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G. W. Kent Moore

Abstract

Observations indicate that frontal zones that form in the presence of strong surface sensible heating have a structure that is markedly different from those that form adiabatically. These differences include: a highly asymmetric low-level jet, the presence of an isothermal pool of warm air in the vicinity of the jet, an enhancement of the baroclinicity on the cold side of the jet, and an extension of the region of low Richardson number into the cold air.

In order to investigate the dynamics responsible for these differences, a Lagrangian model was developed that allowed for the incorporation of a surface sensible heat flux parameterization into the semigeostrophic theory of deformation induced frontogenesis. The model was tested by comparison against the analytic solution that exists in the special case of adiabatic frontogenesis in a constant potential vorticity fluid. Having established the validity of the model, the effects that surface sensible heating has on frontogenesis in a nonuniform potential vorticity fluid were investigated. It is shown that the frontal zones generated by means of the model have the same structural characteristics as those of the observed fronts which formed in the presence of strong surface sensible heating.

It is also shown that surface heating can lead to both an increase in the maximum baroclinicity and a reduction in the minimum stratification associated with a frontal zone. Both of these modifications can be expected to result in an increase in the rate at which instabilities along the front can develop. This then provides for an indirect mechanism by which surface heating contributes to the development of cyclonic disturbances by modifying the background flow rather than by direct interaction with the growing cyclone.

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G. W. Kent Moore

Abstract

The geostrophic momentum approximation and a Lagrangian formulation are employed to consider the nature of the fronts that result from the action of a stretching deformation field in a continuously varying potential vorticity fluid. In such a fluid, the tropopause is represented by a shallow region over which the potential vorticity changes from a representative tropospheric value to a representative stratospheric one. Decreasing the depth of this zone resulted in an increase in the intensity of the upper-level front. Reduction in the cross-front temperature contrast reduced the intensity of both the surface and upper-level front. Most notably it resulted in the elimination of the deep tropopause fold in the vicinity of the upper-level front.

The environment in which the fronts form may have temperature perturbations that are the result of previous or contemporaneous frontogenetic processes. The effects of two different types of perturbations were studied with the continuous model. The inclusion of a barotropic perturbation, i.e. one that is localized in the cross-front direction but that extends throughout the depth of the fluid, resulted in an increase in the intensity of both the surface and upper-level fronts. The upper-level advection of cold air that results in a localized region of reduced potential temperature has been identified as an additional frontogenetic process. In contrast to the effect of the barotropic perturbation, such a localized perturbation resulted in an increase in the intensity of the upper-level front only.

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G. W. Kent Moore

Abstract

Most conventional wave-CISK models have been used in the study of circulations with aspect ratios less than one, and as a result have been hydrostatic. In general, the most unstable waves in the models are the small-scale high frequency ones. For these waves the hydrostatic assumption is invalid. It therefore seems appropriate to consider a model in which the high aspect ratio waves are treated correctly, i.e., a nonhydrostatic one. In this paper, a comparison between a hydrostatic and a nonhydrostatic wave-CISK model is made. In the hydrostatic model, there is no coupling of the horizontal and vertical scales of the waves and this results in its lack of scale selection. The nonhydrostatic model has an explicit coupling in it and this leads to a preferred scale for the growth of the waves. For all the cases considered, the most unstable wave has an aspect ratio of order one.

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G. W. Kent Moore

Abstract

Narrow cold-frontal rainbands are lines of intense precipitation that straddle surface cold fronts. Recent observational work has revealed that the rainfall within the band is organized into regularly spaced ellipsoidal cells called precipitation cores. The rainband is coincident with a line of intense cyclonic shear associated with a low-level jet that lies ahead of and parallel to the surface cold front. Numerous authors have suggested that the organization of cells in the rainband is the result of shear instability of the horizontal wind field. To investigate this hypothesis, a linear stability analysis was performed on an idealized frontal zone consisting of a line of convection coincident with a line of cyclonic shear. To model the convective processes, the air inside the rainband was assumed to be unstably stratified. The presence of horizontal wind shear in an unstably stratified environment resulted in the existence of a mode with a short wave cutoff. The coupling between the convective processes and the shear instability in this mode was strong; its most unstable wave had properties similar to the precipitation cores observed in the narrow cold-frontal rainband.

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G. W. Kent Moore

Abstract

The geostrophic momentum approximation will be employed to investigate the mechanism of tropopause folding that occurs within upper-level fronts formed by the action of a stretching deformation field in a nonuniform potential vorticity fluid. The tropopause, in such a fluid, is represented by a shallow layer in which there is a large change in the potential vorticity. A fold is defined as a region in which the mean tropopause height is a multiple-valued function of the cross-front coordinate. Observational studies have demonstrated that tropopause folds form during frontogenesis as a result of the penetration of stratospheric air down into the troposphere. Earlier studies of this phenomena within the context of semigeostrophic theory have been hampered by the fact that the surface fronts collapse before the upper-level fronts have had a chance to develop. In order to get around this difficulty, previous studies have relied upon unrealistic cross-front temperature gradients, nascent folds, or both to generate any appreciable development at upper levels. In this paper, a different approach is used. It will be shown that the introduction of a cross-front gradient in the potential vorticity field allows one to specify an isentropic lower boundary condition. This allows one to study the frontogenetic processes active in the vicinity of the tropopause in isolation from those active near the surface. As frontogenesis proceeds in such a fluid, deep folds are indeed observed to develop. The structure and evolution of the folds will be shown to be similar to that of observed cases.

