Time integrations of shallow water equations were carried out in order to investigate whether (inertio-) gravity waves are produced or not through upscale energy transfer due to nonlinear wave-wave interactions. Differing from the limitless upscale energy transfer occurred in pure two-dimensional turbulence, the results demonstrate that the upscale energy transfer is weak and is suppressed in the lower wavenumber domain. Initially given gravity waves with random amplitudes and phases produce somewhat inhomogeneous and wave-like patterns at the final decaying stage, which can be identified as large-scale (inertio-) gravity waves. This fact suggests that such nonlinear wave-wave interactions can be regarded as one of the candidates to produce large-scale waves. However, it cannot always be concluded definitely whether the process is an effective mechanism for generation of the large-scale gravity waves in the actual atmosphere or the oceans. Other features of shallow water waves appeared through the process of nonlinear interactions are described in detail.
It has been well-known that extraordinary deep local depressions are frequently observed at Urakawa on the west side of the Hidaka mountain range in Hokkaido (Arakawa, 1960), when the easterly prevails near the surface. Any easterly near the surface has a critical level, because the westerly prevails over Japan usually. The present numerical investigations show that the local depressions are responsible for a shear flow including a critical level. The results are as follows: 1) In the case of a uniform flow, the surface pressure differences between the both feet of the mountain range increase with the wind speed when it is small, and are maintained with small constant values when it is large. 2) The local depressions on the lee side are detected on the mid-slope, not on the foot, of the mountain range and the upper atmosphere is affected significantly by the mountain range. 3) In the case of a shear flow including a critical level, the surface pressure differences increase with increasing wind speed. 4) The local depressions appear on the foot of the mountain range and the atmosphere above the critical level is not affected seriously by the mountain range.
The response of the fluid stably stratified over the infinite slope was investigated by linear theory when finite length 2l in it was heated or cooled. If the angle of the slope was small, the nature of the flow near the slope changed depending on whether the half height of the slope h*S=lsinφ(φis angle of slope) was higher than that of thermal boundary layer h*T=α(υ/N)1/3l1/3 (α-3.5), which was developed over the heat island whose length was 2l, or not. On the real slope h*S>h*T was often satisfied at night and the flow was close to the results from Prandtl theory, but h*S<h*T in the daytime so that the flow behaved like a convection.
Winter averaged forecast error of the 72 hour forecast by the semi-operational version of the JMA spectral model has been studied. The zonally averaged tropospheric errors are characterized by the low level warming in the polar regions (north of 60N), and cooling in the subtropics and tropics. Tonal westerlies weaken in high latitudes and in the subtropics. The subtropical jet tends to strengthen but shifts slightly northwards. The geographical distributions of the errors are characterized by (1) the warming in the polar region at 850hPa over the eastern Siberia and northern Canada, (2) the lowering of the surface pressure over the major continents north of 45N, (3) the exitation of positive 500hPa height error that propagates westward, and (4) the cooling in the subtropical region at 850hPa in the neighborhood of the large-scale mountains. The temperature errors in the polar region and in the subtropics are examined with respect to the error in the heat transport by the transient disturbances. It was found that the error over the eastern Siberia is mainly due to the overprediction of meridional winds, while the error over the northern Canada is explained by the errors of the positions and the tracks of the disturbances. The errors in the subtropics are caused by the overprediction of cold surges at the eastern edge of the Tibetan Plateau, and by the eastward shift of the permanent trough to the east of the Rockies.
A time series of daily data from May 1972 through November 1980 has been used to estimate the annual cycle of the large-scale 500-mb geopotential height fields. The resulting Fourier coefficients, describing the smooth annual cycle, have been used to study the annual cycle of the planetary-scale Southern Hemispere seasonally forced waves and the momentum flux caused by these waves. The results are compared with an analogous study for the Northern Hemisphere. The amplitudes of zonal wave numbers 2 and 3 are far less pronounced in the Southern than in the Northern Hemisphere, and so is the momentum flux caused by these waves. In both hemispheres during the winter and polewards of a given latitude the momentum flux due to zonal wave number 2 is directed towards the equator. The flux is stronger and occupies a wider latitudinal zone in the Northern Hemisphere than in the Southern Hemisphere.
