A Review of the Structure and Dynamics of Upper-Level Frontal Zones

Keyser, Daniel; Shapiro, M. A.

doi:10.1175/1520-0493(1986)114<0452:arotsa>2.0.co;2

Cited by 324 publications

(230 citation statements)

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“…At the actual time and 8 h after the exchange, the cyclonic nature of the flow field is reduced and the exchanges are (pseudo)north of the jet and almost next to it. As indicated already in Figure 6, the pattern of the ω field shows the typical thermally direct (indirect) ageostrophic flow field in the entrance (exit) region of the mean jet streak (Keyser and Shapiro, 1986). The relative difference in the ascending and descending branches of the ageostrophic flow is possibly related to the jet streak's curvature (Moore and VanKnowe, 1992).…”

Section: Relationship To Jet Streak Dynamicscontrasting

confidence: 80%

“…There are several physical mechanisms leading to divergence and, by means of mass conservation, to vertical motions (Bluestein, 1992). For instance, jet streaks are typically associated with a convergence-divergence dipole, leading to a thermally direct (indirect) ageostrophic flow in the entrance (exit) region (Keyser and Shapiro, 1986). It has to be noted that the curvature of the jet streak itself has a significant effect on the kinematic fields, as shown by Moore and VanKnowe (1992).…”

Section: Introductionsupporting

confidence: 66%

“…Deformation is the typical signature of frontogenesis. The mathematical basis for this is concisely reviewed by Hoskins (1982) and the relation to upper-tropospheric fronts is given by Keyser and Shapiro (1986). Some indices of CAT are based upon deformation (e.g.…”

Section: Introductionmentioning

confidence: 99%

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Vorticity, deformation and divergence signals associated with stratosphere–troposphere exchange

Jaeger

Sprenger

2009

Quart J Royal Meteoro Soc

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ABSTRACT:The dynamical tropopause of the extratropics, defined as an isosurface of potential vorticity, acts as a barrier to stratosphere-troposphere exchange (STE). Therefore, any transport across this barrier is necessarily associated with diabatic or frictional processes such as turbulence, radiative forcing or condensational heating. Most of these processes are linked to distinct flow phenomena and hence with distinct signatures of the velocity field. The focus of this study is on the characteristics of the velocity field at the time and location of the tropopause crossing, as given by the flowcomponents divergence (DIV ), deformation (DEF ) and vorticity (VORT ). Firstly, the prevalence of distinct signals is shown for three case studies. Then a Lagrangian climatology of STE is used to assess the climatological aspects of this link for the year 1990. Methodologically, the flow-field components are traced along the STE trajectories, both derived from ERA-40. Some key results of this study are as follows. (1) DEF, positive values of VORT and negative values of DIV are markedly correlated with stratosphere-to-troposphere transport (STT). Therefore, the typical flow field associated with STT is deformative, cyclonic and convergent. (2) For troposphere-to-stratosphere transport (TST), the link with the flow-field components is less pronounced. DEF, VORT and DIV all show small, but distinct local maxima just before the exchange. The flow field during TST is therefore rather divergent and less cyclonic. (3) Sensitivity studies reveal that these findings are robust with respect to temporal and spatial variability.

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Section: Relationship To Jet Streak Dynamicscontrasting

confidence: 80%

Section: Introductionsupporting

confidence: 66%

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Vorticity, deformation and divergence signals associated with stratosphere–troposphere exchange

Jaeger

Sprenger

2009

Quart J Royal Meteoro Soc

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“…These two forms of vorticity play a crucial role in establishing absolute vorticity fields in COLs by converting from one form to the other (Bell and Keyser, 1993). In the NH, the conversions are facilitated by the presence of a jet streak that propagates along the periphery of the amplifying trough (Keyser and Shapiro, 1986;Bell and Keyser, 1993). However, the above composites seem to suggest that the evolution of wind fields differs from the NH.…”

Section: Spatial Structurementioning

confidence: 99%

The link between cut-off lows and Rossby wave breaking in the Southern Hemisphere

Ndarana

Waugh

2010

Q.J.R. Meteorol. Soc.

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The climatological link between cut-off low (COL) pressure systems that occur from 20 to 50 • S and Rossby wave breaking (RWB) in the Southern Hemisphere (SH) is examined for 1979-2008. It is shown that COLs are associated with either RWB events (89%) or with potential vorticity (PV) intrusions where there is north-south advection of high-PV air (11%). In the vast majority of COLs, the RWB events occur upstream, on or before the day of the COL formation. The evolution of the PV, geopotential heights, static stability, absolute vorticity and temperature advection fields during the COLs are consistent with the formation of high-PV anomalies that induce cyclonic circulations as predicted by theory. RWB plays a key role in producing the split flow associated with COLs in the SH, which in turn produces absolute vorticity anomalies by shear-curvature vorticity conversion, and creates static stability anomalies. The COLs associated with RWB at 330 K are deeper and more persistent than those associated with 350 K RWB and surface processes differ depending on the isentropic surface on which the associated RWB occurs. The seasonality of the RWB and COLs are similar, and is linked to the seasonal march of the westerly jets.

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“…As the amount of rainfall associated with cut-offs increases, the cyclonic curvature of the jet also increases in Figure 13. This curvature tends to enhance the divergence and upmotion on the 'exit' side of the jet (Keyser and Shapiro, 1986), thus contributing to the development of the system and higher rainfall rates consistent with the other diagnostics.…”

Section: Jet Streamsmentioning

confidence: 99%

Characteristics and variability of synoptic features associated with cool season rainfall in southeastern Australia

Risbey

Pook

McIntosh

et al. 2008

Intl Journal of Climatology

140

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Cool season rainfall variability in southeastern Australia is investigated via classification and characterization of the predominant types of synoptic systems occurring in the region. These types are frontal systems, cut-off low systems, and other systems. Rainfall in the region is dominated by cut-off systems and these systems are the main influence on the interannual variability of rainfall. Both cut-off systems and frontal systems display an enhancement of thermal (thickness) gradient as rainfall increases, but the mechanisms for intensification differ. Cut-off systems intensify in the region in association with local increases in baroclinicity and the subtropical jet, whereas frontal systems tend to intensify via a confluence of subtropical and polar jets. Interannual rainfall variability is examined for groupings of years based on both clustering of continental rainfall patterns and on El Nino/Southern Oscillation (ENSO)/Indian Ocean Dipole (IOD) years. Cut-off systems exhibit consistent enhancements of thermal gradients for groupings of years in which they produce more rainfall. For ENSO/IOD groupings, the cut-off thermal gradients are consistent with the underlying sea surface temperature (SST) anomalies. Wet years in southeastern Australia are usually produced by cut-off systems, but can also be produced by frontal systems. In those cases the mid-tropospheric flow pattern is reminiscent of the negative Southern Annular Mode (SAM) pattern. The positive SAM pattern is also associated with enhanced rainfall in the southeast via local intensification of blocking and cut-off systems.

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A Review of the Structure and Dynamics of Upper-Level Frontal Zones

Cited by 324 publications

References 0 publications

Vorticity, deformation and divergence signals associated with stratosphere–troposphere exchange

Vorticity, deformation and divergence signals associated with stratosphere–troposphere exchange

The link between cut-off lows and Rossby wave breaking in the Southern Hemisphere

Characteristics and variability of synoptic features associated with cool season rainfall in southeastern Australia

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