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G. W. Kent Moore and W. R. Peltier

Abstract

We address the problem of the stability of nonseparable baroclinic mean states against small-amplitude quasi-geostrophic perturbations. A general numerical methodology is developed that allows us to construct solutions to such problems without requiring any simplifying assumptions.

A previous investigation of frontal stability (Moore and Peltier) which was based upon the use of the primitive equations, demonstrated that frontal zones are unstable to a cyclone-scale mode of baroclinic instability. This mode had not been detected in any previous stability analysis. No evidence of this new mode is found in the results for the quasi-geostrophic problems reported here. In fact, it is shown that the dynamical constraints implied by the quasi-geostrophic approximation forbid the existence of such modes. We also show that recently reported quasi-geostrophic stability analyses that make simplifying assumptions regarding the meridional structure of the basic state have a markedly limited validity.

The present study, therefore, serves to reemphasize the fact that the stability characteristics of nonseparable mean states can be rather complex. The only way to truly understand them is to solve the stability problem without approximating either the field equations employed in the analysis or the structure of the mean state whose stability is under investigation.

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G. W. Kent Moore and W. R. Peltier

Abstract

The accuracy Of the WKBJ approximation to the nonseparable quasi-geostrophic baroclinic instability problem is investigated. By direct comparison with an effectively exact solution, we demonstrate for a particular class of frontal zones that the range of parameters for which agreement is attained is rather limited. Most importantly, the WKBJ solution is shown to be significantly in error for frontal zones representative of typical atmospheric conditions.

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G. W. Kent Moore and W. R. Peltier

Abstract

The problem of the origin of midlatitude cyclonic disturbances having characteristic spatial scales of 1000 km or less has remained outstanding for most of this century. Although the theory of baroclinic instability of Charney and Eady has often been assumed to account for their appearance, Charney himself recognized that this was not so. In an attempt to develop a unique explanation for the observed scale of these systems we have been reinvestigating the classical problem of the baroclinic instability of synoptic-scale frontal zones. This work has shown that when the stability of such regions is analyzed using the hydrostatic primitive equations, a new mode of baroclinic instability is revealed, which has precisely the required scale. The new mode is completely filtered out in both the quasi-geostrophic and semigeostrophic systems.

In the present paper we return to the primitive equations system to further analyze this problem and to refine the numerical formulation so as to enable an accurate investigation of the sensitivity of the growth rate and spatial scale of the new mode to the detailed properties of the background frontal flow. This demonstrates that focusing of the region of low Richardson number near the surface is most conducive to the growth of the cyclone wave.

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G. W. Kent Moore and W. R. Peltier

Abstract

Observational evidence has demonstrated that localized baroclinic zones in the atmosphere are unstable to disturbances with wavelengths of approximately 1000 km. Examples of such disturbances include: polar-front cyclones, Baiu cyclones, polar lows and comma clouds.

In an attempt to understand the dynamical processes responsible for the generation of these diverse but apparently related phenomena a two-dimensional baroclinic zone is tested for stability against small amplitude three-dimensional perturbations. The baroclinic zone employed in this analysis is the Hoskins–Bretherton semigeostrophic model of a synoptic scale front. The fronts am found to be highly unstable and the fastest growing disturbances compatible with an origin through secondary instability in the longwave are those with horizontal length-scales on the order of 1000 km. The energetics of these disturbances demonstrate that they grow by a new “cyclone-scale” mode of baroclinic instability. The structure of the most unstable cyclone-scale baroclinic disturbance has many features in common with Bjerknes” conceptual model of polar-front cyclones. These include: the existence of warm and cold sectors that are bounded by secondary fronts; strong southerly flow in the warm sector, strong northerly flow in the cold sector, and a tendency to exist as members of a cyclone family. In addition, evidence is presented that the incipient disturbance may eventually become unstable to a moist convective instability that occurs in approximately the same location in which prefrontal squall lines are found.

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R. Paul Ford and G. W. Kent Moore

Abstract

A case study of a small-scale polar front cyclone observed during the Canadian Atlantic Storms Program (CASP) is presented. The cyclone forms along an essentially two-dimensional front, which is in approximate thermal wind balance. This cyclogenetic event occurs where the Richardson number near the surface is small. The storm appears to grow in response to favorable low-level thermal advection rather than to any significant upper-level forcing.

As the wave amplifies, the initially two-dimensional frontal zone develops a three-dimensional structure. The cyclone has a horizontal wavelength of 1200 km and a vertical scale of 3–4 km. The wind field associated with the evolved system is strongly unbalanced in comparison with that in the initial state. A strong southerly low-level jet exists in the warm sector and a moderate northerly jet is observed in the air behind the cold front. The low-level warm sector is a region of reduced static stability.

Results will be compared with a frontal zone stability theory that describes how a two-dimensional primary frontal zone will evolve into a three-dimensional structure with secondary fronts. As we shall see, the three- dimensional structure of the observed system compares favorably with the theory.

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