Showing papers in "Journal of the Meteorological Society of Japan in 1982"

Cloud Clusters and Large-Scale Vertical Motions in the Tropics

Abstract: The sensible heat budget of a large-scale area containing an idealized tropical cloud cluster is analyzed. The cluster is assumed to have spatial dimensions and precipitation rates typical of observed cloud clusters. In its early stages of development the idealized cluster consists of isolated deep precipitating convective cells, or \"hot towers.\" A simple model using the assumed precipitation rates as input is employed to compute the condensation and evaporation rates and sensible heat fluxes associated with the precipitating hot towers. The condensation dominates the contribution of the hot towers to the large-scale heat budget, and the net effect of the towers is warming distributed through the full depth of the troposphere. In its mature stage of development, the idealized cluster contains not only convective towers but a widespread cloud shield interconnecting the towers. The cloud shield is dynamically and thermodynamically active, and processes associated with it also contribute significantly to the large-scale sensible budget. Stratiform precipitation falls from the cloud shield, and in the stratiform precipitatioi region, condensation occurs in mesoscale updraft aloft, evaporation occurs in a mesoscale downdraft at low levels and melting occurs in a middle-level layer. The condensation, evaporation and sensible heat transports associated with the mesoscale updraft and downdraft are determined from simple models using the cluster's assumed stratiform precipitation rate as input. The evaporation and melting in the stratiform precipitation region are also estimated from vertical profiles of radar reflectivity in real cloud clusters. The total effects of the stratiform precipitation processes on the largescale heat budget are warming of the middle to upper troposphere, where condensation in the mesoscale updraft is the dominant effect, and cooling in the lower troposphere, where melting and mesoscale downdraft evaporation dominate. The widespread cloud shield present in the mature and later stages of a cloud cluster's life cycle is also an important absorber and emitter of radiation. Radiative transfer models applied to tropical cloud shields show substantial heating effects in the middle to upper troposphere. These effects are nearly as important as the heating by convective towers and the heating and cooling associated with the stratiform precipitation processes. As the idealized cloud cluster progresses from early to mature stages of development, its net effect on the large-scale heat budget changes. As the cloud shield develops, the mesoscale updraft condensation and radiation reinforce the heating by convective towers aloft, while the mesoscale downdraft evaporation and melting counteract the convective-tower heating at low levels. Thus, the net heating by the cluster increases in the upper troposphere and decreases in the lower troposphere as the system develops. Large-scale upward motion, which is required to balance the large-scale heat budget against the effects of the cluster, is thus expected to increase aloft and decrease at low levels. Vertical motions deduced from large-scale wind observations in the tropics confirm this expectation. Thus, it is concluded that the mesoscale stratiform and radiative processes associated with the cloud shields of developing cloud clusters are sufficiently strong to alter the large-scale vertical motion field in the tropics.

How Well do we Understand the Dynamics of Stratospheric Warmings

Abstract: Ever since Matsuno's pioneering numerical simulations of the stratospheric sudden warming there has been little reason to doubt that this spectacular natural phenomenon is essentially dynamical in origin. But theoretical modelling, and the use of satellite observations, are only just reaching the stage where there seem to be prospects of understanding stratospheric warmings in some detail and forecasting them reasonably well. An informal discussion of recent progress is given, and suggestions are made for future work, including a way of avoiding spurious resonances in mechanistic numerical models in which tropospheric motions are prescribed a priori.

Conceptual Evolution of the Theory and Modeling of the Tropical Cyclone

Abstract: Dynamically, the tropical cyclone is a mesoscale power plant with a synoptic-scale supportive system. By the early 1960's, the general structure and energetics of the system and basic components of the supportive mechanism were fairly well documented by the instrumented aircraft observation of hurricanes and through the diagnostic interpretation of the data. The prognostic theory which would have unified these basic findings in a dynamically coherent framework had a more difficult time emerging. When a viable theory finally emerged, a change in the theoretical perception of the problem was necessary. The parameterization of cumulus convection was an important technical factor in the reduction of a multiscale interaction problem to a mathematically tractable form. Nevertheless, it was the change in our perception of the basic problem and the re-arrangement of priorities that made the parameterization a tolerable substitute for real clouds. Even then, the validity and limitation of the new theory, known as CISK, were fully appreciated only through careful experiments with nonlinear numerical models. In the meantime, the mathematical simplicity of certain parameterization schemes enticed many to apply the schemes to other tropical disturbances, including the easterly wave, in the traditional idiom of linear stability analysis. More confusion than enlightenment often ensued as mathematics overran ill-defined physics. With further advances in numerical modeling, the interest in tropical cyclone research shifted from conceptual understanding of an idealized system to quantitative simulation of the detail of real cyclones, and it became clear that the intuitive parameterization of whole clouds would have to be discarded. Now that some models have returned to explicit calculation of the cloud scale, one may wonder if all the exercises with parameterized convection were an unfortunate detour in the history of tropical cyclone modeling. The answer depends on one's philosophical view of \"progress.\

On the Interannual Variability of the Middle Stratosphere during the Northern Winters

