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H. Joe Kwon

Abstract

The problem of genesis of easterly African waves is reexamined as a linear eigenvalue instability problem of a supposedly realistic basic zonal shear flow with and without a cumulus heating. Evidence will be presented to show how the growth rate, the propagation speed, and the overall composite structure of the observed tropical easterly waves can he well reproduced in the model in conjunction with a CISK representation of the latent heating. The degree of resemblance between the structure of the model unstable wave and the observed African waves is found to be significantly dependent upon the secondary features of the basic shear flow. This finding is based on a detailed comparison of the results using different structures for the basic flow including the one used by Mass.

Much of the kinetic energy of the unstable wave is generated via the barotropic instability process associated with the horizontal shear of the Sahara jet centered at the 650 mb level. But the vertical shear of the zonal flow also strongly influences the thermal and vertical velocity fields of the unstable wave. Slightly ahead of the trough, there is a thermally indirect secondary circulation centered at about 550 mb and a direct circulation centered at about 850 mb. From the energetics point of view, the latent heating tends to stabilize the motion because of such a structure at the middle level (400 mb–600 mb). The opposite occurs at the lower level (below 600 mb). The unique structure of this mutual cancellation plays a crucial role in the evolution of the wave disturbance. Its significant implication on the subsequent evolution of the African wave over the Atlantic Ocean is examined in a later paper.

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H. Joe Kwon
and
M. Mak

Abstract

It is hypothesized that few African waves become tropical storms because they typically have an unfavorable thermal structure that cannot be sufficiently transformed during its propagation across the Atlantic Ocean by the condensational heating under normal moisture conditions. This hypothesis is tested in the context of a moist multilevel quasi-geostrophic model. The longitudinal variation of the background zonal flow over the Atlantic Ocean is incorporated in the model as a progressively changing zonal flow that an easterly wave would encounter as it propagates across the oceanic region. The dependence of the evolution of such a wave field for 10 days upon the various model parameters is evaluated by means of numerical integration.

A detailed comparison of the structural and energetic properties of the wave field in nondeveloping and developing cases supports the hypothesis. Specifically, the initial model easterly wave field has a cold core at the trough region centered at the lower tropospheric level, and another cold core ahead of the trough centered at 550 mb. For a wave to be able to intensify subsequently to great intensity within 10 days, the latter must be quickly transformed into a warm core within a few days. This could happen only under the accumulative influence of a sufficiently strong cumulus heating during the westward propagation of the wave. The specific humidity at the top of the surface moist layer in the model required for rapid intensification is found to be ∼13 g kg−1, which is a large but physically meaningful moisture condition.

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H. Joe Kwon
and
R. T. Williams

Abstract

In this paper, nonlinear evolution and equilibration of the barotropic waves in a zonally inhomogeneous Bickley jet are investigated. The equilibrated state consists of a full spectrum of waves with a dominant frequency and its higher-frequency modes. The dominant wave scale in most equilibrated states is wavenumber 1 because of the changes in the local mean flow. Except for wave 1, the dominant wave scale varies with the governing parameters. The wavelength of the dominant wave increases for stronger and longer jets and for weaker dissipation.

The equilibrations are highly dependent on the parametric settings. Steady wavy, limit cycle, and chaotic states are found as with other nonlinear equlibration studies. When the jet is weak but strong enough to produce waves, the equilibration behaves very smoothly. But when the jet becomes stronger, the equilibration process becomes much more complicated and the subregion in parameter space for each type of solution becomes very irregular. When the streamwise length scale of the jet is short, wave activity occurs only for a range of medium values of the jet strength. Multiple equilibrium steady wave states are not found in this study, because all wave components must be cooperative in their growth rather than competitive.

The examination of the local energetics shows that the generation of the local kinetic energy by the barotropic instability process is nearly cancelled by redistribution by the pressure work both in linear and nonlinear cases. The energetics further confirm that the perturbation in the steady wave state with respect to the modified mean flow behaves as a linear wave.

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H. Joe Kwon
and
Gyu-Ho Lim

Abstract

This paper reexamines the structure of ageostrophic winds in a baroclinic wave presented by Lim, Holton, and Wallace. It is found that the nonzero growth rate is indispensible to the compete explanation of the observed structure of ageostrophic winds. For the unstable mode the isallobaric wind shifts slightly westward by φ = tan−1(σ i /σ r ) from the state of the neutral mode both at the upper and lower level. This makes the convergence and divergence patterns shift eastward at the upper level and westward at the lower level, which comes closer to reality where the vertical motion is nearly upright.

