The Eulerian- and Lagrangian-mean flow fields induced around a ‘vertical’ critical surface for a planetary wave propagating latitudinally are discussed by using a simplified model of horizontal wave guide on a beta-plane. It is shown that the Eulerian-mean meridional stream function xE (not zonal mean part of geostrophic stream function ψ) is forced by a delta-function-like divergence of the Reynolds stress. This xE-equation is solved on the basis of the result of classical hydrodynamics for the critical layer problem (e.g., Lin, 1967; Lindzen and Tung, 1978), as done also by Matsuno and Nakamura (1979) for the problem of vertical propagation. It is shown that the Eulerian-mean flow fields are confined in the extent given by the Rossby radius of deformation divided by 2π. Particularly, it is noted that the mean zonal acceleration consists of two parts; one is the easterly acceleration concentrated on the critical surface and is associated with no meridional circulation, and the other is the acceleration due to the Coriolis force acting on the wave-induced mean meridional circulation. The latter acceleration is directed toward west in the mid-level and toward east in the upper and the lower layers. It is also shown that the mean vertical flow is discontinuous at the critical surface and has a four sector structure with upward (downward) branch in the southern (northern) side of the surface in the upper layer, and the flow is reversed in the lower layer. The mean meridional circulation consists of two circulations, opposite to each other, in the upper and the lower layers. Finally, it is shown that the Lagrangian-mean flow fields have the same structure as the Eulerian-mean ones. This is because the second order fields are induced by the divergence of Reynolds stress only, different from the case of vertical propagation (cf. Matsuno and Nakamura, 1979).
he stability of planetary waves in a two-layer fluid on a channel beta-plane is discussed. The lowest mode of baroclinic waves is shown to be always unstable in the sense of triad resonance instability. The lowest mode of barotropic waves, however, is stable if the zonal wavenumber is smaller than the critical value ηc. The transition corresponds to the occurrence of resonating phenomenon between the group velocity of the primary wave and the phase speed of long waves. This phenomenon is important as a mechanism generating relatively strong zonal flow. The coupled evolution equations which govern the phenomenon are derived. Also derived is the evolution equation which governs the modulation for a barotropic wave whose zonal wavenumber is smaller than ηc. These evolution equations have exact solutions. The stability of exact plane wave solutions is examined and related to the long wave resonance instability and the sideband instability reported by Plumb (1977). The exact solutions of a solitary type, which seem to be final states after above instabilities, are also obtained and presented as planetary solitons.
Real data forecast experiments of heavy rainfalls in Baiu front (a front in the Asian subtropical humid region) are made for two typical cases by using a 6-level 77km-mesh regional primitive equation model. We show first that the moist convective adjustment scheme which has bcen used in coarse-mesh models is not always suitable for the present model as the scheme tends to produce heavy rainfalls in the area far south to the frontal zone. In order to confine heavy rainfalls within the frontal zone or in the vicinity of frontal disturbances, we improved the scheme by controling the adjustment through low-level vorticity and by modifying the critical lapse rate used in the scheme. Detailed analysis is made for two cases of Baiu frontal heavy rainfalls predicted by the model including the improved adjustment scheme. The characteristic features of Baiu front associated with heavy rainfalls (i.e., concentration of heavy rainfalls in a zone to the south of subtropical jet stream, formation of a low-level strong wind zone to the south of the heavy rainfall zone, moist tongue and a zone of convective warming in the middle troposphere) are accurately predicted.
Analyses of the hemispherical or global network data of the surface air temperature during recent 100years have been performed by several authors. Some quantitative disagreements are noticed among their results. Most of the previous works indicate no reliability of the results which is indispensable to confirm any time changes. In the present paper, the optimum interpolation is applied to the northern hemisphere network of the seasonal mean surface air temperature anomaly, and the value and its error for each season from 1876 to 1975 are determined at the grid points on 10° latitude and 30° longitude (45° longitude at 80°N only). These grid point values and their errors make possible to compute easily the northern hemisphere mean temperature anomaly, together with the errors. General tendencies of the warming from the 1880s and the cooling from the 1940s are noticed in the time series of the northern hemisphere mean, similarly to those by Mitchell (1963) and Budyko (1969). However, comparing with their results, the magnitude of range of the temperature change in the present work is much small. On this disagreement, some discussions are given.
he vertical structure of the atmospheric boundary layer is studied by a facsimile record of a monostatic acoustic sounder. The echo intensity and pattern on height-time chart changes according to the meteorological conditions. Layered echos come back from the stable stratification of the atmosphere; the inversion layer. The depths of the layered echo are compared with the heights of the surface inversion layer observed by a tethered balloon borne sonde or low-level radiosonde. The same comparison is examined for the top and bottom heights of the lifted inversion layer. The relationships between the vertical temperature gradient and the existence of echoes are examined even if the intensity of the facsimile record changes with the temperature gradient and the gain-offset of the recorder. The stable stratification corresponds to the layered echo return. The super-adiabatic lapse rate or unstable temperature gradient gives the weed type echo. The weak or no echo is due to the temperature gradient between dry adiabatic and equi-temperature gradient. A classification for the facsimile record of the acoustic sounder is proposed according to the atmospheric stability; unstable, neutral and stable. The facsimile record is grouped into three major groups; vertical (weed), no significant echo and horizontal (layered) type.
Observations of middle-level precipitating clouds were made using a vertically pointing radar, a field mill and an optical spectrometer with a charge detector. It was studied how the electrical properties of raindrops varied with the fine structure of clouds and microphysical processes in clouds. Main results are as follows. In a part of the cloud system in which most of generating cells were in a vigorous stage and in which a large increase in echo intensity with decreasing height was found between the levels of -10°C and 0°C, positively charged raindrops were predominant on the ground and averaged charges were observed to be quite large. In the whole cloud system averaged values of positive charge per raindrop varied almost in phase with those of negative charge and both absolute values were nearly equal each other. But the number flux of positively or negatively charged drops did not necessarily vary with time in phase with the averaged charge of each sign. There was a rather good correlation between the time variation of maximum diameter of raindrops and that of the value of raindrop charge. Main results can be accounted for reasonably if we suppose that positively charged particles falling from the generating cell of a vigorous stage were mixed with negatively charged ones falling from the cell of a decaying stage in the region above 0°C level and that precipitation particles grew by aggregational process and they were recharged through friction process.
By using the cloudiness data from 1966 to 1972, a quasi-stationary appearance of 30 to 40 day period was confirmed during the summer monsoon season over India, which is recently found in the data for 1973 as a predominant mode of the monsoon fluctuations (Yasunari, 1979). In the same manner as the result of Yasunari (1979), the fluctuation of this mode shows a northward movement over India-Indian Ocean area in each year. This periodicity seems to appear annually except for the severe drought years such as 1972.