Statistical interpretations of ensemble-time mean forecasts by the use of a dynamical model with unchanging external conditions are discussed. For this purpose, three kinds of variances are defined and their interrelations are clarified. It is proposed to define the predictability limit of the ensembletime mean forecasts as the period when their error variance surpasses that of the climate-time mean forecasts. It is shown that, for a large ensemble of forecasts, this limit is close to Shukla's (1981) limit of individual time mean forecasts. The latter limit is defined as the period when the variance of the time mean forecasts with slightly different initial perturbations approaches that of the time mean forecasts from widely different basic initial conditions. The statistical significance of ensemble-time mean predictability is also discussed and the interprepation of the analysis of variance is clarified. It is emphasized that a null hypothesis of unpredictability should not be readily accepted unless the confidence intervals are sufficiently small. It is shown by the use of confidence intervals that the number of Shukla's predictability experiments with a general circulation model is too small to statistically support his conclusion that the 31-60 day means are not dynamically predictable.
For the purpose of studying the atmospheric response (or sensitivity) to the seasonal forcing, we propose a time-space spectral general circulation model. In the present work we use a low-order equivalent Barotropic vorticity equation with forcing and dissipation terms. The original low-order equation consists of three-component in the space-spectral domain: one purely zonal component and two wave components with the same zonal wavenumber but different mode numbers. Further expanding this equation in the space-spectral domain into that in the time-spectral domain, we finally obtain a time-space spectral model, which is a simultaneous nonlinear algebraic equations system. The model used in the present work consists of one steady and one periodic components in the time-domain. The nonlinear equations system is solved by using the "revised Marquardt method" (Levenberg-Marquardt-Morrison method) and a "Continuation method". Addition of the periodic forcing besides the steady forcing brings about a new stable solution together with the stable solutions corresponding to those under the steady forcing only. This multiplicity of solutions with the moderate intensity of steady and periodic forcings is very interesting for understanding climatic change, and suggests that we can expect to obtain useful results, by extending our model to a further larger timespace spectral general circulation model based on the above-mentioned numerical methods.
A barotropic quasi-geostrophic inviscid β-plane channel model with simple surface topography is constructed to investigate the nonlinear temporal behavior of forced Rossby waves. In particular, an emphasis is placed on the consequent evolution of forced Rossby waves due to growing unstable perturbations, in connection with the wave amplification observed during the blocking and the sudden warming. This paper consists of two parts: In Part I, we investigate linear stability properties of forced Rossby waves. The presence of surface topography is found to have a significant effect to destabilize the basic flow. When the forced Rossby wave has the largest horizontal scale, the flow becomes unstable and produces two kinds of instabilities: One is the topographic instability for the superresonant flow and the other is for the subresonant flow. The former, which can be represented in a loworder model of Charney and DeVore (1979), produces a standing type perturbation characterized by the form-drag through topography, while the latter, which cannot be represented in the low-order model, is characterized by the interaction between wave components of the perturbation and the basic forced Rossby wave. With the decrease of the zonal flow speed, the horizontal scale of the perturbation becomes smaller, as is expected from the necessary condition for instability. It is also found that the topographic instability does not appear when the meridional wavenumber of the forced Rossby wave is even.
The nonlinear temporal evolution of topographically forced Rossby waves and unstable perturbations examined in Part I is investigated analytically and numerically by using a barotropic quasi-geostrophic inviscid model. For the near-resonant flow, a low-order spectral model as in Charney and DeVore (1979) is dealt with analytically. In the linearly unstable region (topographic instability), the evolution of the wave amplitude depends on the sign of the initial zonal flow perturbation: If the zonal flow is decelerated initially, the wave amplifies in a nonlinear oscillation at the expense of the initial zonal kinetic energy through the form-drag; otherwise, the wave amplitude decreases. The period of this oscillation decreases with the increase of the amplitude of the initial forced wave. In the linearly neutral region, on the other hand, the behavior of the motion depends on the magnitude of the initial perturbation. For small perturbations waves show small variations around the steady solution and evolve large trajectories like as in the unstable region for larger perturbations. These properties in the low-order model are also justified in a full nonlinear spectral model with many variables. The relationship between the zonal flow and the forced wave in this evolution shows that the theory of Tung and Lindzen (1979a, b) for the linear resonant growth of the forced Rossby wave is not applicable to this amplification of the planetary waves. It is also found that the results obtained in our nonlinear model essentially confirm the evolution of the waves in the weakly nonlinear theory of Plumb (1979; 1981a, b) for the near-resonant forced Rossby wave. On the other hand, for the off-resonant flow, the result of some numerical integrations of the full nonlinear spectral equation shows that the time-variation of planetary waves is produced by the unstable perturbation with a horizontal scale different from that of the forced Rossby wave; this evolution is completely different from that in the near-resonasnt region.
