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Jonathan E. Martin, John D. Locatelli, and Peter V. Hobbs

Abstract

Interactions between an upper-level frontal system and an initially weak surface cold front resulted in the production of a deep, precipitating frontal structure over the south Atlantic states on 26–27 January 1986. Attendant with the intensification of the frontal circulation was the development of an intense marine cyclone off the Delmarva peninsula. The increase in frontal-circulation strength is attributed to a favorable vertical superposition of the surface frontal trough and the upper-level frontogenetic horizontal deformation field that resulted in a deep column of divergence over the surface frontal trough. The surface cyclone developed partly, and indirectly, in response to the increase in warm-air advection in the lower stratosphere, which was directly related to an increase in the slope of the dynamic tropopause. The increase in the slope of the tropopause is hypothesized to have been the result of the combined effect of adiabatic advection of low tropopause height in the cold air of the upper trough and the latent heating associated with the onset of deep convection during the frontal development.

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John D. Locatelli, Mark T. Stoelinga, and Peter V. Hobbs

Abstract

A numerical simulation using the Pennsylvania State University–National Center for Atmospheric Research fifth-generation Mesoscale Model (MM5) was run on a rainband associated with a cold front aloft (CFA) in a warm occluded structure on the U.S. east coast. The storm originally developed in the lee of the Rocky Mountains as a Pacific cold front overtook a Rocky Mountain lee trough. This formed a warm-type, occluded structure that was essentially maintained as the storm proceeded to the East Coast.

The CFA was a thermal front and therefore dynamically active. The prominence of the CFA in the equivalent potential temperature field was due primarily to the strong upward transport of water vapor from lower levels in the updraft associated with the CFA. The baroclinic zone was characterized by a tipped-forward lower region, where the CFA coincided with a maximum in potential temperature, and a tipped-backward upper region, where the CFA coincided with the leading (warm-side) edge of a zone of enhanced thermal gradient. The tipped-backward upper region displayed many of the characteristics of a vertically propagating gravity wave. In both of these regions, the potential temperature pattern produced a corresponding change in pressure gradient within the baroclinic zone; the imbalance of forces acting on air parcels as they moved through this pressure gradient produced the convergence in the lower baroclinic zone that was responsible for the CFA rainband.

Neither the dry quasigeostrophic nor dry Sawyer–Eliassen diagnosis resolved the details of the simulated mesoscale lifting associated with the CFA rainband. This is because the baroclinic zone of the CFA was mesoscale and structurally complex.

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John D. Locatelli, Mark T. Stoelinga, and Peter V. Hobbs

Abstract

On 8–9 March 1992, a long-lived squall line traversed the state of Kansas, producing hail and damaging winds. It was shown previously that this squall line was part of a synoptic-scale rainband 2000 km in length that was associated with a cold front aloft (CFA). The present study is concerned with the detailed mesoscale structure of this squall line and its relationship to the CFA.

Examination of synoptic-scale cross sections based on rawinsonde ascents, and a mesoscale cross section of winds derived from dual-Doppler radar measurements, shows that the squall line was exactly coincident with the “nose” of the CFA. The dual-Doppler analysis also shows that the inflow of air to the squall line was elevated, drawing in air from the potentially unstable layer within the weak warm frontal–like feature that was being occluded by the CFA. The stability analysis of the air in the pre-squall-line environment shows that when the CFA overtook the surface position of the drytrough, the thermal and moisture structure of the atmosphere was such that a moderate amount of lifting provided by the CFA could have released convective instability within an elevated layer approximately 1–2 km above ground.

The mesoscale structure of the squall line, derived from the radar reflectivity and dual-Doppler wind fields, differs substantially from the “leading line/trailing stratiform” conceptual model for midlatitude squall lines. The lack of a strong cold pool, and the presence of strong low-level shear, indicates that the squall line described here was able to persist in its mature stage in an environment that was “greater than optimal” in terms of the balance of the vorticity of the cold pool to that of the low-level shear. However, in view of 1) the weakness of the surface cold pool, 2) the elevated inflow and convergence associated with the convection, and 3) the collocation of the large rainband in which the squall line was embedded and the CFA, it seems likely that the CFA (rather than the cold pool) provided the driving force for the squall line.

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John D. Locatelli, Peter V. Hobbs, and J. Anthony Werth

Abstract

Three vortices in a polar air stream are analyzed using detailed mesoscale observations and conventional synoptic data. In their mature stages, the vortices exhibited wind, temperature and precipitation patterns similar to the larger extratropical cyclones that form on the polar front. Each of the three vortices interacted with the polar front to form an “instant occlusion.” There is evidence that, in the three cases studied in this paper, a vortex supplied the low-pressure center, occlusion and cold front to the “instant occlusion”.

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Jonathan E. Martin, John D. Locatelli, and Peter V. Hobbs

Abstract

The development of a complex middle-tropospheric frontal structure, the various weather associated with itsprogression across the United States, and its role in the production of precipitation in the eastern third of theUnited States are examined.

The frontal structure consisted of two features: a middle-tropospheric cold front associated with a strong 500mb short wave that moved eastward from the Pacific Ocean, and a leeside warm front that formed in a northwardsloping zone of warm-air advection associated with a trough in the lee of the Rocky Mountains. The middletropospheric cold front overtook the leeside warm front to produce a warm occlusion-like structure'in the middletroposphere. As this system progressed eastward across the United States precipitation (from light rain to convectiveshowers) occurred along the leading edge of the middle4ropospheric frontal zone, well ahead of a decayingsurface trough.

