An advective motion in the lower stratosphere is determined calculating trajectories of a large number of air parcels in a quasi-equilibrium general circulation obtained from a long-term integration of a GCM over the hemispheric domain. A new method is developed for deducing the advective motion of air parcels, i.e., the non-divergent part of the Lagrangianmean motion; the method is to calculate the usual Lagrangian-mean motion of air parcels both forward and backward in time, and then to take the average of the two. It is clearly shown that the advective motions of air mass in the lower stratosphere form one cell with an upward branch at the tropics and a downward branch in high latitudes, being quite different from the usual zonal-mean meridional circulations having direct and indirect cells over the tropical and extratropical regions, respectively. The regions of the upward and downward motions coincide very well with those of the radiative heating and cooling, respectively. This suggests a balance between the adiabatic heating/cooling due to the vertical motion of air parcels and the diabatic heating/cooling. From the dispersion rate of a large number of air parcels initially located along a latitude circle at the same interval of longitude, eddy diffusion coefficients are estimated as -3×109cm2/s in the horizontal and -l×103cm2/s in the vertical. These values are one order or more smaller than those employed in most of 2-D transport models. The results seem to be reasonable in recent theories of Lagrangian dynamics of planetary waves. However, note that the horizontal diffusion as well as the horizontal advection plays an important role in poleward movements of air parcels in the lower stratosphere. Furthermore, a new formulation of a 2-D transport model based on the advective motion and diffusion of air parcels is briefly discussed.
This study addresses the formation of a tropical depression over the northern Bay of Bengal. A medium range numerical wheather prediction experiment is carried out with the observations from the Global and the Monsoon Experiments during 1979. The global spectral model contains 11 vertical levels; horizontally it is described by 29 waves via the so-called rhomboidal truncation. In the present experiment all 29 waves of the nondivergent wind and only 5 waves of the irrotational wind define the initial state. The model physics include air-sea interaction, detailed radiation, cumulus parameterization, large scale condensation, dry convective adjustment and the effects of mountains via the usual sigma frame. The initial state for the case study selected here is on July 1, 1200 GMT 1979. A tropical depression forms by July 5, 1979 over the northern Bay of Bengal. The global model shows considerable skill in predicting its formation and subsequent westward motion. The predicted fields of the total motion field and the streamfunction appear reasonable to day 8 of prediction. On the other hand, the divergent part of the motion field exhibits considerable error beyond day 2, especially on the planetary scale. These errors on the scale of the depression are much smaller since mass and moisture convergence move westward with the stream function minimum. In this region Kuo's scheme prescribes very reasonable rainfall rates for the monsoon depression. Single-level models, run at 700mb up to 48 hours starting on day 3, fail to predict the formation of the monsoon depression. These are respectively based on the conservation of absolute and potential vorticity. In order to understand further the formation of this depression, calculations of detailed energetics of the multi-level spectral forecast are carried out over a limited domain. They show that during the formative stage the barotropic process was most important while the rapid growth and its maintenance is largely attributable to the role of cumulus convection. The storm forms at upper levels (near 500mb) and descends down to the surface in the initial period; this is shown by observation and is reasonably simulated by the prediction. The initial barotropic growth appears to occur over a deep lower tropospheric layer and vertical coupling seems to be an important feature that is not described by the single level experiments. Finally, we present a short summary of results obtained for this experiment over other parts of the globe as well as for other experiments, where some results of data sensitivity studies are addressed.
The influence of heat, moisture and moist static energy (MSE) budget, over the Arabian Sea and adjoining area (0°-30°N and 30°E-75°E), on the onset and activities of Asian summer monsoon has been studied in detail. The data base for this study consists of twice daily FGGE Level IIIb analysis for the period 16 May to 15 July 1979. The pentad mean variation, the vertical distribution and period averages of the various terms in energy budget equations are closely examined to find out their influence on the activities of monsoon. The study indicates the significant increase in the net enthalpy, latent heat energy (LHE), MSE and a number of budget parameters, well in advance of the onset of monsoon over Kerala coast. Further, a decreasing trend is observed in most of the above parameters about 5 days before the break monsoon condition which started over India on 16 July, 1979. The vertical distributions of the budget parameters reveal that during active monsoon period secondary maxima of horizontal heat and MSE flux divergences are observed in the upper troposphere which are replaced by minima during weak monsoon circulation. The broad features of the budget studies over the Arabian Sea are in good agreement with the large scale energetics (Mohanty et al. 1982a, 1982b). Some of the significant departures in the results of the two studies have been discussed.
Seasonal variations of heat budgets in both the atmosphere and the sea in the Japan Sea area are investigated for the period of a year from April 1978 to March 1979. The net oceanic heat flux into the Japan Sea by the Tsushima Current attains the maximum value, which is three times as much as that by Miyazaki (1952) and provides more than a half of the heat stored in the Japan Sea in summer. The heat supply from the sea to the atmosphere is maintained by losing heat content of the Japan Sea in winter. The heat transport from the "warm-current" region to the "cold-current" region increases in winter. The heat transported into the Japan Sea by the Tsushima Current in summer is stored mainly in the "warm-current" region and contributes greatly to the wintertime heat supply from the Japan Sea to the atmosphere. The very different features in heat and moisture budgets in the atmosphere in a year are exhibited owing to the large seasonal variations of the sea-air temperature difference, (Ts-Ta), and the specific humidity difference, (qs-qa). Amounts of sensible heat and latent heat supplied from the sea and the heating rate of the atmosphere are small in spring. The intense heating in the atmosphere is balanced with the condensation of water vapor in summer and autumn. In autumn the value of (qs-qa) is maximum of a year and the condensation in the atmosphere is nearly equal to the evaporation from the Japan Sea. The intense diabatic heating is observed in passages of an extratropical cyclone in autumn and the latent heat supply from the sea is one of the dominant processes in this situation. The increase in the sensible heat supply from the sea is not enough in autumn due to the smaller (Ts-Ta) to give rise to such an intense atmospheric heating as observed in winter.
