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M. A. Shapiro

Abstract

A numerical simulation using the inviscid adiabatic primitive equations for atmospheric motion framed in isentropic coordinates is shown to generate upper-level frontogenesis with physical and dynamical similarity to that shown in real upper-level frontal zones. Calculations of vertical velocity and potential vorticity show evidence of descent of stratospheric air into the upper portion of the simulated frontal zone. An experiment in which six isentropic surfaces intersect the earth's surface produces no numerical instabilities at the intersection, even in the presence of warm frontogenesis. Results suggest the suitability of the isentropic coordinate frame for numerical prediction of real weather systems containing frontal zone-jet stream systems.

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M. A. Shapiro

Abstract

Frontogenesis and geostrophically forced secondary circulations in the vicinity of jet stream-frontal zone systems are treated, assuming that the motions conform to the geostrophic momentum approximation and that the secondary circulations are confined to the cross-front plane. Absolute momentum is used as a frontal defining parameter as its spatial discontinuities are shown to bound the tow hyper-gradient domain of upper jet-front systems. The solution of the Sawyer-Eliassen secondary circulation equation for simulated and observed frontal zones illustrates the importance of the geostrophic shearing deformation of an along-front thermal gradient in forcing strong subsidence within and to the warm side of upper fronts. The Miller frontogenesis equations are rederived with the frontal processes partitioned into those arising from geostrophic and ageostrophic motions. The transformation of the frontogenesis equations into geostrophic momentum, isentropic coordinates is introduced to further simplify the diagnosis of ageostrophic frontogenetical processes. A parameterization for clear-air turbulence within upper fronts is incorporated into the Sawyer-Eliassen equation to illustrate the role of turbulent-scale motions in the forcing of secondary circulations about jet-front systems.

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M. A. Shapiro

Abstract

The distribution of wind, temperature, ozone, and turbulence within a multiple structured frontal zone-jet stream system is described using a combination of direct horizontal measurements by meteorologically-instrumented research aircraft and conventional aerological soundings. Results document the spatial continuity of these zones and associated intrusions of ozone-rich stratospheric air. Aircraft turbulence measurements show the zones with strong vertical wind shear and near-critical values of Richardson number are preferred regions of CAT encounter. Diagnostic calculations of the terms of the gradient thermal wind equation illustrate the importance of the sign and magnitude of the air trajectory curvature and its vertical derivative in maintaining regions of strong vertical wind shear and small Richardson number in the presence of weak horizontal thermal gradient.

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M. A. Shapiro

Abstract

The techniques of scale-analysis and case-study diagnosis are used to establish a theory of meteorological approximations for upper-level frontal zones. A scale-analysis of the governing equations reveals that. 1) the vertical component of motion within upper-level frontal zones is on the order of 10 cm sec−7; 2) the individual terms which comprise the horizontal acceleration are of the same order of magnitude as the Coriolis acceleration and compensate one another to the extent that their sum is one order of magnitude less than their individual contributions to that sum; 3) to the first order of approximation, the horizontal momentum equation reduces to the statement of geostrophic equilibrium; and 4) in the middle-tropospheric portions of upper-level frontal zones, the divergence and tilting terms of the vorticity equation are equal in magnitude and of opposite sign such that their sum is one order of magnitude less than their individual contributions to that sum. These results are verified in a case study of the intense upper-level frontal zone of 0000 GMT 8 December 1963.

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M. A. Shapiro

Abstract

No abstract available.

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M. A. Shapiro

Abstract

Evidence is presented which illustrates the role of jet stream-frontal zone clear air turbulence (CAT) as a mechanism for the exchange of air and chemical trace constituents between the stratosphere and the troposphere. Three-dimensional air motion sensing instrumentation and fast-response ozone and condensation nuclei analysers on board research aircraft permit the quantitative evaluation of the turbulent flux of chemical constituents across the tropopause. The observations reveal that tropopause folds are mixing regions whose chemical characteristics lie somewhere in between those of the troposphere and the stratosphere. The temporal changes of ozone and condensation nuclei brought about through the vertical flux divergence of these quantities suggest that turbulent mixing processes are of first-order importance as a mechanism for stratospheric-tropospheric exchange in the vicinity of jet stream-frontal zone-associated topopause folds.

