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Stanley F. Rose, Peter V. Hobbs, John D. Locatelli, and Mark T. Stoelinga
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John D. Locatelli, Jonathan E. Martin, Jeffrey A. Castle, and Peter V. Hobbs

Abstract

From 8 to 9 March 1992 cold frontogenesis aloft (CFA), which was associated with the development of a vigorous baroclinic wave, triggered a series of squall lines that produced large hail and several tornadoes as they moved across the central United States. The air lifted by the CFA, which produced the squall lines, was made potentially unstable as a result of the circulation associated with a surface drytrough. This study provides further support for the view that in winter and early spring CFA plays an important role in triggering severe weather in the central United States.

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Jonathan E. Martin, John D. Locatelli, Peter V. Hobbs, Peng-Yun Wang, and Jeffrey A. Castle

Abstract

A convective rainband, which was approximately 1500 km in length and affected large areas of the central United States for about 16 h, developed within an evolving winter cyclone. The rainband, which will be referred to as the pre-drytrough rainband, formed approximately 400 km ahead of a developing dryline and lee trough (drytrough, for short) that created an elevated, sloping layer of convective instability. The presence of a deep pool of high-potential-temperature air in the middle troposphere over the south-central United States, advected there from the elevated terrain to the southwest (i.e., an elevated mixed layer), produced a region of warm-air advection downstream of the high terrain. This enhanced the lifting associated with a migrating short wave aloft and generated the pre-drytrough rainband.

In previous studies the dryline, the lee trough, the elevated mixed layer, and the low-level jet in the central United States have generally been viewed as isolated features. Here the authors present a more integrated view, compelled by their common dependence on the interactions of synoptic-scale disturbances with topography.

Mesoscale structures and precipitation distributions similar to those documented in this paper are common in winter cyclones in the central United States and they are responsible for much of the severe weather associated with these systems.

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

On 18 September 1992 a series of thunderstorms in Nebraska and eastern Colorado, which formed south of a synopticscale cold front and north of a Rocky Mountain lee trough, produced a cold outflow gust front that moved southeastward into Kansas, southeastern Colorado, and Oklahoma around sunset. When this cold outflow reached the vicinity of the lee trough, an undular bore developed on a nocturnally produced stable layer and moved through the range of the Dodge City WSR-88D Doppler radar. The radar data revealed that the undular bore, in the leading portion of a region of northwesterly winds about 45 km wide by 4 km high directly abutting the cold outflow, developed five undulations over the course of 3 h. Contrary to laboratory tank experiments, observations indicated that the solitary waves that composed the bore probably did not form from the enveloping of the head of the cold air outflow by the stable layer and the breaking off of the head of the cold air outflow. The synoptic-scale cold front subsequently intruded on the surface layer of air produced by the cold outflow, but there was no evidence for the formation of another bore.

Profiler winds, in the region affected by the cold air outflow and the undular bore, contained signals from nocturnally, southward-migrating birds (most likely waterfowl) that took off in nonfavorable southerly winds and remained aloft for several hours longer than usual, thereby staying ahead of the turbulence associated with the undular bore.

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

Abstract

A forecast of severe weather and the potential for tornadoes associated with a cyclone that developed in the lee of the Rocky Mountains on 19–21 June 2000 is evaluated. The forecasting methods used by the National Weather Service for this case, which focused on the position of a surface trough and the location of favorable quasigeostrophic jet dynamics, poorly predicted the extent and location of the severe weather. Application of a conceptual model for cyclones east of the Rockies, which highlights the importance of cold fronts aloft (CFA), shows that a CFA was an important trigger to convection in the 19–21 June 2000 cyclone. A simple forecasting method is demonstrated that emphasizes the importance of lifting for cases that involve CFA. This method is applied to the 19–21 June 2000 cyclone and is found to improve greatly the determination of where severe weather occurred.

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

Abstract

Analysis of observations and the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) are used to study the development of a forward-tilted cold front off the coast of Washington State. The vertical velocity associated with the cold front produced a wide cold-frontal rainband. In the early stage of development the midtropospheric baroclinic zone (or upper cold front) moved forward with time over the warm sector to produce a structure similar to a split front. The movement of the upper cold front was due to horizontal transport and frontogenetical propagation. The frontogenetical propagation was produced by a combination of tilting and diabatic frontogenesis, which resulted in a negative/positive couplet of frontogenesis straddling the baroclinic zone.

The lower-tropospheric cold front eventually caught up with the warm front to form a classical warm occlusion. In the initial occluding process the converging frontal zones tilted into a warm-type occlusion configuration due to the presence of a background vertical shear of the horizontal wind component perpendicular to the occluded front. Consequently, as the storm moved over the observing network, the occluded front had the structure of a warm occlusion (tilted forward) in the lower levels. Above the occlusion, the cold front was also tilted forward because it retained its split-front-like structure. Thus, the development of the split front and the warm occlusion were separate processes that occurred in sequence.

Although the MM5 captured the basic forward tilt with time of the cold front, some key aspects of the midtropospheric frontal structure were not well simulated. Because diabatic heating was an important contributor to the maintenance and movement of the upper cold front, it is hypothesized that discrepancies in diabatic heating associated with deficiencies in the model’s explicit microphysical scheme may be responsible for deficiencies in reproducing the structure of the upper cold front.

