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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

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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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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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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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, Mark T. Stoelinga, and Peter V. Hobbs

Abstract

A numerical simulation using the Pennsylvania State University–National Center for Atmospheric Research fifth-generation Mesoscale Model (MM5) was run on a rainband associated with a cold front aloft (CFA) in a warm occluded structure on the U.S. east coast. The storm originally developed in the lee of the Rocky Mountains as a Pacific cold front overtook a Rocky Mountain lee trough. This formed a warm-type, occluded structure that was essentially maintained as the storm proceeded to the East Coast.

The CFA was a thermal front and therefore dynamically active. The prominence of the CFA in the equivalent potential temperature field was due primarily to the strong upward transport of water vapor from lower levels in the updraft associated with the CFA. The baroclinic zone was characterized by a tipped-forward lower region, where the CFA coincided with a maximum in potential temperature, and a tipped-backward upper region, where the CFA coincided with the leading (warm-side) edge of a zone of enhanced thermal gradient. The tipped-backward upper region displayed many of the characteristics of a vertically propagating gravity wave. In both of these regions, the potential temperature pattern produced a corresponding change in pressure gradient within the baroclinic zone; the imbalance of forces acting on air parcels as they moved through this pressure gradient produced the convergence in the lower baroclinic zone that was responsible for the CFA rainband.

Neither the dry quasigeostrophic nor dry Sawyer–Eliassen diagnosis resolved the details of the simulated mesoscale lifting associated with the CFA rainband. This is because the baroclinic zone of the CFA was mesoscale and structurally complex.

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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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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, 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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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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