Tropospheric oscillation in the period range of 30-50 days during the 1978/79 winter season was studied using statistical and synoptic approaches. The analysis confirms earlier investigations that this low frequency mode is a broad frequency band phenomenon which involves primarily the zonal wind oscillation. It is very prominent over the Tropical belt of the Indian and Pacific Ocean, in particular, over the convective zones of the winter monsoon. The oscillation differs from its counterpart of the Indian summer monsoon, notably in the absence of divergent meridional propagation over the ascending regions of the winter monsoon circulation. However, prominent poleward phase propagations were observed over the eastern Pacific Ocean; this propagating mode seems to form over the near equatorial region, but is strongly barotropic away from the Equator. It appears that the synoptic scale motions over the Tronical Pacific Ocean are modulated by this low freauencv mode.
Forecast experiment is made using a 13-level 42km-mesh primitive equation model for the extremely intense rainstorm event which occurred over western Japan on 23 July 1982. The rainstorm was associated with a meso-α-scale depression and a low-level convergence line developed in a subtropical frontal zone, but not with an intense synoptic-scale trough. The large amount of precipitation (maximum-500mm) was concentrated in a narrow area (-100km square) and for a short time span (6-12 hours). Several aspects of the rainstorm including the concentration of the rainfall, development of the meso-α-scale depression and the convergence line are simulated in the model. However the predicted precipitation rate (-70mm/6 hour) is much smaller than that observed. Dynamical and thermodynamical processes involved in the occurrence of the rainstorm, such as the moisture convergence, changes in the vertical stability, diabatic heating, changes in vorticity and divergence fields are described on the basis of the experiment. The effect of the increase of the horizontal resolution in the model on the prediction of rainfall rate and the meso-α-scale circulation is also examined by comparing the model predictions with the different horizontal grid sizes (42km, 63km and 127km). The result indicates that both the precipitation rate and the total amount of precipitation are increased with the finer grid sizes. This is attributed to the increase in both magnitude and the degree of concentration of the moisture convergence in the lower layers. The result of a forecast experiment on another intense rainfall event over the western part of Japan in 22-23 July 1983 is also discussed.
The meridional wind compnents and the vertical p-velocities over the east coast of Peninsular Malaysia and Singapore were computed for the period of the heavy rain spells from 13th to 25th December 1982. The relationship between the heavy rain spells and the Hadley cell was investigated. The following significant features are noted : (1) The formation of the heavy rain spells over the east coast of Peninsular Malaysia and Singapore are mainly caused by strong upward motions of moist air in the southern branch of the Hadley cell. (2) The maximum upward velocities in mid-troposphere are caused by the combination of strong low-level convergence and the initial appearance of the equatorial cell in the southern branch of the Hadley cell.
Spectral analysis of long time series of three-hourly barometric observations at three Pacific Island stations was performed. In each case there was evidence for a peak in power near periods of about 33 hours. It appears that this oscillation is reasonably coherent over the roughly 30°longitude separation of the stations. Following Matsuno (1980) this "33 hour wave" is tentatively identified with the gravest, zonal wavenumber one normal mode Kelvin wave. The present analysis makes possible the first determination of the surface pressure amplitude of this oscillation; this was found to be roughly one-fifth that of the more familiar "five day" Rossby normal mode in the tropics as estimated by Madden and Julian (1973).
The height variations of the scale of turbulence and the higher-order moments for the vertical wind velocity have been investigated by tower measurements at three heights (2, 11.25 and 21m) under neutral air. From the present analysis, it is obtained that the scale of turbulence slightly deviates from its linear relation with height. This result is different from the mixing length theory. Moreover, the height dependence of the skewness and the kurtosis of the vertical velocity component were verified experimentally.
