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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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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

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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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

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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Robert A. Houze Jr., John D. Locatelli, and Peter V. Hobbs

Abstract

The dynamics and cloud microphysics of four rainbands in an occluded frontal system were examined. Aircraft, radar, raingage, and serial rawinsonde observations were obtained in addition to standard satellite and synoptic data. Two of the rainbands occurred in the leading portion of the frontal cloud shield and were oriented parallel to the warm front of the system. The other two bands occurred in the trailing portion of the cloud shield and had cold frontal orientations. Mesoscale pressure features were parallel to the rain-bands, except in mountainous areas. Computed air motions showed that the rainhands were supplied with moist air flowing into the rainband region from the south to south-southwest at low levels (below 800 mb). This air was swept abruptly upward in the rainbands just ahead of the cold air mass approaching from the west. Cumulus-scale convection in a layer between 4 and 5 km in clouds associated with these rainbands appeared to enhance the growth the ice particles. However, the ice crystal habits in these regions did not appear to be affected by the presence of the convection. As the ice particles settled below the convective layer, they grew first by vapor deposition and then, just above the melting layer, they began to grow by riming or aggregation. High ice particle concentrations were measured beneath the convective layer. Below the melting layer, very little precipitation growth took place in the rainbands, and in the two warm frontal bands, considerable evaporation of raindrops occurred below the melting layer.

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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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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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