This paper reports on an attempt to use observations to determine the nature of the eddy contributions to the mean zonal momentum balance in the region of the tropical stratosphere that is dominated by the semiannual oscillation (SAO). Since direct observations of the eddy wind fields in this region of the atmosphere are very limited, an indirect procedure was employed. The first step in this process was the computation of diabatic heating rates using observed temperatures and a sophisticated radiative transfer code. These heating rates were then employed to calculate the residual mean meridional circulation. The advection of mean zonal momentum and the Coriolis torque associated with the residual circulation could then be computed. The difference between this contribution to the zonal mean momentum balance and the actual observed acceleration of the zonally-averaged zonal wind was ascribed to the Eliassen-Palm (EP) flux convergence associated with eddies of all types. Meridional profiles of the inferred EP flux convergence were produced for each month of the year at the 1.0 and 0.4mb levels. At both levels there was an indication of the presence of an equatorially-trapped westerly contribution to the EP flux convergence. This is consistent with the suggestion of Hirota (1978) that the westerly accelerations in the SAO are provided by a dissipating equatorial Kelvin wave. In fact at 1mb the total EP flux convergence at the equator is always westerly; this suggests that the dominant contribution to the easterly acceleration near the equator comes from advection by the residual circulation, in accord with the model results of Holton and Wehrbein (1980) and Mahlman and Sinclair (1980). However, there is an indication of significant easterly eddy forcing of the mean flow away from the equator in the winter hemisphere (and in both hemispheres near the equinoxes). This forcing is presumably due to the equatorward propagation of planetary waves generated in the extratropics in the manner discussed by Hirota (1976, 1978, 1979). This easterly eddy forcing becomes more significant at the 0.4mb level, and the total EP flux convergence on the equator becomes easterly during the easterly acceleration phase of the SAO. It thus appears that both the Holton-Wehrbein (1980) and the Hirota (1978, 1979) mechanisms are important for the generation of the easterly accelerations in the SAO. The planetary wave contribution is most significant in the winter hemisphere and at higher levels, and the contribution from the residual circulation predominates in the summer hemisphere and at lower levels.
We have investigated some effects of an envelope orography on the forecast fields by a hemispheric spectral model. For that, a series of 29 experiments is conducted for February 1984. The use of envelope orography improves the daily forecasts. In particular, it remarkably reduces the systematic forecast errors of zonal mean and planetary wave fields. This suggests that the envelopetype enhancement of mountain forcing is very effective in the maintenance of planetary scale stationary fields. In the zonal mean field, the forecast error of latitudinal mass distribution that the surface pressure increases (decreases) in lower (higher) latitudes with increasing forecast period, is reduced. That is, the excessive zonal mean equatorward flow in the lower troposphere at middle latitudes is effectively reduced. This flow change also modifies the predicted zonal mean temperature in the lower troposphere. With regard to the stationary waves, the response of the model atmosphere to the envelope orography varies largely with the latitude. In higher latitudes (60N), the improvement of the wave number 3 pattern is most remarkable. Their temporal variation suggests a remote control of the mountain forcings originating from middle latitudes. In middle latitudes (40N), a significant difference between the two forecasts with standard and envelope orographies appears around 80E. Its temporal behavior suggests that the difference is directly caused by the difference of mountain forcings around the Himalayas. Temperature differences between two forecasts occur mainly through the horizontal advection of sensible heat. The forecasts of cold surge events are also modified by the envelope orography.
A marked periodic oscillation of trochoidal motion was found in the track of the eye fixed by radar and satellite for the typhoon 8019 (WYNNE). Five remarkable oscillations with the period of 5hr-8 hr and the maximum amplitude of 23km were observed for 36 hrs by land-based Miyakojima radar. The amplitude decreased with decreasing period. The analysis of PPI echo image shows that the center of typhoon eye did not agree with the system center of the whole typhoon circulation; and the eye shifted and rotated counterclockwise around the system center. In the occurrence of the phenomenon, the "double eye" structure of the echo and the double wind maxima were observed in the typhoon; the outer and inner eyewalls have diameters of about 260km and 30km, respectively. In particular, elliptical circulation of the pressure and wind fields and resulting the rectangular-shaped echo configuration were found an area inside the radius of the outer eyewall. The rectangle and elliptical circulation rotated counterclockwise around the system center with the same period of that of trochoidal motion. Both features were inscribed in the outer eyewall ring with radius of approximately 130km. The eye traced the subpoint corresponding to one of the focus of the ellipse on the major axis; consequently, the trochoidal motion took places in the typhoon circulation. However, it was not clear why the eye was shifted.