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Peter V. Hobbs, Thomas J. Matejka, Paul H. Herzegh, John D. Locatelli, and Robert A. Houze Jr.

Abstract

Detailed information is deduced on the mesoscale organization of precipitation, the structures of the clouds, the air flows associated with mesoscale rainbands, and the precipitation efficiencies and the mechanisms producing precipitation in the rainbands associated with a cold front. Measurements were obtained with quantitative reflectivity and Doppler radars, two instrumented aircraft, serial rawinsondes and a network of ground stations.

The regions of heaviest precipitation were organized into a complex mesoscale rainband in the warm-sector air ahead of the front, a narrow band of precipitation at the surface cold front, and four wide cold-frontal rainbands. The wide cold-frontal rainbands and the smaller mesoscale areas of precipitation within them moved with the velocities of the winds between ∼3—6 km. The narrow rainband, which was produced by strong convergence and convection in the boundary layer, moved with the speed of the cold front at the surface. A coupled updraft and downdraft was probably responsible for the heavy precipitation on the cold front being organized, on the small mesoscale, into ellipsoidal areas with similar orientations.

The precipitation efficiencies in the warm-sector and narrow cold-frontal rainbands were ∼40–50% and ∼30–50%, respectively. One of the wide cold-frontal rainbands, in which there was a steady production of ice panicles in the main updraft, had a precipitation efficiency of at least 80%, whereas another wide cold-frontal band, in which some precipitation evaporated before reaching the surface, had a precipitation efficiency of ∼20%.

Ice particles from shallow convective cells aloft played important roles in the production of precipitation in the wide cold-frontal rainbands and in some regions of the warm-sector rainband. These “seed” ice particles grew by aggregation and by the deposition of vapor as they fell through lower level “feeder” clouds. About 20% of the mass of the precipitation reaching the ground in the wide cold-frontal rainbands originated in the upper level “seeder” zones and ∼80% in the “feeder” zones.

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

Abstract

Observations and numerical model simulations associate rising motions below the right-entrance and left-exit quadrants of an upper-level straight jet streak with the development of convection and severe weather. The occurrence of tornadoes in relation to the jet quadrants is investigated for the continental United States for the spring months of 1990–99. Tornadoes occurred primarily within the two exit quadrants, with the left-exit quadrant favored over the right-exit quadrant. While fewer tornadoes were located below the two entrance quadrants, the right-entrance quadrant was favored over the left-entrance quadrant. For those days on which many tornadoes occurred (“outbreak” days), a greater percentage of tornadoes occurred below the left-exit and right-entrance quadrants than for those days on which only a few tornadoes were reported. Composite diagrams are presented to clarify the relationship between the quadrants of a jet streak, severe weather, and synoptic features such as low pressure centers and frontal boundaries.

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

Abstract

On 13–14 December 2001 a vigorous cyclonic storm passed over the Pacific Northwest, producing heavy orographic precipitation over the Cascade Mountains. This storm was one of several studied during the second field phase of the Improvement of Microphysical Parameterization through Observational Verification Experiment (IMPROVE). A wide variety of in situ and remotely sensed measurements were obtained as this storm passed over the Oregon Cascades. These measurements provided a comprehensive dataset of meteorological state parameters (temperature, pressure, humidity, winds, and vertical air velocity), polarization Doppler radar measurements, and cloud microphysical parameters (cloud liquid water, particle concentrations, size spectra, and imagery).

The 13–14 December case was characterized by the passage of a tipped-forward lower-tropospheric front that extended upward to a preceding vigorous upper cold-frontal rainband, which produced clouds up to ∼8–9 km. An important difference between this storm and those studied previously over the Washington Cascades was that the prefrontal low-level airflow over the Oregon Cascades was characterized by strong westerly (as opposed to weak easterly) cross-barrier flow. Consequently, as the upper cold-frontal band passed over the Oregon Cascades there was both strong ice particle production aloft and significant production of liquid water at lower levels in the orographic lifting zone. Airborne in situ measurements, ground-based microwave radiometer measurements, and observations of snow crystals showed the simultaneous presence of high ice crystal concentrations and relatively large values of cloud liquid water aloft, and heavily rimed particles reaching the ground. Analyses indicate that a synergistic interaction occurred between the frontal and orographic precipitation.

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

Abstract

A nonhydrostatic, three-dimensional, mesoscale model, including cloud physics, is used to simulate the structure of a narrow cold-frontal rainband (NCFR). The model simulations reproduce the observed “core–gap” structure of the NCFR. Trapped gravity waves, triggered by regions of stronger convection on the cold front, induce subsidence and regions of warming aloft. In these regions, precipitation is suppressed, thereby creating precipitation gaps along the front separated by precipitation cores. The advection of hydrometeors is responsible for the parallel orientation and the elliptical shapes of the precipitation cores.

Gravity waves produce pressure perturbations just behind the cold front, which modify the wind and thermal structure. Parts of the front behave locally like a gravity current, traveling at the theoretical gravity current speed in a direction perpendicular to the local orientation of the front, but the motion of the front as a whole is not well described by the gravity current speed calculated from quantities averaged along the length of the front.

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