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Akira Kasahara

Abstract

Roles of the horizontal component of the earth's rotation, which is neglected traditionally in atmospheric and oceanographic models, are studied through the normal mode analysis of a compressible and stratified model on a tangent plane in the domain that is periodic in the zonal and meridional directions but bounded at the top and bottom. As expected, there exist two distinct kinds of acoustic and buoyancy oscillations that are modified by the earth's rotation. When the cos(latitude) Coriolis terms are included, there exists another kind of wave oscillation whose frequencies are very close to the inertial frequency, 2Ω sin(latitude), where Ω is the earth's angular velocity.

The objective of this article is to clarify the circumstance in which a distinct kind of wave oscillation emerges whose frequencies are very close to the inertial frequency. Because this particular kind of normal mode appears only due to the presence of boundary conditions in the vertical, it may be appropriate to call these waves boundary-induced inertial (BII) modes as demonstrated through the normal mode analyses of a homogeneous and incompressible model and a Boussinesq model with thermal stratification. Thus, it can be understood that the BII modes can coexist with the acoustic and inertio-gravity modes when the effect of compressibility is added to the effects of buoyancy and complete Coriolis force in the compressible, stratified, and rotating model.

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Akira Kasahara
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Akira Kasahara

Abstract

The linear response of model normal modes in a stratified atmosphere to tropical thermal forcing is investigated by using global primitive equations which are linearized with respect to a resting state and include a prescribed thermal forcing and momentum dissipation. By the method of separation of the variables, the basic equations are split up into vertical and horizontal equations. The homogeneous parts of these equations are solved spectrally to obtain the model normal modes. The forced problem is then solved by using a normal mode expansion.

For a parabolic form of heating in the vertical, it is shown that the internal modes corresponding to the equivalent height of a few hundred meters are favorably excited. This implies that the disturbances created by diabatic heating tend to have a typical baroclinic vertical structure. Numerical results are presented for the forced solutions generated by stationary and transient beat sources. For the case of stationary tropical heating, most of the excited energy goes into the rotational modes, but a significant portion also goes to the Kelvin modes, while other gravity wave modes play insignificant roles in general. For the case of transient tropical heating, the generation of gravity wave modes, except for the Kelvin modes, depends strongly on the time scale of heating, while the rotational modes and the Kelvin modes are dependent only weakly on the heating rate. The unique behavior of the Kelvin modes may be explained by the resemblance of the heating pattern to the horizontal structure of Kelvin modes and the closeness of their frequencies to those of rotational modes.

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Akira Kasahara

Abstract

This paper describes further improvement in a new spectral model of the global barotropic primitive equations (Kasahara, 1977) which utilizes Hough harmonies as basis functions. A review is presented on a method of constructing Hough harmonics (normal modes of Laplace's tidal equations) with new results of the eigensolutions for the logitudinal wavenumber zero case.

Applying this complete set of orthonormal Hough harmonics, we formulate a spectral model of the nonlinear, barotropic primitive equations (shallow-water equations) over a sphere which eliminates separate treatment of the zonally averaged component equations in the previously proposed model by the author. An example of the model calculation with Haurwitz wavenumber 6 initial conditions is presented.

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Akira Kasahara

Abstract

A new spectral model is formulated using Hough harmonics as basis functions to solve numerically the nonlinear barotropic primitive equations (shallow water equations) over a sphere. Hough harmonics are eigensolutions of free oscillations (normal modes) for linearized shallow water equations over a sphere about a basic state of rest and a prescribed equivalent height. Hough harmonics are expressed by Θ l s exp(isλ) with zonal wavenumber s, longitude λ and meridional index l. Hough vector functions Θ l s consist of three components-zonal velocity U, meridional velocity V and geopotential height Z, all functions of latitude. There are three modes with distinct frequencies for s≥1: eastward and westward propagating gravity waves and westward propagating rotational waves of the Rossby/Haurwitz type.

The advantage of using Hough harmonies for a spectral barotropic global primitive equation model is that the prognostic variables are efficiently represented because Hough harmonics are normal modes of the prediction model. Initialization is no longer a separate procedure but is built into the forecasting scheme. The characteristics of wave motions are associated with the expansion functions, so that the filtering of unwanted wave components can be performed easily.

The nonlinear advection term in the spectral equation is calculated by the transform method–a combination of Fourier transform and Gaussian quadrature. The results of a test calculation with a balanced non-divergent initial state (a Haurwitz wave) compare favorably with those of an integration using a fourth-order finite-difference model.

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Akira Kasahara

Abstract

Solutions of the linearized global shallow-water equations (Laplace tidal equations) including the effect of a mean zonal flow are obtained by the Galerkin-transform method. Free oscillations of the first kind (gravity-inertia modes) are little affected by the zonal flow. Solutions of the second kind (rotational modes of the Rossby-Haurwitz type) are significantly affected by a zonal flow different from solid rotation. Only a few lowest rotational modes, whose angular phase velocities are less than the minimum velocity in the zonal flow, appear as discrete. The remaining angular phase velocities fall into a continuous spectrum which covers the interval between the minimum and maximum zonal velocities. An approximate, but accurate, frequency formula is obtained for the discrete modes of free oscillations under the effect of a mean zonal flow.