Lorenz (1968, 1976) stated that regime transition in almost-intransitivity of nonlinear climatic system may play an important role in climatic change, and he suggested that climatic change associated with the transition may appear in interannual variabilities. Referring to Lorenz's suggestion, we will treat abrupt changes of time mean, designated as climatic jumps. A quantitative definition of jump and simple method of its detection are presented, noting that the time of jump appearance can be specified within a margin of several years. Some jumps are detected in time series of seasonal mean data of surface air temperature, sea level pressure, precipitation, sunshine duration and maximum depth of snow-cover averaged spatially over Japan. The fact that jumps appear commonly in various climatic elements around 1950 suggests an association of these jumps with some abrupt changes of the atmospheric general circulation. Concerning the cause of the jumps around 1950, we survey some change in external forcings. Big explosions of several volcanoes over the world occurred almost simultaneously with the jumps around 1950 after a pause of about 30 years. It is inadequate to assume that this volcanic activity would directly cause the jump in transitive system, because the possible climatic effect of volcanic eruption is mainly cooling and the jump of temperature is warming over Japan. However, further studies are needed for any definite conclusion on problem whether this reopening of volcanic activity would be a triggering action of the regime transition or not.
Three-dimensional distributions of wind speed and temperature in a V-shaped valley covered with a pasture and scattered shrubs were continuously observed. The typical drainage flows were observed in the cold layer when the wind speed at a ridge is less than 2.6m/s and mainly 79 cases of such a drainage flow were analyzed. The cold layer in the valley is divided into two parts. The upper part is a drifted cold layer and a lower one is a primary cold layer. The vertical profile of potential temperature in the drifted cold layer is represented by a linear function of height. The wind profile of a typical drainage flow is parabolic rather than Prandtl-like. The wind speed below the level of maximum wind does not seriously change with a fetch, but the wind speed in the upper part of the cold layer outstandingly increases with a fetch. The maximum wind speed of drainage flow increases in proportion to a square root of cooling strength. Also, the height of cold layer at the fetch of 800m slightly increases with a cooling strength. The horizontal heat flux toward the downstream along the valley axis changes with a fetch and took the values of -1.01MW and -1.77MW at the fetches of 400m and 800m, respectively. From the difference of horizontal heat flux between two cross sections of valley, the bulk coefficient of heat transfer, CH, and the reciprocal of the interfacial Stanton number, BH-1, were estimated at 0.01 and 20, respectively.
The lidar measurement on stratospheric aerosols made at Syowa Station (69°00'S, 39°35'E), Antarctica in 1983 is described. In early winter, stratospheric aerosol content began to increase rapidly corresponding with temperature decrease. During this period, depolarization ratio of aerosol layer was low. After then, not only aerosol content but also depolarization ratio were in high level in winter 1983. These suggest that nonspherical particles were actively formed in the cold winter stratosphere, and confirm that the suggestion by Steele et al. (1983), in which increase of particulate matter in the winter polar stratosphere is assumed to be due to the growth of individual ice crystals through sublimation of water vapor molecules, is important factor controlling the winter enhancement of polar stratospheric aerosols. However, the measurement in the early winter showed that the increase in particulate matter in the winter stratosphere is not only due to the growth of individual nonspherical particles but due to some other processes. Additionally balloon measurement made on June 3rd showed that there were lots of large particles (about 15 particles cm-3) in the lower stratosphere. The growth of Aitken particle to large particle is speculated as another possible process causing the increase in particulate matter content in the winter polar stratosphere.
The important feature of the present study is to consider the specific concept of thundercloud motion and quantify the sets of microphysical processes operating within and around a thundercloud. The present study shows that saturation space charge density at the upper boundary of a thundercloud reduces due to upward motion of the cloud. The obtained results also show that growth rate of electric field inside a moving thundercloud is declined with respect to a motionless thundercloud. The effect of the reduced electric field growth rate on charging of precipitation particles and precipitation development has also been discussed.