Thirteen radiosondes for measuring the electric charge on precipitation elements were launched in cumulus clouds during showers and in anvil clouds. During showers, positively electrified raindrops were observed in the clouds. Relatively strong electrified precipitation elements were measured in the anvil clouds extending from the thunderstorm.
The frequency distribution of the wave length of mesoscale disturbances is studied for a 3-day period from July 8 to July 11, 1968, when an active "Baiu front" was situated over the Kyushu district. The predominant wave length of mesoscale disturbances is shown to be about 150∼200km. A series of mesoscale disturbances accompanied by heavy precipitation are studied in detail particularly for the case of July 9. Significant rainfall amounts were limited to the southern part of Kyushu where a low-level jet stream was located, while the configuration of the associated mesoscale pressure system was elongated in the north-south direction to a large extent. Stages of precipitation intensity depend on the spatial phase relationship between the pressure and the divergence fields. The synoptic-scale divergence and vorticity of the surface wind are found to fluctuate with a very short period of the order of 1.5∼3 hours. The phase difference between the fluctuation of divergence and that of vorticity changes from "in phase" to gout of phase" when a cyclone passes over Kyushu. This seems to be due to the convective exchange process in the area of convergence.
A detailed comparison is made between the rainfall distribution over land at intervals of 20 minutes and the radar echoes which were associated with Typhoon Vera in September 1959. Even at this short time scale, the association of the isohyet pattern with the radar echo distribution is not easily seen because the rainfall amount is very much enhanced in the mountain range and the orographic rain is not well detected by radars. An analysis of time changes of the 20-min rainfall amount reveals the existence of zones of strong rainfall intensity corresponding to radar rain bands. It is found that not only the rainfall amount but also its variability with time are dependent upon the elevation of rain-gauge stations. This suggests a mutual interaction between the orographic rain and the convection associated with the typhoon rain bands.
In order to simulate numerically one of the most remarkable phenomena of airflow over a mountain, i. e., the hydraulic jump on the lee side of the mountain, several numerical experimen tsare conducted with the shallow fluid flow system. Since the jump is a kind of shock phenomenon in the fluid, we need to use a highly accurate finite difference scheme. In the present study the one-step Lax-wendroff method is adopted. The following four cases of uniform flows are given as the initial conditions over the whole domain, i. e., two cases of subcritical flows and remaining two cases of supercritical flows. The results are in good agreement with those of the one-dimensional flow obtained by Houghton and Kasahara (1968). In the present experiment the jump is set up on the lee side of an isolated bell-shaped obstacle in one out of two subcritical flows and in one out of two supercritical flows. In another case of the subcritical flow the wave of wave length which is close to the width of the obstacle predominates. The flow pattern similar to that of the one-dimensional flow is observed in the remaining case of the supercritical flow. The zonal flow crossing over the obstacle is more predominant than that passing around the obstacle.
A study is made of stability properties of perturbations superimposed on an unstably stratified plane parallel flow with variable vertical shear. Two different types of instability which may take place in the flow are found: one is a thermal instability modified by a shear flow, and the other is an inertial instability modified by a thermal stratification. Unstable perturbations of the thermal type are distinguished from those of the inertial type in terms of the Richardson number. The thermal instability is most favorable for development of a three-dimensional longitudinal perturbation whose wavelength in the direction parallel to the basic flow is much longer than that in the direction perpendicular to the basic flow. A preferred perturbation of the inertial instability, however, is of a two-dimensional transverse mode. Amplification of transverse perturbations of thermal origin is reduced by the influence of a shear flow regardless of the presence of variable shear in the basic flow. We conclude that a shear flow in general is responsible for the formation of longitudinal convection roll, and a variable shear slightly affects characteristics of thermal instability of a constant shear flow.
For the purpose of extending the theoretical interpretation of the large-scale equatorial waves discovered by Yanai and Maruyama (1966), a three dimensional linearized primitive equationmodel of unstable waves is constructed on an equatorial beta plane. It is assumed that the release of latent heat due to moist convection in the troposphere is disturbed in proportion to the horizontal convergence in the subcloud layer responding to large-scale equatorial waves. It is found that Mode HB in the theory of unstable low latitude disturbances by Yamasaki (1969) is characterized by complex equivalent depth which simulates many aspects of YanaiMaruyama waves. This mode is further classified into various meridional modes, following Matsuno (1966). When the heat in the upper troposphere exceeds a critical value, unstable waves of free internal mode come into existence. Their growth rate increases with increasing heat in the upper troposphere. With the exception of inertio-gravity waves and Kelvin waves, mixed Rossby-gravity waves are the most unstable. The period corresponding to the observed wavelength of 10, 000km coincides with the observed period of about 4 days. The e-folding time is of the order of 10 days which, however, decreases with increasing wavelength. The energy and momentum budgets are also examined in detail. It is shown that a nonlinear forcing by Rossby type waves results in easterly acceleration of the mean zonal wind in the lower stratosphere, while Kelvin waves accelerate a westerly flow.
A direct method was designed to solve the Poisson equation for an arbitrarily shaped domain with the boundary conditions of Dirichlet or Neumann. The present method is essentially an extension of the one-dimensional “sweep-out method„ to the two-dimensional case by using the matrix calculus. The procedure of the present method consists of two parts, i.e., (1) the calculation of the residual maxtrix R and its inverse matrix R-1, and (2) the correction of the initial guess matrix Φ by the use of R-1. Since R depends only on the shape of the domain, this method is powerful in repeating to solve the equation for the same domain but with different boundary conditions and load functions. An estimate of the number of arithmetic operations required for solving the equation indicates that the present method requires less machine time than those of other methods. Some remarks on the practical computations are also given.