Free-fall patterns and the variations in the vertical and horizontal velocities of unrimed plate-like snow crystals were analyzed by means of a stereo-photogrammetric method. Whether the crystals exhibit a stable falling motion or not mainly depends on the Best number, which involves the mean vertical velocity (fall velocity), and the non-dimensional moment of inertia of the crystals. Unstable fall patterns were roughly classified into three types: nonrotation, swing, and rotation or spiral. For dendritic shaped crystals, the distinction among these types is approximately determined by the combination of the non-dimensional moment of inertia and the Reynolds number, which involves the mean vertical velocity. Although the standard deviation of the vertical velocity for dendritic shaped crystals was very small (1 to 3% of the mean vertical velocity), that of the horizontal velocity was considerably larger, measuring 5 to 20% of the mean vertical velocity. Accordingly, it appears likely that the variation in the horizontal velocity plays an important role in the random aggregation of plate-like snow crystals having almost the same shape and size.
A snow band, formed on the lee side of Mt. Akaiwa at Takashima Peninsula, Hokkaido, was observed by a three-dimensional scanning Doppler radar. The snow band was composed of meso-γ scale (-20 km) radar echoes, whose interval of appearance was about 40 minutes. Inside the meso-γ scale echoes, were two or three convective echoes with 10 km in size. Further, each convective echo was composed of several convective cells 1-2km in size. The snow band elongated from west to north and convective cells existing along the southern part of the snow band always appeared the most active and the highest. The convective cells successively formed only along the northern part of the snow band. The width of the snow band was dependent on the number of convective cells which composed the convective clouds. Thus, the snow band was asymmetrical in shape in the vertical plane, perpendicular to its elongated direction. On an average, the band width increased proportional to (distance)1/3. The band thickness linearly increased with increase in band width, and the ratio between them increased, that is, the snow band flattened as distance increased. Averaged snow water content continued to increase over the ocean, for the convective clouds which continued to grow. But it decreased near the sea coast. This can be explained as follows: When a cloud approaches land, snow particles which are sustained in the air over the sea begin to fall near the sea coast because the averaged speed of updraft in a cloud decreases there. After landfall, the convective clouds regenerated, owing to horizontal wind convergence created by Mt. Asoiwa and weakened rapidly on its lee side.
A midlevel vortex which occurred during the period of 16-17 June 1987 in the Taiwan Area Mesoscale Experiment (TAMEX) is analyzed in this paper. All of the intensive observational data collected in this period were employed to reveal the structure and evolution of the vortex. These included the conventional surface and upper air observations, conventional and Doppler radar data, and the P-3 aircraft measurements. The cloud top temperatures were also analyzed using enhanced Geostationary Meteorological Satellite (GMS) IR imageries. Analyses showed that this midlevel vortex had a cold, moist core in the middle and lower troposphere (900-550 hPa; 1-5 km) capped by a warm upper troposphere (300-200hPa) and tilted slightly northwestward towards the mid-tropospheric cold air in the early stage. It developed upward to the 300 hPa (-10 km) warm region and downward to 950 hPa (0.5 km) in the later stage. The horizontal scale of this vortex was about 100-200 km in diameter and the life span was possibly more than 24h. The heavy rainfall (>125 mm/24h) over northeastern Taiwan appeared to be due to the combined effects of the enhanced terrain lifting and the particular structure of the maximum wind over northeastern quadrant of this vortex. Based on the overall results, it is proposed that the vertical vortex stretching process coupled with the local topographical effect and the pre-existing vorticity associated with the westward propogating trough was primarily responsible for the formation of this vortex. The reintensification of the vortex during its northward journey over land in the later stage was perhaps mainly due to the intensification of the mesoscale convective systems over the vortex area.
on 13 October 1980 five tornadoes occurred in association with typhoon 8019 WYNNE in the Miyazaki City area on the east coast of Kyushu Island, Japan, injuring 15 people, destroying about 400 houses and causing crop damage of sixteen hundred million yen. This tornado event was selected for study because of the availability of detailed surface data allowing for the examination of the mesoscale flow fields near the tornadoes. Emphasis is placed on the detection of the tornado cyclones and description of the tornadogenesis. Around the time of the tornado outbreaks, there existed vertical wind shear of about 20ms-1 km-1 between the surface and 1km height, and saturated moist air with positive buoyancy prevailed over most of the troposphere. The tornadoes occurred during the passage of west-northwestward to northwestward moving echoes in the rainbands of the typhoon. Four tornadoes originated from one long-lived echo. Analysis of perturbation wind fields revealed that tornadoes occurred generally in the rear of the tornado cyclones which moved inland from the Hyuga-Nada at a speed of about 13ms-1.
When studying snow-particle formation and precipitation mechanisms, it is important to examine the ratio of the contribution of each process-such as deposition, riming, and aggregation-to the growth of a snow particle. In the winter monsoon season, measurements were made of the contribution ratio of the riming process to snow-particle growth by the method of Harimaya and Sato (1989). Snowfall in which the riming process was predominant in the growth of snow particles (i.e. snowfall having a rime mass of more than 50 % of the total mass) accounted for about 70 % of total snowfall frequency along the coastal areas toward the Japan Sea. Over the coastal areas, the deposition process did not contribute to strong snowfall intensity, being controlled instead by the riming process. It has generally been considered that snow clouds in the developing and mature stages pass over coastal areas more frequently than over inland areas, as snow clouds move from the sea toward the land. It has been observed that the riming process was predominant during the developing and mature stages of snow clouds. It follows from these that the riming process is predominant in snowfall along coastal areas where such snow clouds often pass.
