Abstract

The recent suggestion that lower-tropospheric cyclogenesis is predominantly a result of column stretching associated with the updraft portion of the shearwise quasigeostrophic (QG) vertical motion is quantified through direct calculation of 900-hPa height tendencies via the QG vorticity equation. Comparison of the separate lower-tropospheric height tendencies associated with the shearwise and transverse portions of QG omega in a robust cyclogenesis event demonstrates that the shearwise updraft drives the largest part (>80%) of the cyclogenetic height falls at least through the end of the mature stage of the life cycle. The lower-tropospheric height falls and vorticity production near the sea level pressure minimum of the occluded surface cyclone are driven nearly equally by shearwise and transverse updrafts.

Abstract

A numerical model-based analysis of the quasigeostrophic forcing for ascent in the occluded quadrant of three cyclones is presented based upon a natural coordinate partitioning of the Q vector into its along- and across-isentrope components, Q _s and Q _n , respectively. The Q _n component describes the geostrophic contribution to the rate of change of the magnitude of ∇ _p θ (traditional frontogenesis), whereas the Q _s component describes the geostrophic contribution to the rate of change of direction of ∇ _p θ (rotational frontogenesis). It is shown that convergence of Q _s simultaneously creates the isobaric thermal ridge characteristic of the thermal structure of occluded cyclones and provides the predominant dynamical support for ascent within the occluded quadrant. The absence of significant Q _n convergence there suggests that quasigeostrophic (Q-G) frontogenesis plays a subordinate role both in forcing vertical motions and in affecting three-dimensional structural changes in the occluded sector of post-mature phase midlatitude cyclones.

A cyclonically ascending, cloud- and precipitation-producing airstream that originates in the warm-sector boundary layer and flows through the trowal portion of the occluded structure is supported by the upward vertical motions implied by the identified Q-G forcing. This airstream is referred to as the “trowal airstream” and it is shown to be responsible for the production of the “wrap around” cloud and precipitation commonly associated with occluded systems. The relationship of the trowal airstream to previously identified cloud and precipitation producing airflows in cyclones is discussed.

Abstract

Separate vector expressions for the rate of change of direction of the potential temperature gradient vector resulting from the geostrophic vorticity and geostrophic deformation, referred to as Q _VR and Q _DR, respectively, are derived. The evolution of the thermal structure and forcing for quasigeostrophic vertical motion in an occluded cyclone are investigated by examining the distributions of Q _VR and Q _DR and their respective convergences.

The dynamics of two common structural transformations observed in the evolution of occluded cyclones are revealed by consideration of these separate forcings. First, the tendency for the sea level pressure minimum to deepen northward and/or westward into the cold air west of the triple point is shown to be controlled by the convergence of Q _VR, which is mathematically equivalent to thermal wind advection of geostrophic vorticity, a well-accepted mechanism for forcing of synoptic-scale vertical motion. Second, the lengthening of the occluded thermal ridge and surface occluded front are forced by the nonfrontogenetic geostrophic deformation, which rotates the cold frontal zone cyclonically while it rotates the warm frontal zone anticyclonically. The net result is a squeezing together of the two frontal zones along the thermal ridge and a lengthening of the occluded thermal ridge. The associated convergence of Q _DR along the axis of the the thermal ridge also forces vertical motion on a frontal scale. This vertical motion accounts for the clouds and precipitation often observed to extend from the triple point westward to the sea level pressure minimum in the northwest quadrant of occluding cyclones.

Abstract

The frontal structure and occlusion process in a cyclone of moderate intensity that affected the central United States in January 1995 is examined. The deep warm-frontal zone associated with this cyclone had a lateral extension to the southwest of the sea level pressure minimum that, although characterized by cold-air advection near the surface, had many of the characteristics of a warm front aloft. In fact, this feature had a structure similar to the so-called bent-back fronts previously documented only in association with explosively deepening maritime cyclones.

The development of a warm-occluded structure was investigated with the aid of a numerical simulation of the event performed using the University of Wisconsin–Nonhydrostatic Modeling System. The development of the warm-occluded structure was asynchronous in the vertical; occurring first at midtropospheric levels and later near the surface, in contrast to the classical occlusion process. Near the surface, the warm-occluded front was formed as the warm front was overtaken by the frontogenetically inactive portion of the historical cold-frontal zone. At midtropospheric levels, the warm occluded structure formed as a result of the cold-frontal zone approaching, and subsequently ascending, the warm-frontal zone in accord with a component of the classical occlusion mechanism.

