The amplitudes of stationary Rossby wave packets propagating in a super-rotational zonal flow are studied with the non-divergent linear barotropic vorticity equation. Discussions are made concerning various pitfalls in determining wave amplitudes on the basis of ray theory. It is recalled that the change of wave amplitude along a ray in a slowly varying medium can be calculated by ray theory only by specifying the ray configuration neighbouring the ray considered (owing to the ∇•Cg term). The poleward increase of stream function amplitude of stationary Rossby wavetrains as emphasized by Hoskins and Karoly (1981) is a consequence of a particular ray configuration implicitly assumed (all longitudinally neighbouring rays pointing the same direction). In their problem, a caustic forms at each turning latitude where northward-directed rays turn southward or vice versa. Although ∇•Cg is important for amplitude calculation in general, a spherical harmonic solution cosυθ'eiυφ', which is often referred to as a wavetrain along a great circle (θ'=0 in a tilted spherical coordinate system), is shown to be an exception. It is suggested that this solution should be interpreted to consist of an isolated single great circle ray. The direct application.of ray theory is impossible in this case so that a slight modification of the method is suggested. It is also noted that attention must be paid to zonally symmetric components (k=0) of solutions, especially when the total wave number of stationary wave is small. This is because steady state solutions of the linearized vorticity equation with basic zonal flows contain an ambiguity of arbitrary function of latitude. A physical consideration that the zonal mean flows do not change at the first order of wave amplitude leads to a modification of spherical harmonic solution.
Non-linear resonance of external Rossby wave in a continuously stratified atmosphere under cyclic condition in the meridional direction is studied by the perturbation expansion method. The feed-back effects of the mean zonal flow induced by the wave-topography (and/or heating) and wave-self interaction are included. As the forcings, a sinusoidal mountain and/or heating are assumed, while the Ekman damping, Ray Leigh friction and Newtonian cooling are included as the dissipations. In the case of topographic forcing alone, it is shown that for long (short) waves the amplitude response curves incline toward negative (positive) side of the detuning parameter δ (deviation of the basic zonal flow from the linear resonant condition), to give the socalled multiple (three) equilibrium solutions for a certain range of δ for small damping coefficients (e.g., Charney and DeVore, 1979; hereafter referred to as CD). The positions of maximum response on δ-axis shift in proportion to 2nd power of the magnitude of forcing and -2nd power of the damping coefficients. At a 'critical' wave number, the amplitude response curve is similar to that of linear resonance as a result of the cancellation between non-linear terms; the multiplicity of equilibrium vanishes. The stabilities of equilibrium solutions are examined by perturbation method, to show that the upper and the lower branches are stable, while the middle branch is unstable as has been pointed out by CD. All the equilibrium states outside the multiple equilibrium region are stable. If thermal forcing is incorporated, the response becomes complicated, depending upon its magnitude and its phase relative to the topography; for example, the amplitude response curve forms a loop if plotted against δ and the largest amplitude state is shown to be unstable. This loop becomes larger and wider as the heating increases. It is noted that such a loop can be found also in CD's model if the mountain becomes high, though it seems to have been overlooked so far. Finally, we show the time-dependent behaviors of the solutions by numerical integrations of the equation obtained by the present method.
The role of a temporary amplification of planetary waves in transport of the stratospheric ozone in mid-winter is studied with the use of a semi-spectral hemispheric primitive equation model. The results of a simulation involving zonal wavenumber 1 are presented.The wave forcing at the lower boundary (5km) is turned on at day 0, reaches the maximum at day 5, and then decays after day 20 and to become zero at day 25. The three-dimensional Lagrangian motions are investigated by tracing a large number of marked particles placed along latitude circles at the initial time. The Lagrangian motions in the middle and lower stratosphere are irreversible owing to the dissipations and the lower latitude critical surface. The particles in higher latitudes are found to return to the initial latitude after the wave has damped except for permanent downward displacements. In contrast, a material line consisting of air parcels at lower latitudes is cut off somewhere in the course of time evolution and some particles migrate poleward largely, and finally the members are widely scattered in both the meridional and horizontal planes. The ozone continuity equation is integrated for two types of ozone models: the Chapman and the inert models. When the planetary wave develops, large ozone wave is induced in middle latitudes followed by the total ozone enhancement in high latitudes and a little decrease in the tropical region. The latitudinal pattern of the tonal mean column ozone at the active stage of the planetary wave is almost maintained even after the wave has disappeared when the ozone distribution becomes axi-symmetric again. This net ozone change is largely responsible for the observed spring ozone maxima. The divergence (convergence) of the photochemically induced ozone eddy flux is almost cancelled by the mean photochemical source in the mid-latitude transition region, in the results the Chapman cycle acts only as diffusion.
