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Peter V. Hobbs and John D. Locatelli

Abstract

The mesoscale organization and structure of precipitation in a cyclonic storm have been studied using satellite, radar, airborne and ground measurements. The large mesoscale regions, which were mainly in the form of rainbands, contained within them smaller mesoscale regions (preciptation cores) which were characterized by higher rainfall rates. It is shown that the precipitation cores in warm frontal bands originated in generating cells aloft which provided “seed” ice crystals which grew by collection as they fell through lower cloud layers. The generating cells were probably produced by the lifting of shallow layers of potentially unstable air which were situated above warm fronts. There is also some evidence that the precipitation cores within cold frontal bands originated within layers of potentially unstable air.

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John D. Locatelli and Peter V. Hobbs

Abstract

On 22 June 1947, Holt, Missouri, experienced a world-record rainstorm when 304.8 mm (∼1 ft) of rain fell in 42 minutes. In this paper, evidence is presented that this extremely heavy rain may have been produced by cold frontogenesis aloft (CFA). It is shown that what was earlier analyzed as a surface cold front was probably a drytrough, and that CFA was located at 700 hPa east of the drytrough, close to the location of the squall line, that produced the record precipitation rate.

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John D. Locatelli and Peter V. Hobbs

Abstract

A technique is described for obtaining detailed horizontal winds in a “two-dimensional,” steady-state frontal system from a single-Doppler radar. Measurements obtained from this technique in a cold frontal system are presented.

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John D. Locatelli and Peter V. Hobbs

Abstract

Mesoscale measurement from radars, aircraft and rawinsondes, and synoptic and satellite data are used to provide a detailed description of a warm front as it approached the Washington Coast. In many respects, the warm front was consistent with the classical model: temperature rises were concentrated within a forward-sloping frontal zone, winds veered with height and lapse rates were more stable within the frontal zone, clouds and precipitation were produced by upglide over the warm-frontal surface and, as the warm front approached, clouds lowered and precipitation generally increased. However, in several important respects the warm front differed from the classical picture. Air flowed through the warm front and the warm-frontal zone. Also, the warm-frontal zone had a “staircase” profile, with some segments nearly horizontal and other segments with steep slopes. Finally, precipitation was by no means uniformly distributed; instead, it occurred in both irregular and banded-shaped mesoscale features.

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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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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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Owen Hertzman, Peter V. Hobbs, and John D. Locatelli

Abstract

The three-dimensional structure of a warm front and its precipitation features are caused using due-Doppler radar data and supporting mesoscale measurements. Evidence is presented to support a staircase-like structure of the warm-frontal surface and significant flow of air through front from the warm side.

The cyclonic vertical vorticities within both the principal banded and nonbanded precipitation features were very weak. The primary source of the vertical vorticity appears to have been advected horzontally from behind the frontal zone by a strong low-level inflow. Vortex streteching was generally weak. Tilting terms is in the vorticity budget were primarily sinks.

Kinematic factors that must have played a role in the formation of the banded and irregular precipitation feature associated with this front are discussed and some generalizations are made to other preciptations systems.

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

Abstract

The origins of a rainband of moderate intensity that occurred over the eastern Carolinas is investigated. It is concluded that the band formed in the updraft portion of a thermodynamically direct vertical circulation about an upper-level frontal zone in a region of conditional symmetrical instability (CSI). The release of CSI is presumed to have been responsible for the dimensions of the band and its orientation relative to the shear vector. An adiabatic mechanism for destabilization of the environment of the upper-level front to CSI was explored but found to be insignificant in this case.

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

Abstract

Recent studies that have classified ice particles from airborne imaging probe data have concluded that the vast majority of ice particles in stratiform precipitation systems are of an “irregular shape.” This conclusion stands in contrast to the findings from microscope observations of snow particles at the ground during the Improvement of Microphysical Parameterization through Observational Verification Experiment from November to December 2001 in the Oregon Cascade Mountains (IMPROVE-2), which show that most snow crystals (either single crystals or the component crystals of snow aggregates) are readily identified as regular types within established crystal classification systems. This apparent contradiction is rectified by examining the definition of the term irregular as applied to ice particles and by considering limitations of different methods for observing ice particles. It is concluded that the finding of the airborne probe-based studies is a consequence of both limitations of the observing technology and an overly broad definition of irregular shape that is not consistent with the more restrictive definition established in well-known snow crystal classification schemes. When detailed microscope analysis of snow crystals is performed at the ground, and all regular types are included in the classification, the vast majority of snow crystals are of an identifiable regular type, rather than an irregular type.

The classification of the vast majority of particles as irregular implies that there is little hope to describe the important properties of these particles (such as their scattering properties, fall speeds, and temperature and humidity conditions in which they grew), when in fact, many of these particles are of known types with known properties. Instead of using the term irregular, classification studies should use a term that focuses on the limitation of the observation method as being the defining characteristic of the category, such as “unidentified” or “undetermined.”

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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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