We examine here the convection in a conditionally unstable atmosphere as an eigenvalue problem. If we take account of the effect of evaporation in a downdraft of liquid water which has condensed in the updraft in a convection, we have various types of convection. One of them has an updraft only in the cloud body and the others have both an updraft and a downdraft. We deal with these convections as an open system, that is, the air-mass flow through lateral boundaries is taken account of. The stability properties are discussed in connection with the mean vertical motion associated with the air-mass flow. It is shown that the convection followed by mean upward motion has a smaller lowest Rayleigh number, wider preferred cloud area and smaller compensating downdraft area while the convection followed by mean downward motion has a larger lowest Rayleigh number and smaller preferred cloud area than those for a closed convection. These results may explain the fact that the activity of cumulus convections increases in the low level convergence field or the upper level divergence field. Furthermore we discuss the energy conversion process of convection with mean vertical motion and the heating effect of convection on the large scale field.
Numerical experiments concerning the medium-scale disturbances were performed with use of a dry model in a channel flow and three typical patterns of disturbances were obtained. When the initial Richardson number (Ri) of the basic field was about 2.0, ordinary baroclinic waves* developed. When Ri was about 0.4 initially, symmetric type of disturbances grew. While, when there existed statically unstable domain initially, a different type of organized disturbances developed. The pattern of the surface pressure resembles the pattern of Case B presented by the numerical experiment of Gambo (1970b). Convective redistribution of heat at small Ri atmosphere played an essential role in the development of this wave.
The vertical structure of the barotropically unstable wave in a tropical easterly current is studied by solving linearized quasi-geostrophic equations numerically. An easterly current with a sine-profile jet is assumed to exist in a belt between 5N and 25N with a maximum velocity of 10m sec-1. The vertical shear of the easterly current and surface friction are taken into account. It is found that the vertical structure in the lower troposphere represents some features of the easterly wave such as eastward tilt of the trough axis, upward motion located to the east of the trough and negative correlation between upward motion and temperature. While surface friction is an indispensable factor to these features, the vertical shear and the decreasing hori-zontal shear with respect to height in the lower troposphere act to suppress them. As suggested in a previous paper (Yamasaki and Wada, 1972), an extremum of frictional convergence is located to the east of the trough in a region where Z (meridional gradient of the absolute vorticity of an easterly current) around 900mb is negative, and another extremum is located slightly to the west of the trough in a region of positive Z. The effect of parameterized convection on the barotropic unstable wave is also discussed.
Spectrum analysis is applied to satellite observed cloud data over the tropical region of the northern Pacific during the period from October, 1968, through September, 1969. Power spectra of the zonally averaged cloud amount between 160E and 120W are first computed along every 2.5 degree latitude band from 30N to the equator. They reveal three prononuced peaks in the period ranges of 16.7-25.0 days, 7.1-10.0 days, and 3.3-5.0 days. The shorter period fluctuation (3.3-5.0 days), which appears to be a feature of the low latitudes (5-10N) only, is less significant than the others with longer time periods. The pronounced spectral peaks at 7.1-10.0 day period are encountered in low latitudes near 5 to 10N. The spectra with a period range of 16.7-25.0 days reveal a significant feature of two maxima in the substropics near 20N and the tropics near 5-10N, separated by a minimum near 15N. Spectra at individual localities are computed at every 2.5 degrees latitude and longitude square mesh. They also exhibit three pronounced peaks at the period ranges of 16.7-25.0 days, 7.1-10.0 days, and 3.3-5.0 days, respectively. From the analysis of the coherence and phase difference with respect to several reference points, one can estimate the wavelength of disturbances associated with major spectral peaks. The wavelength estimated for disturbances with the 16.7-day period is about 7, 000km (10, 000km) at tropical latitudes near 7.5-10N, and about 15, 000km (13, 000km) at subtropical latitudes near 22.5-25N during the winter (summer) months. The wavelength for the 10-day period disturbances is measured only at 7.5-10N, as the fluctuation with this period is not primarily important in more northerly latitudes. It is 7, 000km for the winter months, and 12, 000km for the summer months. As for the 4.5-5.0-day disturbances, the estimated wavelength is about 4, 000km at 7.5-10N during the winter months, while it is unreliable for the summer months because of somewhat poor coherence.
Spectrum analysis is applied to satellite observed cloudiness data over the northern and southern Pacific. Power spectra with periods up to 50 days are computed by using continuous data series during the four year period from October, 1965 through September, 1969. They reveal the existence of four pronounced peaks in the period ranges of 33.3-50.0 days, 16.7-25.0 days, 7.1-10.0 days, and 4.0-5.0 days. Spectra in the long period end (33.3-50.0 days) exhibit a significant feature of two maxima (one in the vicinity of 20 N and the other between 5-10 N) over the northern Pacific, while they are not in zonal alignment in the southern Pacific. The shorter period fluctuation (4.0-5.0 days) appears to be significant in low latitudes only. Upon eliminating annual mean variations from the original data series (Jones, 1964, 1971), the daily filtered data are further separated into two groups : four winter half year data and four summer half-year data. An attempt is then made to compute the 4-year mean cloud spectral power with periods up to 25 days in both seasons. This computation proves the existence of three marked peaks with periods 16.7-25.0 days, 7.1-10.0 days and 4.0-5.0 days. The analysis of coherence and phase difference with respect to a reference point (22.5 N, 155 W) reveals the zonal wavelength of cloud disturbances (16.7-25.0 day period) to be about 12, 200km (30, 000km) in the winter (summer) half year. Attempts to obtain wavelength and phase speed for both the 7.1-10.0 day period and the 4.0-5.0 day period were unsuccessful because of a low coherence.