The frequencies and latitudinal structures of a few lowest rotational modes under the effect of a mean zonal flow are examined in detail and compared to observational evidence of westward propagating wavenumber 1 long-period oscillations in the atmosphere. The 5-day wavenumber 1 oscillation (the lowest symmetric rotational mode I R +1) is found insensitive to the presence of zonal flows. Other discrete modes are relatively sensitive to and their periods increased by the zonal flow effect. In particular, the period of the second symmetric rotational mode I R +3 (zonal wavenumber 1) increases to about 16–19 days in favor of the observations summarized by Madden (1978).

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Akira Kasahara

Abstract

The steering flow of a hurricane is obtained by eliminating the vortex pattern from the total flow field. The evolution of the steering flow is predicted by solving the barotropic nondivergent vorticity equation. Based upon the steering-flow prediction, a forecast of the hurricane movement is obtained with the use of an equation which provides for interaction between the hurricane and the steering flow.

In the geostrophic model, the geostrophic-wind assumption is used in solving the vorticity equation. In the nongeostrophic model, the stream function governed by the ‘balance equation’ is adopted. With the above two prediction models, 45 pairs of predictions of the 24-hr and 48-hr movement of hurricanes Diane and Connie (August 1955) and Betsy (August 1956) at the 500- and 700-mb levels were prepared by the use of an electronic computer.

A detailed comparison between performances of the two prediction models is presented. Generally speaking, there is a remarkable similarity between the nongeostrophic and geostrophic forecasts of hurricane movement. The accuracy of the 700-mb hurricane forecasts appears to be comparable with that obtained from the 500-mb forecasts. However, for both prediction models, the ‘resultant’ forecast, which is the vector mean of the 700- and 500-mb predicted displacements, seems to provide a significant improvement over the accuracy of a single-level barotropic forecast. These analyses suggest that further improvements are possible by advancing the prediction model from the barotropic model to the equivalent barotropic or the baroclinic model.

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Akira Kasahara

Abstract

A steering method of predicting hurricane movement is formulated based upon a two-level baroclinic model. The upper steering-flow field is constructed from the pressure-weighted mean of the 200- and 500-mb steering-height fields, and the lower steering field is constructed from the pressure-weighted mean of the 700- and 1000-mb steering-height fields. Here, the steering flow of a hurricane is defined as the residual field after eliminating the vortex pattern from the total-flow field.

The evolutions of the upper and lower steering flows are predicted simultaneously by solving the two-level steering-flow vorticity equations. Based upon those steering-flow forecasts, the movement of a hurricane is predicted with the use of an equation which is derived from a solution of two vortex vorticity equations. A side condition is imposed that the upper and lower vortex patterns should move with the same velocity in the corresponding steering flows.

Ten cases of predicting the movement of hurricane “Betsy” (August 1956) up to 48 hr are presented. A preliminary comparison of the forecasts with those obtained from the barotropic model is also made.

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Akira Kasahara

Abstract

In order to increase the accuracy of prediction of hurricane movement beyond that which has been obtained with methods now in use, a numerical procedure based upon the barotropic model is presented here.

A vortex field is separated from the total flow to obtain the residual-steering flow field. By solving the equation for the steering field, which does not include any parameters related to the vortex, the prediction of the steering flow is executed in the ordinary manner. By solving the other equation, which includes the interaction terms between the hurricane and the steering flow, the movement of the vortex pattern is predicted. To make the latter problem more tractable, a velocity formula for the movement of the vortex center is derived on the basis of a few reasonable assumptions concerning the structure of the vortex and of the steering field. The variables in the formula are expressed only in terms of quantities determined by the steering field and a single characteristic parameter related to the scale of the hurricane. By this means, a forecast of the hurricane movement can be programmed as a subroutine which is executed in the course of the prediction of the steering flow.

Five cases of predicting the 24-hour and 48-hour movements of hurricane “Diane” and “Connie” (August 1955) at the 500-mb level are presented here. These results have been obtained by the use of a high-speed computer, from initial maps which were be-analyzed using all available data.

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Akira Kasahara

Abstract

A model of a tropical cyclone is constructed which is based upon conservation of momentum, mass,water vapor and heat in the hydrostatic system. The horizontal and vertical eddy-exchange processes for momentum, moisture and heat are included in the equations in order to incorporate the planetary frictional (Ekman) layer into the model. The effects of the surface boundary (Prandtl) layer are simulated by the boundary conditions for the equations, which permit the evaluation of surface stress, the sensible heat transport and the evaporation of water vapor from the earth surface. The energy sources of the model are the latent heat of condensation released during the ascent of moist air and the sensible heat transported from the ocean surface.

The formulation of the finite-difference equations for the axially-symmetric case is presented, together with an examination of the computational stability. By means of a high-speed computer, two independent computations with and without the supply of latent heat were made from the same initial wind and temperature fields.

A comparison of the two cases reveals an important effect of latent heat of condensation upon the development of the tangential motion as well as its warm-core radial circulation. It is shown that the formation of cellar convective bands in the system is a manifestation of the gravitational instability which does not occur without the latent-heat supply.

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