The cancellation between the isallobaric wind and the advective part of ageostrophic wind for unstable mode differs from place to place so that zonal asymmetry with respect to the center of each high pressure and low pressure region appears, which results in a dramatic change from the results of the neutral mode. At the upper level the orientation is mainly zonal. But the magnitude of the zonal component of the ageostrophic wind is stronger on the right side both of the ridge and the trough. On the left side of the ridge, the ageostrophic winds that blow up the pressure gradient are weaker than those that blow down the pressure gradient on the right-hand side. The same is true for the trough region. At the lower level, on the other hand, the orientation of the ageostrophic wind is mainly meridional. But there are additional components directed down the pressure gradient nearly everywhere. This results in a slight modification of the lower-level orientation of the ageostrophic wind, in which zonal components enter. The overall feature is that the ageostrophic wind that blows down the pressure gradient overwhelms the counterpart. The implication of this feature is that this is another indication that the mode is indeed baroclinically unstable. The above features are also verified in the numerical model framework.

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H. Joe Kwon
and
Mankin Mak

Abstract

This paper investigate the equilibration of finite amplitude waves in a forced, dissipative, high resolution, barotropic beta-plane model. The external forcing is prescribed in the form of an easterly Bickley jet and the dissipation is due to Rayleigh friction. Under moderate supercriticality, the system evolves towards a steady wave state consisting of one dominant wave and its higher zonal harmonics after a complex transient stage. The wavelength of the dominant wave is longer for a wider and stronger jet, a weaker viscosity, and/or a smaller beta effect. The dominant wave has a symmetric meridianal structure with respect to the jet and its next higher harmonic an antisymmetric structure. The latter is energetically sustained by the primary wave. All wave in a steady state are phase-locked as indicated by a common zonal propagation speed. The zonal flow is modified by the nonlinear feedback until the equilibrated waves become dynamically neutral. This system is also demonstrated to have the property of hysteresis. These results complement nicely with the laboratory results of Niino and Misawa (1984).

When the forcing is sufficiently large, a state of vacillation of all the waves would emerge with one subset of the waves fluctuates out of phase with respect to the complementary subset of waves. The wave-wave interaction plays a central role in such a state. A detailed energetics analysis is also presented. When the forcing is further increased, the equilibrated state is chaos. The most favorable conditions for the vacillation states to prevail are intermediate forcing and damping as in a baroclinic system (Mak, 1985).

The intensity of the jet required for vacillation is considerably stronger than that of typica; jets in the earth's atmosphere, implying that barotropic dynamics alone are likely to give rise to steady waves rather than vacillation in the atmosphere.

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Hui Yu
and
H. Joe Kwon

Abstract

Using large-scale analyses, the effect of tropical cyclone–trough interaction on tropical cyclone (TC) intensity change is readdressed by studying the evolution of upper-level eddy flux convergence (EFC) of angular momentum and vertical wind shear for two TCs in the western North Pacific [Typhoons Prapiroon (2000) and Olga (1999)]. Major findings include the following: 1) In spite of decreasing SST, the cyclonic inflow associated with a midlatitude trough should have played an important role in Prapiroon’s intensification to its maximum intensity and the maintenance after recurvature through an increase in EFC. The accompanied large vertical wind shear is concentrated in a shallow layer in the upper troposphere. 2) Although Olga also recurved downstream of a midlatitude trough, its development and maintenance were not strongly influenced by the trough. A TC could maintain itself in an environment with or without upper-level eddy momentum forcing. 3) Both TCs started to decay over cold SST in a large EFC and vertical wind shear environment imposed by the trough. 4) Uncertainty of input adds difficulties in quantitative TC intensity forecasting.

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H. Joe Kwon
,
Seong-Hee Won
,
Myung-Hwan Ahn
,
Ae-Sook Suh
, and
Hyo-Sang Chung

Abstract

The Geophysical Fluid Dynamics Laboratory (GFDL) hurricane initialization algorithm is implemented in the community fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5). This work is applied to the MM5-based Regional Data Assimilation and Prediction System model (RDAPS), the Korea Meteorological Administration's regional forecast model. The bogus procedure starts by initializing the winds within the bogus area. The main difficulty lies in the generation of other variables, such as humidity, temperature, geopotential, etc., which are dynamically consistent with the prescribed wind. However, it was found that there is a simple and practical way of tropical cyclone (TC) initialization. It is achieved by the use of the built-in function of MM5, the four-dimensional data assimilation (FDDA). In order to do so, a miniature RDAPS is constructed. After the initialization of wind within the filter region, all other variables are generated by the model through a strong 24-h nudging to the prescribed wind.

It is found after careful analyses that there is an improvement over the no-bogus model. Failures are mostly due to the fake vortex or the spurious deepening of the vortex, which have been problems of the original RDAPS model. The bogus RDAPS never cures the failure of the original model.

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