If three assumptions shown in the text are adopted, we can replicate the δ18O-derived observations of ice mass evolution. Our results suggest that the large free oscillations with 100 kyr period may be governing the most predominant oscillations in the actual climate, and that the effect of variation in CO2 which would be induced by the link between biological action and the Milankovitch insolation variation would be superposed non-linearly on the free oscillation system with phase lock of the major variation, and play an important role in an apprearance of the real δ18O data.
A new dynamic initialization scheme for a primitive equation model is proposed. In the proposed scheme, the balanced initial state is obtained by forward-backward integration of the linear part of the model equations. During the integration the nonlinear part is kept constant. After the integration, the nonlinear terms (including physical processes) are updated. The updating of the nonlinear terms'and the linear integration are repeated several times like the iteration procedure of nonlinear normal mode initialization (NMI). The scheme is proved to be equivalent to NMI with a frequency dependent underrelaxation factor, and therefore, it is called dynamic normal mode initialization (DNI). Since only the linear part is integrated during the dynamic integration, unlike the conventional dynamic initialization, the proposed scheme is efficient. The scheme has been successfully applied to the FSU multilevel global spectral model. The scheme is expected to be applicable to limited area models without difficulty.
A spectral limited-area primitive-equation model with a time-dependent lateral boundary condition is described.The model is nested in one way to a low resolution model.A spectral representation of the horizontal fields of prognostic variables is performed by introducing a modified double Fourier series.This series is composed of the usual orthogonal double Fourier series, that is suitable for prediction in the limited-area with a free-slip wall boundary condition, and a few additional bases by which the boundary condition which does not belong to the wall condition will be satisfied.Fluxes of mass, momentum, energy, etc.across the boundary can be specified using the additional bases.The proposed spectral method prescribes the amplitude of the additional bases, using the historical data from a coarse-mesh model forecast.The model predicts the amplitude of only the orthogonal bases that satisfy the wall boundary condition.A boundary relaxation technique is incorporated in order to reduce the amplitude of a spurious solution near the boundary due to ill-posed lateral boundary condition. A one-dimensional, linear advection equation is integrated by the present spectral method and by a grid-point method.The comparison of these two methods reveals that the spectral method give far better results than those of the grid point method.This is mainly due to the improved accuracy in estimating horizontal derivatives in the spectral method i.e., the computational dispersion and the systematic phase error that are unavoidable in the grid-point method are completely excluded in the spectral method. The spectral formulation of a limited-area model is applied to the Japan Meteorological Agency (JMA) operational grid-point 12-level fine-mesh limited-area model (12L-FLM; March, 1983 -) and the forecasts of the two 12L-FLMs are compared.The spectral 12L-FLM almost perfectly duplicates the forecast of the grid-point 12L-FLM for relatively large-scale disturbances, within the resolution limit of the latter.However, the forecast of the spectral 12L-FLM is superior to the grid-point model for relatively small-scale disturbances.The compared case includes a well-organized intense typhoon in the initial field.The horizontal scale of the typhoon is almost marginal to be resolved by the grid-point 12L-FLM.The predicted typhoon by the spectral model is more realistically intense and symmetric than that by the grid-point model. The amount of computation required by the spectral 12L-FLM, whose transform grid is the same as that of the grid-point model, is 20∼30 per cent larger than the amount required by the grid-point 12L-FLM.We conclude from this that the computational economy of the spectral 12L-FLM is practically superior to the grid-point 12L-FLM, since the effective resolution of the spectral model is about 1.5times better than the grid-point model.We plan to apply the spectral method to the next operational limited-area model in JMA.
The initial-value problem corresponding to perturbed viscous shear flow in shallow water over topography is solved both analytically and numerically. A formal solution is obtained analytically by using the Fourier-Laplace transform. On the other hand, a numerical solution is obtained for Froude number Fr=0.1 and a basic flow U=tanh(y) by time integration. Both spatial and temporal behavior of the solution are studied. The stability of shear flows which are unstable in an inviscid fluid over a flat bottom changes with the strength of the friction; it varies from unstable to stable through a resonance between the topographic forcing and a barotropic wave. The structure of the disturbance is very similar to the unstable barotropic wave as long as the friction is slightly greater than the resonance point. It is supplied with energy from the basic shear flow through the Reynolds stress. Furthermore, a vortex remains in the basic shear zone when the topography moves across the flow. The structure of this vortex is also similar to the unstable barotropic wave and its energy is supplied from the shear flow. Thus, the vortex has a long life time against the friction. If the friction is large, the disturbance directly reflects the topographic forcing. The structure is similar to gravity waves and the energy is supplied from the topographic forcing. A comparison with the vortex observed in the atmosphere is also described.
Stationary and stable baroclinic eddies have been produced in an anticlockwise rotating fluid annulus subject to internal heating and outer-wall heating. Thermal and dynamical structures of the flows dominated by such eddies of wavenumber 3 are presented. Characteristics of the flow field are as follows. 1) Distinct and elliptical anticyclonic eddies are observed at the top-surface. 2) The mean zonal flow near the mid-radius has a parabolic profile in the vertical direction with a westerly maximum below the mid-level. 3) Cyclonic and anticyclonic disturbances have their centers at the places almost coinciding with the westerly maximum. 4) There exist a warm core above the center of the cyclonic disturbance and a cold anomaly below the center. 5) Upward motion locates to the west of the warm core and downward motion to the east. It can be pointed out that these features of the stationary cyclonic disturbance are similar to the characteristics of a monsoonal mid-tropospheric cyclone, which is also quasi-stationary (e.g. Miller and Keshavamurthy, 1968; Krishnamurti and Hawkins, 1970).
