The preferred horizontal plan form of Benard convection is examined by a numerical integra-tion of Boussinesq system of equations. We carried out the computations of the cases of R/Rc=2 and 6 for air and water, where R is a prescribed Rayleigh number and Rc is the critical Rayleigh number. The steady rolls are obtained under a nearly random small initial temperature perturbation except the case of R=6Rc for water. In the case of R=6Rc for water we have a steady three-dimensional convection. However the final steady convection with a finite amplitude initial perturbation has a different horizontal plan form from one with the nearly random small initial temperature perturbation. The vertically coherent oscillation whose period is almost the same as that observed in the laboratory experiments by Krishnamurti (1970) and Willis and Deardorff (1970) was found in the case of R=6Rc for air.
Large-scale aspects of the air-mass transformation over the East China Sea during AMTEX '74 (February 14-28, 1974) are studied based on the synoptic and heat energy budget analysis. Both the synoptic and budget situations over the East China Sea vary according to the alternation of the subtropical air-mass and the polar air-mass. The synoptic situations in the earlier period are characterized by the small air-sea temperature difference, the southerly wind and very weak thermal advection in the lower layer. The amount of the total heat energy supply from the sea is small (200ly/day) during this period. The moisture and heat supply from the sea primarily contributes to the local time change of moisture and temperature in the lower layer. The synoptic situations for the latter period are characterized by the large air-sea temperature difference, the northerly wind, and strong cold advection in the lower troposphere below the inversion layer. The amount of the total heat energy supply from the sea is as large as 1200 ly/day. In spite of the strong cold advection, the temperature fall (∂T/∂t) is small, because the cold advection is primarily balanced by the diabatic heating due to the energy supply from the sea. The results of the budget calculations also indicate that the diabatic heating is mainly confined in the lower layer below the inversion. The time variation of the vertical stability in the lower troposphere is also analysed in relation to the formation of the inversion layer. The results of analysis indicate that the differential thermal advection is the primal factor to control the vertical stability. It is also shown that the differential diabatic heating cancels some part of the differential advection during the period of cold air outbre
Detailed analysis on the field of horizontal moisture flux and moisture budget is made in the vicinity of the Japan Islands for the 16 days period between 29 June and 14 July 1972, which includes the period of extremely heavy rainfalls. The results of the analysis indicates that the strong precipitation is primarily related to the increase of the horizontal moisture convergence in the lower layer below 700 mb, while the role of the moisture convergence in the upper layer above 700mb is insignificant. The areal mean amounts of the observed precipitations, which are accurately estimated from the data of about 2000 rain-gauge stations, are well agreed with the amounts of the calculated precipitations (apparent moisture sink). By comparing the situations of moisture budgets for subdivided periods (i.e., light, heavy and extremely heavy rainfall periods), we are able to show that the situations in the extremely heavy rainfall period is remarkably different from those in remainders. During the extremely heavy rainfall period, the low-level moisture convergence mentioned above is mainly caused by the southerly transversal moisture influx across the lateral boundary of the cloud zone rather than the longitudinal flux within the cloud zone. It is also concluded that the moisture flux from the subtropical Pacific, i.e., from the outside of the cloud zone, into the cloud zone is primarily important to the occurrence of heavy and extremely heavy rainfalls.
The synoptic scale structure of the ITCZ over mid-oceanic regions is deduced from observations over the Pacific and the Atlantic oceans. The temperature distribution indicates that the ITCZ is warm-cored in the lower troposphere and cold-cored in the middle troposphere. The moisture distribution is characterized by large moisture content throughout all levels in the central core; a region of low moisture content is found near the 700-mb level just outside of the central core. The wind is relatively unperturbed at the surface layers but significant perturbations are indicated in the middle troposphere. The physical processes which may be responsible for the observed structure are proposed. On the basis of these physical processes, the energetics of the ITCZ as a synoptic system is discussed.
Simultaneous observations of the surface electric field, shape and charge on snow crystals and melted diameters of the crystals were carried out at the seashore of Ishikari near Sapporo where snow clouds of falling snow accompanied by northwesterly winds were observed during the winter monsoon season. From the observational results, it was confirmed that in general dendritic crystals were negatively charged under a positive electric field and relatively large dendritic snowflakes, densely rimed crystals and soft hail were positively charged under a negative electric field. Furthermore, it was found that when positive snow crystals fell, the mode value of the size distribution of melted diameter at that moment shifted to smaller values as compared with that when negative snow crystals fell alone. Therefore, it was considered that snowflakes and densely rimed crystals were positively charged when small fragments were torn off from fragile flakes and crystals during their descent and at such times small fragments acquired negative charges. Moreover, suspended small fragments acquired negative charges when they collided with the falling flakes and crystals. As a result, the main crystals acquired positive charges as a matter of course.
A cold chamber in which a carrier gas can be compressed up to 150 atm was designed and constructed to study ice crystal growth in high pressure carrier gas. Under high pressure conditions, the diffusion coefficient of water vapor is reduced inversely proportional to the pressure of the carrier gas, while the thermal conductivity of the gas is unchanged with pressure. Therefore, the effect of the diffusion coefficient of water vapor on ice crystal growth can be studied independently upon the thermal conductivity. The details of the chamber and an example of experimental results are shown in this paper.
Turbulent properties of longitudinal wind velocities were studied from wind records observed in 150 m meteorological tower on the coast. It was recognized that the wind from the sea was inflenced by the terrain features near the tower and the magnitude of turbulent velocities increased with decreasing height. The effect of terrain roughness was found clearly on the intensity of turbulence, but was hardly noticed on the scale of turbulence. The low-frequency part of power spectrum was greatly influenced by unknown causes except terrain roughness, and a considerable discrepancy was noticed from one observation to another. It would be hard to represent whole power spectra by one simple algebraic expression. The discrepancy was less between vertical correlations than between longitudinal scales. It might be expected from vertical correlations between various heights that a vertical scale was increasing and phase of gusts was advancing with increasing height.
The equation of eddy diffusivity K given by the product of the square of mixing length l and wind shear is not valid in the flow with small wind shear. If the wind shear used in the equation of K is replaced by a value averaged over relevant finite time Tl, the equation may become valid in the flow with small shear. The wind shear averaged over Tl can be written as the summation of the expected value and the deviation. Assuming that the probability distribution of the deviation is Gaussian, and that the structure function of wind velocity averaged over Tl obeys the 2/3 power law, we have K=max(l2|dV/dz|, αl4/3). The value of a is estimated as 0.l≤α≤0.2 (MKS units).