Abstract: The variability of the stratospheric winters is investigated. Monthly mean 30-mbar temperatures over the North Pole which are available for a 26-year period, have been grouped according to the phase of the equatorial QBO at the 50-mbar level, following a suggestion of Holton and Tan (1980). It is shown that the winters are \"relatively undisturbed, cold\" with an enhanced tendency for the development of a pronounced \"normal wave 2 pattern\" when the equatorial winds at the 50-mbar level are from the west. Major midwinter warmings seem not to take place in this category, except near the sunspot maxima. In contrast, during the \"easterly\" phase of the equatorial QBO there is a tendency for an enhanced development of height-wave 1 already in early winter, which leads often to the development of a major warming during midwinter and hence to a generally warmer polar

A Semi-Lagrangian and Semi-Implicit Numerical Integration Scheme for the Primitive Meteorological Equations

Abstract: A semi-Lagrangian algorithm is associated with the semi-implicit method in the integration of the shallow water equations on a rotating sphere. The resulting model is unconditionally stable and can be integrated with rather large time steps. Truncation errors remain reasonably small with time steps 25 times as large as those used with explicit integration schemes. An analysis of the proposed method is performed and it indicates that the scheme is stable. Also, the results of a few integrations are presented and from these we conclude that the model is not very sensitive to the size of the time step provided that it does not exceed a value of the order of two or three hours.

Space-Time Spectral Analysis and its Applications to Atmospheric Waves

Abstract: Space-time spectral analysis methods and their applications to large-scale atmospheric waves are reviewed. Space-time spectral analysis resolves transient waves into eastward and westward moving components and is mathematically analogous to rotary spectral analysis which resolves twodimensional velocity vectors into clockwise and anticlockwise components. Space-time spectral analysis can also resolve transient waves consisting of multiple wavenumbers into standing and traveling wave packets. Space-time energy spectra are governed by space-time spectral energy equations which consist of linear and nonlinear energy transfer spectra. Space-time spectra can be estimated by either the lag correlation method, direct Fourier transform method or the maximum entropy method depending on the length of the time record. By use of the modified space-Fourier transform these spectra can be estimated correctly from polar-orbiting satellite data which are sampled globally at different hours of the day. Space-time spectral analysis has been extensively applied to data generated by GFDL general circulation models to determine the wave characteristics, structure and energetics of transient planetary waves, to verify the model with observations and to clarify their generation mechanisms by means of controlled experiments.

A numerical model study of turbulent airflow in and above a forest canopy

Abstract: A simplified second-moment urbulence closure model, which has been reasonably well tested in various geophysical problems, is used to simulate effects of a tall tree canopy on air circulations in the atmospheric boundary layer. Qualitative simulation of a canopy flow, with nearly constant and low wind speeds in a canopy, but large wind shears near a treetop, and unstable (stable) temperature layers within a canopy during the night (day) are all satisfactory. Strong couplings between the mean and turbulence variables are obvious when simulations performed with and without a canopy in the model are compared with one another.

Three Dimensional Propagation of Planetary-Waves

Abstract: The ideas of ray tracing from geometrical optics and wave propagation in a slowly varying medium are used to study the propagation of plaentary waves in the atmosphere. Kinematic wave theory is applied to wave solutions of the linearised quasi-geostrophic potential vorticity equation on the sphere. An index is defined for planetary wave propagation in the vertical-meridional plane and it is shown that wave activity is refracted towards larger values of this index. Ray solutions for stationary planetary waves are calculated for simple basic states and for basic states representative of Northern Hemisphere summer, autumn and winter conditions. The results agree with those from observational and numerical model studies of stationary planetary waves in the atmosphere. It is shown that the sphericity of the Earth and the curvature of the zonal flow are important factors determining the propagation of

北半球中部成層圏における冬季極渦の年々変動;北半球中部成層圏における冬季極渦の年々変動;Interannual Variability of the Wintertime Polar Vortex in the Northern Hemisphere Middle Stratosphere

Abstract: Interannual vartability of the Northern Hemisphere polar vortex at the 30mb during 21 winter seasons is examined making use of two indices which may be viewed as measures of the intensity of the vortex. The signature of stratospheric warmings is clearly evident in time series of the indices, but these events account for only part of the interannual variability. Alleged relationships between the intensity of the wintertime stratospheric polar vortex and the phase of the equatorial stratospheric quasi-biennial oscillation and the tropical tropospheric Southern Oscillation are examined, making use of the same indices. Both relationships show up quite distinctly in our analysis but neither one is consistent with the anomalies in the intensity of the polar vortex during all winters.

Low-order Models of Atmospheric Circulations

Abstract: : Low-order models (LOM's), which are systems of ordinary differential equations which have been simplified by extreme reduction of the number of dependent variables, are often capable of representing atmospheric processes in a qualitatively correct manner. With a LOM it is generally possible to obtain a much more extended time-dependent solution, or a much larger ensemble of solutions, than would be economically feasible with a larger model. A general procedure for constructing LOM's is described. A selection of LOM's is presented, to illustrate the many forms which these models may take and the many uses to which they may be put. The step-by-step construction of a LOM is illustrated with a model of the large-scale circulation of a moist atmosphere.

Structure and Aanalysis of the Eye of a Numerically Simulated Tropical Cyclone

Abstract: A tropical cyclone has been simulated in a qu , druply nested mesh model with finest grid resolution of about S km. At the center of the vol1ex, a compact eye was maintained. Azimuthal means as well as asymmetry of the ye and the eye wall !,tructure are de