Mean tropospheric soundings over different regions of the tropical belt lying between 30°N and S during different periods of the year are constructed and used to compute mean static stability (SS) over all regions and mean Richardson number (Ri) over monsoon regions in different isobaric layers. The results show that the vertical distribution of moist static stability based on values of equivalent potential temperature is remarkably similar in all regions, with high negative values near surface and high positive values in the upper troposphere, the level of transition from negative to positive value occurring at a level between about 700 and 500mb. However, the investigation reveals the following regional and seasonal characteristics in values of mean static stability: (a) in any particular region, the lower troposphere is statically more unstable and instability extends to greater heights during the summer than during the winter; (b) during a particular season, the regions lying in the summer hemisphere show greater static instability extending to greater heights than those lying in the winter hemisphere: and (c) in any particular hemisphere, the atmosphere over the region/area lying near the equator is statically more unstable than that lying away from the equator. The vertical distribution of Ri as judged by values in monsoon regions appears to exhibit similar variation with seasons and between regions as static stability.
Attempt has been made to identify equatorial waves, of the type described by Yanai and Maruyama in the tropical Pacific, over the region east of the Indian Ocean. The study has shown that occurrence of such waves in the lower stratosphere is a strong possibility in the region under question.
Twenty-four years of typhoon data (1945-1968) were evaluated to determine if there are geographic and seasonal variations in very large and very small tropical cyclones of typhoon intensity. The results show that there are indeed distinct geographic and seasonal preferences for both large and small storms.
An effect of the earth tide on the observation of surface barometric lunar tides is discussed If corrected to the rigid earth's surface z=0, the amplitude of the statistically observed tide should be 20 to 40 percent enlarged, while the phase lag is reduced. Seasonal variation of the phase of the tide is found to be significantly smaller than so far believed.
Wind distribution in the neighborhood of an obstacle near the ground surface was measured. In the present study there are used a circular cylinder, a square pillar, the Marine Tower of our Institute, and a model of the tower which is reduced on the scale of one to twelve. Wind decrement on the windward side and wind increment on the transverse side of the obstacle are noted. The distribution of wind velocity around a square pillar corresponds to that around a circular cylinder with a diameter from 1.55 to 1.6 times as large as the side length of the square. At some height above the ground surface the wind pattern on the windward side of the circular cylinder is almost represented by the potential solution for an ideal inviscid fluid, but in the layer close to the ground surface the effect of an obstacle in the wind field is considerably greater than that predicted by the potential solution.
Balloon measurements of negative small ion density in the atmosphere from ground to 30km have been made in September of 1970. Vertical profiles of negative small ion density have been obtained for daytime and nighttime conditions. The difference between the profiles obtained during daytime and nighttime are discussed. The rapid changes in the ion density are observed in the altitudes below about 3km. A remarkable decrease in the ion density is observed in the altitudes from 16 to 25km. During nighttime a second ion density increase is observed in the altitudes of about 30km. The profiles obtained are compared with those determined from the data of ionization measurements in the atmosphere, assuming ionization equilibrium with small ions. The results obtained from the measurements at a constant level of about 30km are also described.
Snow and rain water samples were collected at a site on the coast of the Hokuriku district in February and March 1970 and rain water samples were collected at a site on Mt. Odaigahara in July 1970. Solid particles contained in snow and rain water samples were analyzed by means of X-ray, infrared and electron optical methods. The results are summarized as follows: (1) Most of the solid particles in snow sample were found to be soil particles originated from the arid or semiarid regions of North China and Mongolia; these soil particles are considered to play an important role as ice nuclei in the formation of snow crystals in the season of north west monsoon. (2) At both sites, rain water samples precipitated from the air mass which did not pass through the arid or semiarid regions of the continent of Asia and stayed over the ocean for a long time contained a small quantity of solid particles. (3) Rain water samples collected from rainfall due to Typhoon 7002 contained a very small quantity of solid particles. Their materials were identified as α-quartz, α-cristobalite, pyroxene and unidentified mineral. It is suggested that these particles were of volcanic and other origins and suspended in the upper troposphere or the stratosphere.
We have to see by direct observations whether the meridional circulation of the thermally controlled and frictionally driven motion in a steady rotating dishpan has a direct sense or an indirect sense. A reverse circulation accompanied by a front was observed in some experiments also, under the jet stream observed at the water surface.