The observed asynchronous evolution of the occluded structure is proposed to result from the vertical variation in vortex strength associated with the upper-level potential vorticity (PV) anomaly that controls the cyclogenesis. It is suggested that the occlusion process begins aloft, where the associated vortex strength is greatest, and gradually penetrates downward toward the surface during the cyclone life cycle. Additionally, a characteristic“treble clef” shape to the upper-level PV anomaly is shown to be a sufficient condition for asserting the presence of a warm-occluded structure in the underlying troposphere.

Abstract

The production of a narrow, heavy, occasionally convective snowband that fell within a modest surface cyclone on 19 January 1995 is examined using gridded model output from a successful numerical simulation performed using the University of Wisconsin–Nonhydrostatic Modeling System. It is found that the snowband was produced by a thermally direct vertical circulation forced by significant lower-tropospheric warm frontogenesis in the presence of across-front effective static stability differences as measured in terms of the equivalent potential vorticity (PV_e). The sometimes convective nature of the snowband resulted from the development of freely convective motions forced by frontal lifting of the environmental stratification.

Model trajectories demonstrate that a stream of warm, moist air ascended through the trowal portion of the warm-occluded structure that developed during the cyclone life cycle. The lifting of air in the trowal was, in this case, forced by lower-tropospheric frontogenesis occurring in the warm-frontal portion of the warm occlusion. This trowal airstream accounts for the production of the so-called wrap-around precipitation often associated with occluded cyclones and, in this case, accounted for the northern third of the heavy snowband.

Abstract

It is a common diagnostic, synoptic practice to consider the Trenberth–Sutcliffe approximation to the quasigeostrophic (QG) omega equation, which relates upward vertical motion to regions of cyclonic vorticity advection by the thermal wind. Use of this approximate form of the QG omega equation requires the neglect of the so-called deformation term, which is often described as important only in frontal regions. Here, an alternative expression for the deformation term is derived that clearly illustrates its relationship to the mathematical forcing function in the Q-vector form of the QG omega equation.

The magnitude of the deformation term in the middle troposphere is traced throughout the life cycle of a typical midlatitude cyclone. It is found that this term is generally small at midlevels in the early stages of the cyclone life cycle. As the cyclone approaches and passes its mature stage, however, the deformation term exerts a comparable, locally predominant influence on the total QG forcing for vertical motion. Particularly interesting is the large magnitude this term acquires in the axis of high potential temperature, characteristic of a post–mature stage cyclone’s horizontal thermal structure. The large magnitude of the deformation term in such regions demonstrates that there are nonfrontal, midtropospheric regions within cyclones in which the deformation term may not be small.

Abstract

The total quasigeostrophic (QG) vertical motion field is partitioned into transverse and shearwise couplets oriented parallel to, and along, the geostrophic vertical shear, respectively. The physical role played by each of these components of vertical motion in the midlatitude cyclone life cycle is then illustrated by examination of the life cycles of two recently observed cyclones.

The analysis suggests that the origin and subsequent intensification of the lower-tropospheric cyclone responds predominantly to column stretching associated with the updraft portion of the shearwise QG vertical motion, which displays a single, dominant, middle-tropospheric couplet at all stages of the cyclone life cycle. The transverse QG omega, associated with the cyclones’ frontal zones, appears only after those frontal zones have been established. The absence of transverse ascent maxima and associated column stretching in the vicinity of the surface cyclone center suggests that the transverse ω plays little role in the initial development stage of the storms examined here. Near the end of the mature stage of the life cycle, however, in what appears to be a characteristic distribution, a transverse ascent maximum along the western edge of the warm frontal zone becomes superimposed with the shearwise ascent maximum that fuels continued cyclogenesis.

It is suggested that use of the shearwise/transverse diagnostic approach may provide new and/or supporting insight regarding a number of synoptic processes including the development of upper-level jet/front systems and the nature of the physical distinction between type A and type B cyclogenesis events.

Abstract

Employing reanalysis datasets, several threshold temperatures at 850 hPa are used to measure the wintertime [December–February (DJF)] areal extent of the lower-tropospheric, Northern Hemisphere, cold-air pool over the past 66 cold seasons. The analysis indicates a systematic contraction of the cold pool at each of the threshold temperatures. Special emphasis is placed on analysis of the trends in the extent of the −5°C air.