We study the stratospheric solid particles collected at 24 km altitude over Japan using a balloonborne impactor. The elemental compositions of 200 particles (1-30μm in diameter) are analyzed with an energy-dispersive X-ray analyzer. The size distribution and concentration of the particles are also assessed. In our sample, particles rich in Al are rarely found. Since the predominance of Alrich particles were frequently observed in previous samplings, it is suggested that the composition of stratospheric solid particles varies greatly as a function of time, location and/or altitude. On the other hand, we have found 14 particles composed of Cr, Fe and Ni. These particles (probably, most of the other particles as well) are considered to have been transported from the ground to the stratosphere through convective clouds.
The mechanism of mesoscale cyclogenesis in winter monsoon air streams is studied by data analyses of observations and linear instability analysis. Mesoscale cyclones, with a diameter of 200-700 km, occasionally develop off the west coast of Hokkaido, Japan. Based on the analysis of satellite images, the mesoscale cyclones are classified into two types according to their horizontal scale: Type I (200-300 km in diameter) and Type II (500-700 km). The observational studies suggest that the baroclinicity in the lower troposphere is important for the mesoscale cyclogenesis. The quasi-geostrophic equations are used to investigate an instability problem of a baroclinic flow with both the zonal and meridional components. The basic flow of the model expresses the essential elements of the observed wind profiles. The linear instability analysis showed that two unstable modes are found: Mode I with a wavelength of 200-300 km and Mode II with 500-700 km. The former is caused by the vertical shear of the meridional wind component and the latter mainly by that of the zonal wind component. The comparisons between theoretical and observational aspects indicate that Mode I and Mode II can account for the characteristic properties of Type I and Type II, respectively. The energetics shows that both Mode I and Mode II are maintained by the increase of the eddy available potential energy and its conversion into the eddy kinetic energy. Based on the data analyses of observations and the linear instability analysis, we concluded that the mesoscale cyclones are baroclinic instability disturbances developing in a particular baroclinic flow.
Regional changes in precipitation are studied by means of a statistical hypothesis test. Precipitation data used are records at 52 meteorological stations in Japan where observation has been carried out since before 1900. The test used is the Lepage test, which is a distribution-free two-sample test. The detection is made in the following way: Two data sets of precipitation records during N-year period till (Y-1) year and after Y year are statistically tested. The Y year is moved so as to cover the time series data at each station, and when the test rejects the null hypothesis of no difference with a 1% or 5% significance level, the change is determined. The time variation of the number of the stations where the change in the annual precipitation amount is detected with a 1 % significance level shows that the number of the stations sharply increases around 1924, 1949 and 1960 when N=25. This large number indicates that the change occurred in a wide area, and the sharp variation of the number indicates that these changes are discontinuous. A change in 1915 can be added to these by referring to a previous study. Spatial distributions of the difference between the means for 25-year periods prior to and from the years mentioned above illustrate that every change shows a trend of being consistent within one region but different among other regions. Thus, regional discontinuous changes are presented. The same test is applied to the seasonal data. The change in 1949 is strongly detected in the AprilJuly season when it falls on Baiu, an East Asian rainy season. The change in 1915 is also detected in the November-March season, an East Asian winter monsoon season. The changes in 1924 and in 1960 were dominant only in the annual precipitation amount. Dependence of the detection upon the sample size is also studied. Because a few papers reported that discontinuous climate changes occurred on a global scale in 1920s and around 1950, regional changes in precipitation corresponding to these global changes are shown in Japan.
The mean characteristics of the phase function and of the coefficients of its Legendre series expansion were studied within a sample of 239 observed fog droplet size spectra, by computing their mean value, dispersion coefficient, skewness and kurtosis, at 86 wavelengths in the λ=0.35-90μm spectral interval. The results showed that the mean spectral behavior of the above quantities retains all the features pertaining to single spectra, and that the values of the optical quantities obtained for the different spectra are widely dispersed about their mean values. In particular, the dispersion for the phase function, at visible wavelengths, has a minimum close to zero (disp=l.3%) around 40°, surrounded by a wide interval (15-55°) of low dispersion; for the asymmetry parameter the dispersion increases with wavelength from 1.7% at λ=0.35μm up to 48% at λ=90μm. The data produced can serve as reference values for both experimental and theoretical needs.
The occurrence and amount of graupel precipitation over the Japan Islands are studied using routine surface observation data at 150 stations. The results of statistical analysis of the annual and monthly number of days with graupel for 1971-1986 in the geographical map and in annual cycles show that graupel fall is predominant in winter over the Japan Sea coastal area in Tohoku and Hokuriku District. Furthermore, monthly precipitation amount of graupel in January for 1982-1986 at five stations in the coastal area are statistically estimated to be at least one-fourth to one-third of the monthly precipitation amount. Based on the statistical results of graupel precipitation, weather modification potential by cloud seeding is discussed.