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

When a shortwave trough moves eastward over the Rocky Mountains and into the central United States, the following important features may form: a drytrough (i.e., a lee trough that also has the characteristics of a dryline), an arctic front, a low-level jet, and two synoptic-scale rainbands (called the cold front aloft rainband and the pre-drytrough rainband) that can produce heavy precipitation and severe weather well ahead of the drytrough. These features are incorporated into a new conceptual model for cyclones in the central United States. Use of this model can aid the interpretation of observational data and numerical model output, and it may also help to improve short-range forecasting in the central United States.

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

Abstract

Brief descriptions are given of four cases that illustrate the important role that cold fronts aloft (CFA) can play in producing significant weather cast of the Rocky Mountains. In all four cases, the CFA, and its associated short wave, were located ∼200–300 km ahead of a surface trough. Precipitation (and in some cases severe weather) developed along the leading edge of the CFA. The nested grid model generally did a good job of locating the CFA. Analysis of absolute momentum confirms that these features were fronts, in a dynamic, as well as a thermodynamic sense.

A conceptual model for CFA is presented. In the cases examined, this model provides not only a useful picture of the distribution of clouds and precipitation associated with CFA, but also means for locating them. It also helps to define a major class of systems that do not fit the Norwegian cyclone model. Therefore, it should help in the identification of CFA and in improving the forecasting of precipitation and severe weather associated with them.

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John D. Locatelli, Joseph M. Sienkiewicz, and Peter V. Hobbs

Abstract

The frontal structure of a cyclone that developed in the lee of the Rocky Mountains and moved eastward across the United States is examined. The evolutions and interactions of three frontal features are traced: the primary cold front, a shallow secondary arctic front, and a leeside trough. The zone of warm advection associated with the lee trough became more concentrated with time, and eventually resembled a warm front. The primary cold front had a tipped-forward structure, with cold advection aloft preceding cold advection at lower levels. This front overran the trough to form on the East Coast a structure that was similar to a warm occlusion or a split cold front. Two rainbands, parallel to and approximately 225 km ahead of the surface front, formed and dissipated within the inner network of the Genesis of Atlantic Lows Experiment. These rainbands developed at the leading edge of cold advection aloft, and they dissipated as they approached a region of strong convection over the Gulf Stream.

This study provides some insights into the role of a lee trough in the development of a warm occlusion or split cold frontlike structure, the formation of squall lines, and the potential for misanalyzing dry cold fronts. It also highlights the need for some clarifications and/or redefinitions of current terminology associated with occlusions.

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John D. Locatelli, Peter V. Hobbs, and Kumud R. Biswas

Abstract

Cloud and precipitation processes in a stratocumulus cloud layer (∼1 km thick) were investigated by means of airborne, radar and ground observations for three situations: 1) the stratocumulus alone, 2) fallstreaks from altocumulus falling into the stratocumulus, 3) regions of stratocumulus not appreciably affected by fallstreaks but strongly affected by artificially seeded dry ice.

In case 1) the cloud was composed primarily of supercooled droplets. In cast 2) dendritic ice crystals in the fallstreaks increased their mass by riming as they passed through the stratocumulus; derived precipitation rates for this case were ∼0.02–0.08 mm h−1. In addition, it appeared that the dendrites provided a source for high concentrations of needle crystals in the stratocumulus; these crystals were estimated to give a precipitation rate of ∼0.01–0.03 mm h−1. In case 3) high concentrations of needle crystals were produced by the dry ice seeding and it was deduced that these also produced precipitation rates ∼0.01–0.03 mm h−1. Some implications of the results for areal artificial seeding experiments are discussed.

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

Abstract

Particle size spectra collected by the University of Washington’s Convair-580 research aircraft at a variety of altitudes and temperatures in winter frontal and orographic precipitation systems during the Improvement of Microphysical Parameterization through Observational Verification Experiment (IMPROVE) are analyzed in this study. The particle size spectra generally appeared to conform to an exponential size distribution, with well-correlated linear fits between the log of the number concentration and particle diameter. When the particle size spectra were grouped according to the habit composition as determined from airborne imagery, significantly improved correlations between the size spectrum parameters and temperature were obtained. This result could potentially be exploited for specifying the size distribution in a single-moment bulk microphysical scheme, if particle habit is predicted by the scheme. Analyses of “spectral trajectories” suggest that the rime-splintering process was likely responsible for the presence of needle and column habit types and the positive shift in both N 0s and λs at temperatures warmer than −10°C.

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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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