec can affect the tropical atmosphere most intensely when it has a period of about 12 days, in case of no critical latitude. In the case with critical latitude, the wave becomes evanescent in the south of that latitude and cannot have any influence upon the equatorial disturbances. However, in the model including non-linear interaction with zonal flow, this conclusion is greatly altered. The critical latitude can no longer be a barrier to the southward energy flow from the middle latitude and a new equatorial wave is instigated in the experiment. It is also revealed that the structure of the newly instigated wave is similar to the equatorial Kelvin wave.
Zonal harmonic analysis of the structure of quasi-stationary planetary waves in the winter stratosphere of the Northern Hemisphere made by Hirota and Sato (1969) is extended to the cases of eight winters (1963-1970) inclusive of 500mb data. Each winter is formed of three months: January, February and March. From the analysis of monthly mean states of the amplitude, the position of trough and the wave energy, it is found that the ″node-like structure″ of planetary waves of zonal wavenumber 1 appears at about 100mb level at 60°N in many months among 24 months. The presence of nodes and the intensity of the zonal westerlies at upper levels are highly correlated; the node appears when the intensity of the mean zonal wind at 10mb level exceeds about 40meter/sec. This suggests that the node appears as the result of the interferences of waves propagating upwards and downwards. In such situation, reflective indices of amplitudes are simply estimated to be between 35 and 60%. The presence of the node-like structure is not evident in the wave 1 at 40°N. It is not found in waves 2 and 3. The vertical distribution of the wave energy of waves 2 and 3 shows the maximum at about 300-500mb level and decreases with increasing height above the level. The year to year variation of the vertical structure of planetary waves is discussed, and is also compared with the result of van Loon et al. (1973).
To investigate the oscillation of ultra-long waves with periods longer than 12 months, a lowpass time filter and periodogram analysis are applied to zonal harmonics of wave numbers 1 to 5 of monthly mean 500mb geopotential fields at latitudes 30, 40, 50 and 60°N from January 1946 to December 1970. Statistically significant quasi-biennial period is found in the oscillation of amplitude for some of wave numbers at each latitude. Some correspondence is found between the oscillation of amplitude and that of monthly mean zonal wind in the equatorial stratosphere.
Successive observations of the radar echo which starts from random distribution of convective cells about 200km off the coast of the Hokuriku district to form longitudinal multiple echo lines, then transverse lines, and finally but less frequently, vortical echo pattern, denote typical evolution of the echo pattern of heavy snowfall on the Japan Sea side. The present paper aims to clarify, in more detail, properties, synoptic situation and formation process of the organized snow echo associated with the mesoscale severe phenomenon such as heavy snowfall, using materials obtained by the radar. Firstly, we measured to find that the mean spacing of the multiple echo lines was 30km, which is ten times as large as the mean height of the echo top. The observation supports Asai's theoretical result with respect to the spacing of longitudinal cloud bands. Secondly, the synoptic situation of the mesoscale vortical echo has been specifically examined. Synoptic survey of the environmental condition of the vortical echo was performed for seven samples. Main features commonly observed in the surface pressure pattern are a sharp ridge with the radius of curvature of 100km over the mountainous region of Jadan Island, a cyclonic circulation over the Japan Sea and predominadt difference of the vertical wind shear in the lower atmosphere between northern and western parts of the mesoscale vortical disturbance. Thirdly, the echo band nearly along the discontinuity line of the surface wind field and transverse echo lines intersecting the band were observed as a sign of the subsequent vortical echo. A model of the formation process of the vortical echo is presented in this paper.
A large echo band, whose length is about 1, 000km, developed over the East China Sea adjacent to southwestern Japan on 7 July 1970. The three dimensional structure of the echo band is analyzed by using the data of the Experiment of Severe Rainstorms Research Project. The echo band is associated with strong precipitations and a remarkably developed ageostrophic low-level jet stream. The analyzed moisture field around the jet stream coincides well with the model of the low-level jet's circulation proposed by Matsumoto and Ninomiya (1971). It is inferred about the mechanism of the echo band that the convective mixing of the horizontal momentum in the echo band maintains the ageostrophic low-level jet. The unbalanced solenoidal field caused by the ageostrophic wind field accelerates the vertical circulation and thus maintains the convective activities of the echo band. It is also found that the echo band is not a zone of uniformly distributed echo cells but a row of mesoscale echo clusters whose spacing is about 150km. Two other cases of large-scale echo bands, one over the Pacific coastal area and another over the Midwest of U. S. A., are also presented as similar examples.
This paper describes the three-dimensional structure of precipitation and some associated mesoscale dynamical features observed in frontal depressions. The material is derived mainly from a recent series of case studies in the British Isles undertaken jointly by the Meteorological Office and the Royal Radar Establishment. These studies have given fresh insight into the structure and mechanism of such features as rainbands, line convection, low level jets, and orographic rain.
The collection efficiency of circular disks, hexagonal plates and plates with broad branches simulated to snow crystals for water droplets was experimentally obtained. The capture of supercooled droplets by the models of snow crystals was examined in the free falling state of them through a layer of droplets in order to make the experiment analogous to natural condition. The collection efficiency of natural snow crystals of hexagonal plate type was also obtained by the same method. The collection efficiencies of models were generally smaller than Ranz and Wong's theoretical value based on the assumption of simplified flow around the circular disk except the region of their dimensionless parameter Ψ_??_0.9, where Ψ is a measure of the inertia of droplets. In the region of Ψ_??_1/16, where the theoretical collection efficiency of circular disk becomes zero, experimental values were finite. The collection efficiencies of natural snow crystals were smaller than values of models in the region of Ψ>0.3.