Energy budgets are analyzed for a tropical cyclone simulated previously in a quadruply nested mesh model (Kurihara and Bender, 1982). It will be shown that the eddy kinetic energy within the eye is comparable in magnitude to that of the mean kinetic energy. It is supplied by import from the eye wall regions as well as by the conversion from total potential energy. At the same time it is converted to the kinetic energy of the mean flow and alsc lost by the dissipation. The influx of mean kinetic energy from the outer radii to the eye wall region and the export of potential energy both to the outer radii and to the eye region play important roles in the energetics of the eye wall region. Many obtained features agree well with those of a coarser resolution model (Tuleya and Kurihara, 1975) in which the eye of the vortex could not be resolved. This suggests that the eye structure has little impact on the energetics in the eye wall and outer regions of a tropical cyclone.
Numerical experiments show the following: (1) upward motion induced by a heat island promotes the development of a cumulus, and it can be a trigger to initiate a cumulus; (2) the initiation and the development of a cumulus are so directly affected by conditions of a lower layer modified by the heat island that cumuli which form over heat islands with different horizontal scales behave differently. Result (1) explains the urban enhancement of cumulus activity shown by climatological studies.
A simplification is made of the nonorthogonal Fourier transform method (Salby, 1982) for estimating space-time spectra from uneven twice-daily data sampled by a polar-orbiting satellite. The modified method transforms frequency only by the use of the Galilean transformation, while Salby's method transforms both frequency and wavenumber. Space-time spectra are obtained from the time-Fourier transform with respect to the Doppler-shifted frequency as viewed from the satellite. They wavenumber-frequency aliasing characteristics are examined and the computer code is exemplified.
In order to estimate space-time spectra correctly from uneven twice-daily data sampled by a polar-orbiting satellite, the wavenumber transform method (Hayashi, 1980) is modified by the use of a nonorthogonal Fourier inversion. The space-time spectra are obtained from the time-Fourier coefficients of the space-Fourier transforms of the asynoptic field with respect to its frequency-shifted wavenumber. Since this method requires the spatial interpolation of asynoptic data, it is effective only for ultralong waves. The wavenumberfrequency aliasing characteristics are examined and the computer code is exemplified.
An economical explicit scheme and its application to a primitive equation model is described. In the present scheme, the low-frequency advection terms are integrated by the leap-frog scheme with a time step interval Δta which is decided by a wind speed of meteorological waves, and not by a gravity wave phase speed. In order to get stable numerical solution, the high frequency gravity wave terms are integrated with a shorter time interval Δtb(≅Δta/M). Time integrations are carried out by combining low frequency terms which are assumed constant during 2Δta leap-frog time extrapolation interval with high frequency terms which are calculated at every Δtb interval. The scheme is three-time-level for low frequency terms. The present scheme is different from the splitting technique proposed by Marchuk (1965). The present economical explicit scheme has the advantage of the easiness in programming and the second order accuracy in the integration of low frequency terms. The present scheme has been applied to the 4L-NHM, operational 4-level northern hemispherical model used in 1978-81 in JMA. The results show that the economical explicit version model produces almost the same prediction as the original (explicit scheme) 4L-NHM. The present scheme saved the computation time as much as the semi-implicit version of 4L-NHM developed by Kudoh (1978) did.
Fraunhofer diffraction has been explicitly formulated for a finite hexagonal cylinder and a spheroid in any arbitrary orientation. The diffracted intensity was computed for hexagonal cylinders and spheroids oriented randomly in a three dimensional space and in a horizontal plane. The diffraction by hexagonal cylinders in 3D random orientation can be well approximated by that of spheroids of the same aspect ratio. For horizontal orientation, the diffracted intensity is a function not only of the scattering angle but also of the azimuth angle, and the diffraction patterns for hexagonal cylinders and spheroids are quite different from each other. The diffraction patterns of hexagonal cylinders in horizontal orientation strongly depend on the source elevation, showing patterns highly anisotropic with respect to the azimuth angle at low source elevations. Applications to optical phenomena due to ice crystals in the atmosphere are also discussed.
The following recent research results of the Mei-Yu phenomenon in southeastern China are reviewed in this paper. 1) Case studies of a Mei-Yu frontal system during June 10-15, 1975. This system was studied in detail for its rainfall characteristics, kinematic and dynaniical structures, budgets of vorticity, moisture, and kinetic energy, and the accompanying mesoscale systems. The results suggested the dynamical processes for maintaining a Mei-Yu trough and the role of a low level southwesterly jet in the formation of mesoscale cb clusters. 2) Composite studies of 8 cases of Mei-Yu frontal systems during the more active years of 1975 and 1977. The structure variation in different stages and different segments of the Mei-Yu trough observed in the individual cases was substantiated. 3) The climatology of Mei-Yu front and its associated frontogenesis and low level jet over southern China and Taiwan for 1968-77. 4) Numerical simulation of the Mei-Yu frontal system. One of the more interesting conclusions in the model simulations is the formation of a low level jet to the south of the Mei-Yu front due to a secondary circulation induced by the convective latent heat release.