This study highlights the importance of middle-tropospheric frontal structures in the organization and distribution of precipitation. The study also provides some insights and speculations concerning the similaritiesbetween lee troughs and drylines, the generation of squall lines by middle-tropospheric cold fronts, and the needfor better conceptual models for the evolution and structure of middle-tropospheric fronts.

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Mark T. Stoelinga, John D. Locatelli, and Peter V. Hobbs

Abstract

A cyclonic storm that moved over the central United States on 8–9 March 1992 developed two convective rainbands, namely, a pre–dry trough rainband and a cold front aloft (CFA) rainband. This study extends the results of previous investigations of these two rainbands by examining their initiation with the use of a nested-grid mesoscale model simulation with spatial resolution down to 8.3 km. The model simulation reproduced the synoptic-scale setting in which the rainbands developed, as well as the mesoscale processes that initiated the rainbands.

The pre–dry trough rainband was produced by the gradual ascent of a convectively unstable airstream above a gently sloping warm-frontal zone east of the dryline. After sufficient lifting, the instability was released through upright convection. The gradual ascent is well estimated by quasigeostrophic diagnosis, but the location and timing of the rainband are very sensitive to the convective stability characteristics within the airstream.

The CFA rainband was initiated by a Pacific cold front that occluded with the warm-frontal surface. This mesoscale occlusion process produced a narrow region of enhanced ascent at the dryline, which resulted in the lifting of the western edge of an air mass with high convective available potential energy. The lower-tropospheric mesoscale occlusion process was not resolved by a quasigeostrophic vertical velocity diagnosis. Also, although an upper-level front and tropopause fold were present, the CFA was separate from that feature.

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John D. Locatelli, Mark T. Stoelinga, and Peter V. Hobbs

Abstract

The outbreak of tornadoes from the Mississippi River to just east of the Appalachian Mountains on 2–5 April 1974 is analyzed using conventional techniques and the Pennsylvania State University–National Center for Atmospheric Research fifth-generation Mesoscale Model (MM5). The MM5 was run for 48 h using the NCEP–NCAR reanalysis dataset for initial conditions. It is suggested that the first damaging squall line within the storm of 2–5 April 1974 (herein referred to as the Super Outbreak storm) was initiated by updrafts associated with an undular bore. The bore resulted from the forward advance of a Pacific cold front into a stable air mass. The second major squall line within the Super Outbreak storm, which produced the strongest and most numerous tornadoes, was directly connected with the lifting associated with a cold front aloft. This second squall line was located along the farthest forward protrusion of a Pacific cold front as it occluded with a lee trough/dryline. An important factor in the formation of this occluded structure was the diabatic effects of evaporative cooling ahead of the Pacific cold front and daytime surface heating behind the Pacific cold front. These effects combined to lessen the horizontal temperature gradient across the cold front within the boundary layer. Although daytime surface heating and evaporative cooling are considered to be essential ingredients in the formation and maintenance of organized convection, the MM5 produced a strong squall line along the leading edge of the Pacific cold front even with the effects of surface heating and evaporational cooling removed from the model simulations.

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Mark T. Stoelinga, John D. Locatelli, Ralph D. Schwartz, and Peter V. Hobbs
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Mark T. Stoelinga, John D. Locatelli, Ralph D. Schwartz, and Peter V. Hobbs

Abstract

On 19–20 June 1979 a cyclone moved through the central United States. This cyclone contained a squall line associated with a cold front aloft (CFA), which caused significant damage. The fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5) was used to diagnose the role of the cold pool in the maintenance of the squall line. A control simulation with “full physics” was run, at 18- and 3.6-km grid spacing. Both simulations produced a squall line that was similar in location, orientation, and speed to the observed squall line, and displayed several characteristics that differed from the “leading line–trailing stratiform” paradigm for midlatitude squall lines. Sensitivity test simulations were run for both grid spacings, with diabatic cooling due to evaporation and melting of precipitation withheld to prevent the formation of a cold pool. These simulations produced a squall line similar to that in the control simulation, in terms of the location, orientation, and movement of the squall line. The simulations showed that the CFA provided the primary lifting responsible for the maintenance and movement of the simulated squall line by means of the hydrostatic surface pressure pattern it induced. The cold pool did not play a critical role in the maintenance of the simulated CFA squall line, but it did retard the progression of the synoptic-scale trough that trailed the simulated squall line, thereby increasing the forward tilt of the Pacific cold front.

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John D. Locatelli, Mark T. Stoelinga, Ralph D. Schwartz, and Peter V. Hobbs

Abstract

The magnitude of surface convergence, produced by the movement of cold fronts aloft and prefrontal surges, is derived by applying the linear divergence equation to observed surface pressure traces for Pacific Northwest warm occlusions and from a mesoscale model simulation of a warm occlusion–like structure in the central United States. Convergence values of approximately 10−4 s−1 are found to be generated locally for periods of about 1 h, yielding vertical displacements of 10–50 hPa. It is hypothesized that such convergences should noticeably enhance condensation rates in the widespread lower stratiform clouds associated with warm occlusions and could be a key mechanism for the triggering of squall lines by cold fronts aloft in cyclones in the central United States.

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