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

The classical definitions of warm and cold occlusions are updated in light of recent observations and numerical modeling of occluded fronts within midlatitude cyclones. Mesoscale model simulations of occluded fronts show that they are first-order discontinuities in potential temperature, with broad zones of fairly uniform horizontal potential temperature gradient and static stability on either side of the frontal surface. Under this assumption, an analysis of the relationship between the slope of an occluded front and the potential temperature distribution on either side of it yields a “static stability rule,” namely, that an occluded front slopes over the statically more stable air, not the colder air.

The static stability rule leads to modifications of the classical definitions of warm and cold occlusions, in which the term “colder air” is replaced by “statically more stable air.” Examples are given of occluded fronts in which this distinction is important, and the static stability rule is used to examine possible reasons why warm occlusions are the predominant type of occlusion in midlatitude cyclones. The static stability rule also leads to a better understanding of forward-tilting cold fronts, including cold fronts aloft in the central United States.

The static stability rule, and its implications for occluded and other frontal structures, suggests that greater emphasis be placed on the effects of horizontally nonuniform static stability in theoretical and modeling studies of frontogenesis, frontal interactions, and the occlusion process—an emphasis that has been largely absent from such studies in the past.

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

Abstract

Conventional data and mesoscale model simulations are used to analyze two cyclones that developed east of the Rocky Mountains in June and November 1998. Both cyclones formed when a Pacific cold front overtook a lee trough/dryline east of the Rockies. In one case the leading edge of the Pacific cold front was on the surface, as depicted in the classic Norwegian model of a cyclone. In the other case, which is referred to as a cold front aloft (CFA) cyclone, the leading edge of the Pacific cold front was aloft and in advance of the lee trough. The lifting and severe weather associated with the Pacific cold front was along the leading edge of this front in both the Norwegian-type and CFA-type cyclones.

To obtain an estimate of how often CFA cyclones, with a coincident CFA rainband, occurred in the central United States during the period 15 September 1994 through 15 September 1995, 70 cyclones that maintained closed surface low pressure centers for at least 24 h, and produced precipitation in the United States east of the Rockies, were analyzed. Analysis of these cyclones revealed that 46% were CFA type, 23% were Norwegian type, and the remaining 31% could not be readily classified into either type. Lee troughs were common features east of the Rocky Mountains during this period. They were present in 62% of the 70 cyclones analyzed.

The 1000–500-hPa thickness field is suggested as a useful tool in locating the leading edge of a Pacific cold front, and in determining whether a cyclone is a Norwegian type or CFA type. The issue of how the frontal structures in CFA-type cyclones should be analyzed on surface weather charts is discussed, and some suggestions offered.

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

Abstract

On 1–2 February 2001, a strong cyclonic storm system developed over the northeastern Pacific Ocean and moved onto the Washington coast. This storm was one of several that were documented during the first field phase of the Improvement of Microphysical Parameterization through Observational Verification Experiment (IMPROVE). In the 1–2 February case, soundings and wind profiler measurements showed that a wide cold-frontal rainband was coincident with the leading edge of an upper-level cold front in a classical warm occlusion. Ground-based radar observations revealed the presence of subbands within the wide cold-frontal rainband and two layers of precipitation generating cells within this rainband: one at 5–7 km MSL and the other at 9–10 km MSL. The lower layer of generating cells produced fallstreaks that were traced from the cells down to the radar bright band at 2 km MSL. Observations suggest a connection between the subbands and the lower layer of generating cells.

A research aircraft, equipped for cloud microphysical measurements, passed through at least two generating cells in the 5–7-km region. These cells were in their formative stage, with elevated liquid water contents and low ice particle concentrations.

The microphysical structure of the wide cold-frontal rainband was elucidated by particle imagery from a Cloud Particle Imaging (CPI) probe aboard the research aircraft. These images provide detailed information on crystal habits and degrees of riming throughout the depth of the rainband. The crystal habits are used to deduce the temperature and saturation conditions under which the crystals grew and, along with in situ measurements of particle size spectra, they are used to estimate particle terminal fall velocities, precipitation rates, radar reflectivities, and vertical air motions. The radar reflectivity derived in this way generally compared well with direct measurement. Both the derived and directly measured parameters are used to determine the primary particle growth processes in the wide cold-frontal rainband. Above the melting layer, vapor deposition was the dominant growth process in the rainband; growth of ice particles by riming was small. Significant aggregation of ice particles occurred in the region just above the melting layer. A doubling in the air-relative vertical precipitation mass flux occurred between the region where sheath ice crystals formed (−3° ≤ T ≤ −8°C) and the surface. Substantial amounts of liquid water were found within the melting layer where growth occurred by the accretion of cloud droplets and also by condensation. Growth by the collision and coalescence of raindrops was not significant below the melting layer.

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

Abstract

The structure and evolution of a shallow but intense cold front (commonly referred to as an arctic front) and its associated precipitation features that passed through the central United States from 0000 UTC 9 March to 0000 UTC 10 March 1992 are studied with the aid of observations and outputs from a numerical simulation using the Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model MM4.

Located above the arctic front was a region of midtropospheric, frontogenetical confluence that was attended by a thermally direct vertical circulation. A large banded precipitation feature, for the most part located behind the arctic front, was produced by ice crystals from upper-level clouds (formed by the frontogenetical confluence) falling into low-level stratocumulus associated with the arctic front. The arctic front at the surface separated a region where the precipitation reaching the ground was solid from an adjacent region where the precipitation was liquid. A westward-moving, low-level jet behind the arctic front produced upslope flow over the high terrain of the northern Great Plains, which contributed to heavy snowfalls in this region.

A portion of the arctic front that moved southward, west of a low pressure center, was characterized by sharp drops in temperature and dewpoint and an increase in wind speed. However, the arctic front was not associated with either a pressure trough or much change in wind direction. The proximity of arctic fronts to such nonfrontal features as lee troughs and/or drylines often leads to the latter being misanalyzed as cold fronts.

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