Inter-correlations between the active/break cycles of Indian summer monsoon and the circulation change in the northern middle and high latitude westerlies are investigated particularly relevant to the low frequency (30-50 day period) mode. Empirical orthogonal functions and composite analysis revealed the standing-type east-west oscillations of the geopotential height field between central and far-east Asia with the node over Tibet. Lag-correlations between the monsoon trough and the 500mb heights in the northern hemisphere suggest that this east-west oscillation is part of the response of the mid-latitude westerlies to thenorthward-moving monsoon heat source. The response in the higher latitudes seems to reach its maximum when the heat source approaches to the southern periphery of the westerlies; i.e., near to the break phase of monsoon. A plausible mechanism of this interaction between the monsoon and the westerly flow in the higher latitudes are also briefly discussed.
The wide band high resolution radiometer on the NIMBUS 7 satellite provides a detailed view of the net outgoing long wave radiation around the 11.5μm band. The availability of this data during 1979 made it possible to explore a statistical relationship among the radiance data and the divergent circulations based on the 200mb wind from the analysis of the Global Experiment data sets. This study reveals a very strong relationship between these fields at a horizontal resolution of wave number 8 (two dimensional). Examples of the monthly averaged and daily maps of the divergent wind, observational and statistically derived at this resolution, are compared. The use of this relationship, especially over the data sparse tropical latitudes, for numerical weather prediction and climate studies is suggested.
Observations from the International Winter Monsoon Experiment (Winter MONEX) have been used to determine temperature changes in the lower troposphere beneath the stratiform components of tropical mesoscale convective systems. It is found that while the apparent heat source Q1 in the lower troposphere beneath the stratiform cloud systems is negative (due to hydrometeor melting and evaporation), the time tendency of dry static energy (∂s/∂t) is positive. This low-level warming is a result of mesoscale downdrafts within the mesoscale convective systems. The warming and drying at low levels in such systems may contribute importantly to temperature and moisture changes in large-scale tropical disturbances. The effects, if important, will need to be considered in the development of schemes for the parameterization of cumulus convection.
Heat budget observations were carried out at Akaigawa, Hokkaido, to investigate the mechanism of extreme nocturnal cooling in a basin. The results of vertically integrated heat budget studies of the NBL (nocturnal boundary layer) show that when the general current above the NBL is weak, about 80% of the observed cooling of the whole NBL is attributed to horizontal advective cooling which is due to downslope cold air from surrounding mountains. Cooling due to sensible heat flux divergence accounts for less than 10% of the observed cooling and can be dominant only in the lowest layer. However, when the wind speed of the general current exceeds a critical value (about 10m/s at a height of 1000m), advective cooling decreases and turbulent cooling begins to dominate the observed cooling. Cooling due to atmospheric radiative heat flux divergence remains unchanged during the night and varies little from night to night. It accounts for about 10% of the observed cooling under weak wind conditions and its relative importance increases under strong wind conditions when the observed cooling is reduced.
Spectral and angular distributions of the intensity of solar aureole were evaluated accurately for several realistic models of the turbid atmosphere. Three cases of the size distribution, two cases of the imaginary part of the index of refraction and four cases of the columnar amount were assumed for aerosol models. The reflection of light from the ground surface was taken into account by assuming a Lambertian surface. Computations were carried out by means of the matrix-doubling method, but evaluating the intensities of singly and multiply scattered radiations separately. An advantage of this algorithm is to provide accurate solutions without consuming large computation time, because the intensity of singly scattered radiation can be evaluated exactly and only ten or twenty terms are necessary for the Fourier expansion of the phase function to evaluate the intensity of multiply scattered radiation with sufficient accuracy. Results of computation show that the effect of multiple scattering is so significant as compared with the multiple scattering due to air molecules that the single scattering theory for aerosols breaks down for almost all wavelengths and turbidity conditions. The contribution of light reflected from the ground surface is also significant. The effect of absorbing aerosols becomes noticeable for scattering angles larger than 10°, as a result of multiple scattering.
Number-size distribution of submicrometer particles was measured in the daytime in the urban atmosphere of Nagoya by using an electrical aerosol analyzer. It was confirmed that two characteristic types of size distribution reported by Okada (1985) appeared very often in the urban atmosphere of Nagoya. The occurrence of these distributions was discussed in association with the stages of gas-to-particle conversion and the speed of air passing over the urban area was concluded to be an important factor in the formation of these two distributions. The supersaturation spectra of cloud condensation nuclei (CCN) were also measured on some occasions together with size distribution of particles.
Measurements of direct and diffuse solar radiations were carried out in cooperation with in situ measurements of aerosols during two periods of Nov. 4-7, 1978 and Nov. 1-3, 1979 in Sendai, Japan. Intercomparison of volume spectra of aerosols obtained by several kinds of instruments showed that bimodal volume spectra are preferred to interpret the observed data. Values of the complex refractive index of aerosols were also investigated using a polar nephelometer and the diffuse to global radiation ratio measured by spectro-pyranometers.
Examined is the barotropic instability problem of a discontinuous zonal flow with a boundaryon a 5-plane as well as the steady wave problem. It is found that instability occurs but over-reflectiondoes not. This result is different from that in the Kelvin-Helmholtz (K-H) instability problem of astably stratified Boussinesq fluid (Lindzen and Rosenthal, 1976). The difference is due to the absenceof an outgoing neutral normal mode in the present problem.The piecewise-linear barotropic shear layer on a /3'-plane is also examined in the similar manner.Again instability occurs but over-reflection does not. Further, the K-H instability problem with arigid wall above the critical level is re-examined. These results suggest that there is no exact correspondencebetween over-reflection of a neutral wave and a shear instability.