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M. A. Shapiro

Abstract

measurements from the Boulder Atmospheric Observatory meteorological research tower are used to describe the structure and physical processes of a strong surface cold front. Analysis reveals that the horizontal gradients in temperature and wind velocity at the front are concentrated within a 200 m distance and measured vertical velocities exceed 5 m s−1 at the leading edge of the front. Calculations with the Miller frontogenesis equation show the magnitude and relative importance of cross-frontal confluence versus differential vertical motion (tilting) in forcing frontogenesis.

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M. A. Shapiro

Abstract

Results from three case study investigations of upper-level jet stream systems document the existence of stratospheric mesoscale cyclonic wind shear in the layer of maximum wind. Anomalously high values of potential vorticity are shown to coincide with the mesoscale cyclonic shear zone. The high values of potential vorticity within an upper level frontal zone were shown to result from shearing vorticity in the mesoscale high potential vorticity region of the stratosphere which is transported downward into the tropospheric frontal zone and becomes transformed into curvature vorticity with little change in thermal stability. The vertical gradient of diabatic temperature change resulting from vertical shear-induced turbulent heat flux, in layers of CAT, is proposed as the generation mechanism responsible for large values of potential vorticity on the mesoscale. It is proposed that turbulent-scale mixing processes are of fast order importance in the evolution of jet stream frontal zone systems.

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M. A. Shapiro

Abstract

Recent aircraft observations of the mesoscale and turbulent structure of upper level frontal zone-jet stream systems provide further evidence of stratosperic mesoscale cyclonic wind shear and associated anomalously high values of potential vorticity in the layer of maximum wind. Measurements of turbulent heat flux in regions of clear air turbulence above and below the layer of maximum wind (LMW) document the first-order importance of turbulent-scale processes in the generation and dissipation of potential vorticity. Ozone concentration measurements illustrate the intrusion of stratospheric air into the troposphere and give evidence of the effect of turbulent mixing processes in the LMW. It is proposed that the nonconservative property o@ potential vorticity permits air parcels to enter the stratosphere by direct transport across the potential vorticity discontinuity in the LMW, in agreement with earlier isentropic trajectory calculations.

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Daniel Keyser and M. A. Shapiro

Abstract

This article presents a review of upper-level fronts with the intent of synthesizing observational and modeling studies into a conceptual and dynamical description of these fronts and their evolution relative to the life cycle of midlatitude baroclinic waves. The discussion begins by tracing present-day concepts concerning the structure of upper-level frontal systems, which are based on composite analyses of radiosonde and aircraft data, from their origins in the pioneering analyses of upper-air data in the 1930s. Perspectives from scales both smaller and larger than upper-level frontal systems are provided respectively by considering the effects of turbulent processes on frontal structure and dynamics and by relating variations in frontal structure to the evolution of the baroclinic waves that provide the dynamical environment for upper-level frontogenesis.

The dynamics of upper-level fronts are shown to comprise the interactions between the primary (geostrophic) and secondary (ageostrophic) circulations. To elucidate the mechanisms and feedbacks contributing to the evolution of upper-level fronts in relation to their setting within baroclinic waves, the two-dimensional theory of forced secondary circulations in the cross-front plane developed by Sawyer and Eliassen is presented and interpreted, and theoretical and numerical examples of the formation of upper-level fronts in idealized two-dimensional flows are reviewed. In the three-dimensional case, the presence of along-front ageostrophic circulations superimposed upon the cross-front ageostrophic circulations treated by the two-dimensional theory is discussed in terms of the gradient wind. The relative contribution of the along-front ageostrophic circulation to upper-level frontogenesis is considered in the context of the results from three-dimensional β-plane channel models of baroclinic wave growth.

Directions for future observational, diagnostic and theoretical investigation are identified, including the scale interactions between upper-level fronts, their environmental baroclinic waves and related low-level cyclones, and between upper-level fronts and mesoscale convective systems. The review concludes with a discussion of the potential role of recent innovations in remote-sensing technology and trends in numerical weather prediction using mesoscale models in motivating continuing interest and future advances in frontal research.

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