Based on FGGE Level IIIb data, the structural features of 40-50 day oscillations over ann extensive region (30°S-30°N, 30°E-150°W) during the 1979 summer are detailed. The analysis confirms earlier investigations that these low frequency modes are primarily associated with the zonal wind oscillations. These 40-50 day perturbations propagate northward and eastward, which is most clearly defined over the monsoon region north of the equator from 60° to 150°E. The monsoon region is characterized by prominent spectral peaks in the 850mb meridional winds with periods shorter than 10 days, probably reflecting the activities of monsoon disturbances. However, the local Hadley circulation, as defined by averaging the meridional component of the wind between 60° and 150°E, exhibits a distinct spectral peak in the period range of 40-50 days. Similarly, the square of the meridional winds, which is a measure of synoptic-scale disturbance activity, also changes with a period of 40-50 days. These features, which are similar to the group velocity phenomena, are pronounced only over the central monsoon region (10°-20°N, 60°-150°E). The low frequency modes propagate northward and become most intensified near 10°-20°N through mutual interaction between synoptic-scale disturbances, the local Hadley circulation, and the tonal mean flows over the monsoon region. At the equator, the 40-50 day zonal wind pertubations propagate systematically eastward (500km/day) and upward (0.7km/day). In the equatorial region, the low frequency oscillations owe their existence to a lateral geopotential wave-energy flux from the monsoon region, which represents the major energy source for 40-50 day perturbations via the conversion from potential to kinetic enegy. Compared to the equator, the phase propagation of zonal wind perturbations along 15°N, although moving eastward, is not as systematic. At this latitude, zonal wind perturbations are pronounced in the lower troposphere over the monsoon region, and also in the upper troposphere over the western Pacific. As an integral part of E-W interaction between these two regimes, there occurs downward progression of westerly (or easterly) perturbations over to the Arabian Sea region. The downward phase of westerly (easterly) modes corresponds to the commencement of "active" ("break") monsoons over South and Southeast Asia.
In July 1979, large-scale apparent heat sources exceeded 2°C per day over the eastern Arabian Sea, the northern Bay of Bengal, the central South China Sea, and the equatorial Pacific near the dateline. These heating centers are embedded in the monsoon trough at 850mb and are in good agreement with regions of strong upward motions, apparent moisture sinks, and small outgoing longwave radiation values. In May, the moisture supply for the rainfall near the west coast of Burma and Malaysia comes primarily from the Bay of Bengal, not from the Southern Hemisphere. In midsummer (June to August), the cross-equatorial moisture flux off the east coast of Kenya is not large enough to maintain the rainfall over South and Southeast Asia. Thus, evaporation over the Arabian Sea constitutes the key contribution to the moisture supply for monsoon rains. The northward and eastward passage of 40-50 day perturbations is related to the phase changes between active and break monsoons over central South and Southeast Asia. When active monsoons begin, the large-scale apparent heat sources Q1 and moisture sinks Q2 become above normal over the Arabian Sea. About 5 to 7 days later, both Q1 and Q2 reach their maxima over the Bay of Bengal. This is followed by the intensification of Q1 and Q2 over the South China Sea region about 5 days later. Regions of above normal Q1 and Q2 also propagate northward across the monsoon region. Similarly, regions of break monsoons with below normal Q1 and Q2 propagate eastward and northward.
Evolution process and fine structure of a medium-scale (∼1000km) cloud cluster developed over the western portion of the Baiu front (Continental China region) and propagated to the eastern portion (the Japan Islands and the Pacific) are investigated. In this paper (Part I), special attention is put on variations in cloud/rainfall features, associated disturbances and environmental situations throughout evolution process of the cloud cluster.