Composite differences in lower-tropospheric temperature, midtropospheric geopotential height, and tropopause level jet anomalies between the five coldest and five warmest years are considered. Cold years are characterized by an equatorward expansion of the jet in the Pacific and Atlantic sectors of the hemisphere and by invigorated cold-air production in high-latitude Eurasia and North America. Systematic poleward encroachment of the −5°C isotherm in the exit regions of the storm tracks accounts for nearly 50% of the observed contraction of the hemispheric wintertime cold pool since 1948. It is suggested that this trend is linked to displacement of the storm tracks associated with global warming.

Correlation analyses suggest that the interannual variability of the areal extent of the 850-hPa cold pool is unrelated to variations in hemispheric snow cover, the Arctic Oscillation, or the phase and intensity of ENSO. A modest statistical connection with the East Asian winter monsoon, however, does appear to exist. Importantly, there is no evidence that a resurgent trend in cold Northern Hemisphere winters is ongoing. In fact, the winter of 2013/14, though desperately cold in North America, was the warmest ever observed in the 66-yr time series.

Abstract

The polar jet (PJ) and subtropical jet (STJ) often reside in different climatological latitude bands. On occasion, the meridional separation between the two jets can vanish, resulting in a relatively rare vertical superposition of the PJ and STJ. A large-scale environment conducive to jet superposition can be conceptualized as one that facilitates the simultaneous advection of tropopause-level potential vorticity (PV) perturbations along the polar and subtropical waveguides toward midlatitudes. Once these PV perturbations are transported into close proximity to one another, interactions between tropopause-level, lower-tropospheric, and diabatically generated PV perturbations work to restructure the tropopause into the two-step, pole-to-equator tropopause structure characteristic of a jet superposition.

This study employs piecewise PV inversion to diagnose the interactions between large-scale PV perturbations throughout the development of a jet superposition during the 18–20 December 2009 mid-Atlantic blizzard. While the influence of PV perturbations in the lower troposphere as well as those generated via diabatic processes were notable in this case, tropopause-level PV perturbations played the most substantial role in restructuring the tropopause prior to jet superposition. A novel PV partitioning scheme is presented that isolates PV perturbations associated with the PJ and STJ, respectively. Inversion of the jet-specific PV perturbations suggests that these separate features make distinct contributions to the restructuring of the tropopause that characterizes the development of a jet superposition.

Abstract

The effect of latent heat release on the development of the occluded thermal structure in a major winter storm is examined through comparison of full physics (FP) and no-latent-heat-release (NLHR) simulations of the event performed using the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5). Though both simulations possess a well-developed occluded thermal ridge near the surface, the 3D structure of their respective occluded quadrants is quite different. In particular, the FP simulation depicts the canonical, troposphere-deep warm occluded thermal structure, whereas the NLHR simulation produces only a shallow, poorly developed one. Consistent with these differences in tropospheric thermal structure, the FP cyclone displays a robust treble clef potential vorticity (PV) distribution in the upper troposphere in its postmature phase, while a considerably less robust version characterizes the NLHR simulation. The PV minimum of the treble clef overlies a poleward sloping column of warm, weakly stratified air that extends through the depth of the troposphere and is a signature of the trowal, the essential structural feature of warm occluded cyclones. Consequently, examination of the role played by latent heat release in production of the occluded thermal structure in this case is made through consideration of its influence on the evolution of the upper-tropospheric PV morphology.

It is found that direct dilution of upper-tropospheric PV by midtropospheric latent heat release initiates formation of a local, upper-tropospheric PV minimum, or low PV tongue, to the northwest of the surface cyclone center. The production of this PV minimum initiates a cutting off of the upper-tropospheric PV anomaly associated with the surface development. The upper-tropospheric circulation associated with this cutoff anomaly, in turn, forces the advection of low (<1 PVU) values of PV into the developing PV trough. This combination of kinematic and diabatic processes acts to produce both the tropopause PV treble clef as well as the underlying warm occluded thermal structure in the FP simulation. In contrast, though an adiabatic kinematic tendency for production of a treble clef PV morphology operates in the NLHR simulation, the resulting PV and thermal structures are weaker and slower to evolve than those produced in the FP simulation. Thus, it is suggested that latent heat release plays an indispensable role in the production of the characteristic occluded thermal structures observed in nature.