The vertical distribution of precipitation charge in snow clouds was measured with electric charge sondes in the winter of 1976 in Sapporo. The following results were obtained. 1. In the case of stratiform snow clouds which included no graupel pellets, the precipitation was composed of negatively charged snow crystals. Their negative electrification is considered to be made by the selective ion capture processes. 2. It was confirmed that the strong positive space charge observed in the lower portion or below the cloud base was carried by positively charged graupel pellets. The pellets are considered to be positively charged by colliding with unrimed snow crystals at temperature regions warmer than about -10°C. 3. The local alternative space charges due to charged precipitation particles were observed. They corresponded to the rapid alternative change in the surface electric field.
The possibility of formation of ammonium sulfate particles in the stratosphere is examined by comparing the absorption rate of ammonia gas into a sulfuric acid droplet with the condensation rate of sulfuric acid vapor onto the droplet. When the concentration of ammonia gas is sub-ppb level, the conversion time from a sulfuric acid droplet of 0.1μm radius into an ammonium sulfate particle is one to two orders of magnitude shorter than the characteristic time scales of eddy diffusion and sedimentation of particles of 0.1μm radius in the stratosphere. The contribution of coagulation to the growth of particles is smaller than that of the condensation and negligible. When the concentration of ammonia gas is low enough for the influence of the absorption to be neglected, the conversion time from an ammonium sulfate particle of 0.1μm radius into an entire liquid droplet is comparable to or longer than the characteristic time scale of eddy diffusion and sedimentation in the stratosphere.
The latitudinal and seasonal variations of ozone photochemical process are analyzed using the monthly means of ozone density observed by ozonesondes at Sapporo, Tateno and Kagoshima. The estimated photochemical change rate (creation minus destruction) and observed ozone density variation are used to evaluate the divergence of ozone. The seasonal variation of total ozone amount at Kagoshima is mainly due to the density variation in the quasi steady region, where the photochemical change rate of ozone density is effectively balanced with its divergence. On the other hand, the variation at Sapporo is dominated by the dynamical process in the non-steady region. In this case, the primary effect in the dynamical process is the seasonal variation of tropopause height and the secondary one is the accumulation in the lower stratosphere due to the meridional circulation in winter. The former tends to set the time of maximum of total ozone amount in mid-winter and the latter shifts it to early spring. Especially, the continuous increase in the total ozone amount during winter is an evidence that the distribution of ozone remains at a transient state for the winter circulation. Such a remarkable difference between the mechanisms of the seasonal variation of total ozone amount in the two stations is originated from the variation of tropopause structure. The major tropopause that effectively separates the stratospheric air mass from the tropospheric one is the tropical tropopause through all seasons over Kagoshima, but it is the tropical one in summer and the polar one in winter over Sapporo. Finally, from the view point of the longitudinal dependence, the ozone distribution over Japan is discussed.
After the volcanic eruptions of Mt. El Chichon in Mexico, increase of stratospheric aerosols has been measured by a UV lidar at a wavelength of 340.5nm at Fukuoka (33.65°N, 130.35°E) since October 1982. The altitude of the maximum backscattering ratio (Rmax-3.5) was 24km in October 1982, and it decreased to 20km in January 1983. The value of Rmax-1 decreased by a factor of 2.4, but large changes of aerosol backscattering coefficient integrated over an altitude range of 15-30km were not recognized during this period. A brief consideration is given on this lowering of the aerosol layer.
Rainfall series in two large sectors of Northeast Brazil (Nordeste) and indicative atmospheric/oceanic fields over the tropical Atlantic are studied in terms of their characteristic time and space variability. Spatial couplings between rainfall and surface circulation are first explored using simple correlation. Spectral analysis of rainfall reveals relative maxima of power in the frequency bands of 12.7-14.9, 4.5-4.9, and 2.2-2.4 years. The spatial linkages are further examined by maps of the phase relationships in these three preferred bands.