Observation features and evolution process of a long-lived medium-scale cloud cluster have been analyzed in Part I of the present paper. Thermal and kinematic fields and heat budget of this cloud cluster are investigated in Part II. The special attention is given in its formation and developing stages over the Continental China region. The following results were obtained. 1). A cloud cluster developed in the uniform unstable tropical air over the Continent. Thermal fields in and around the cloud cluster exhibited local variations due to convections and 3-dimensional moisture advection. 2). Kinematic fields in the cloud cluster over the Continent also showed particular localized features. That is, medium-scale cyclonic circulation in the lower troposphere, anticyclonic circulation in the upper layer and upward motion are found. They indicated time variations throughout the formation and developing stages. 3). An apparent moisture sink in the lower tropospere and an apparent heat source in the upper troposphere existed in the cloud cluster. The both quantities reached the maxima at the time when observed rainfall reached the maximum (∼80mm/6 hour). 4). Calculated rainfall (vertically integrated apparent moisture sink) showed similar time variation to that in the observed rainfall: The calculated rainfall amount showed a variation of about one day period, that was consistent with the observed variation analyzed in Part I of the present paper. 5). Thermal and kinematic fields in and around the cloud cluster investigated in this study, which developed in the uniform unstable air over the Continent, showed similar features to those of medium-scale cloud clusters frequently developed in the Baiu front (subtropical baroclinic zone) over the Japan Islands and the Pacific Ocean, in spite of different environmental situations. The present study indicates that the cloud cluster transformed into a frontal depression under the influence of a short wave trough in the subtropical baroclinic zone. It is conjectured that in the atmosphere without baroclinicity the cloud cluster remained as a cloud cluster without transforming into a depression, even though a large amount of heat was released and redistributed through convections.
Convergent Cloud Bands over the central Japan Sea were observed by photographing clouds from an airliner in the winter monsoon period. The cloud structure of the band was analyzed by the use of the aerial photographs. Considering the data of radar pictures and daily amount of snowfall, it was shown that the Convergent Cloud Bands have a cloud bank line in its south-west boundary, and that the bank line brought heavy local snowfall in San-In District. The formation mechanism of the cloud band was discussed, and a structural model of the Convergent Cloud Band was proposed.
The structure of the land and sea breeze and its diurnal variation were studied by using the data of special field observation performed in the Sagami Bay area in summer of 1980 and 1981. A dynamical analysis was made in order to examine the balances of heat and momentum and the effect of the land slope on the land and sea breeze. It is found that the sea breeze is driven mainly by the land-sea surface temperature difference and has many features of a typical sea breeze. The sea breeze in the developing stage forms a small-scale vertical circulation centered over the coast line, while that in the mature stage forms a part of the "regional-scale sea breeze" which covers the whole Kanto plain. On the other hand, the land breeze is largely affected by the land slope, because the reduced vertical heat transport in the nocturnal atmosphere suppresses the land-sea breeze effect.
Growth of ice crystals from the vapour phase generally occurs in three processes : volume diffusion process of water molecules, surface kinetic process characterized by the dependence on surface supersaturation of the kinetic coefficient and conduction process of the latent heat of sublimation. General expression for the growth rate of ice crystal is derived by taking into account these processes. This expression uses the supersaturation at infinite distance from the crystal surface to describe the macroscopic driving force for the growth and the sum of resistances for the individual processes. By estimating the values of the resistances, we can investigate the role of each process as rate determining process under the given growth conditions specified by temperature, supersaturation, crystal size, air pressure and so on. We also discuss the supersaturation determined by temperature and pressure of water vapour at the crystal surface under various growth conditions.
Growth rates of basal and prism faces of ice crystals from the vapour phase have been measured as functions of the supersaturation at infinity from the crystal surface at -30°C and at air pressures 250 and 0.3 Torr*. Contributions of the volume diffusion process of water molecules, the surface kinetic process and the conduction process of the sublimation heat to the measured growth rates have been quantitatively discussed on the basis of theoretical consideration in the companion paper (Kuroda, 1984). The dependence on supersaturation at the crystal surface and air pressure of the kinetic coefficient or the condensation coefficient has been obtained by combining the theory of the companion paper with the experiments. It should be noted that an increase in air pressure makes not only the diffusion process slower, but also the surface kinetic process inactive.
The complex index of refraction, scattering cross section and albedo for single scattering have been estimated from measurements of the angular distribution of light scattered by aerosol particles, by an inversion library method. The humidity dependence of these optical properties has been examined in compiling 250 samples for the period FebruaryNovember 1978. It is found that optical properties of aerosol particles change systematically according to the change of relative humidity. The humidity dependence of the complex index of refraction is explained by Hänel's theory introducing the mean mass increase coefficient for atmospheric aerosols consisting of both water-soluble and insoluble substances and the value of 1.58-0.04 i for the complex index of refraction of dry particles. Preliminary experiments of controlled relative humidity have also been performed to confirm the above results.