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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

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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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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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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Richard R. Weiss Sr., John D. Locatelli, and Peter V. Hobbs

Abstract

A technique is described for deducing, from vertically pointing Doppler radar measurements, whether the predominant ice particles just above the melting layer are graupel or aggregates of ice crystals. Under certain conditions, the type of graupel and the type of ice crystals which comprise the aggregates can be deduced.

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

Abstract

Previous model simulations indicate that in stratiform precipitation, the precipitation rate can increase by 7% in the melting layer through direct condensation onto melting snow and the resultant cooled rain. In the present study, a model simulation of stratiform precipitation in a wide cold frontal rainband indicates that the precipitation rate can also increase by 5% in the melting layer through accretion, by melting snow and rain, of additional cloud water produced by the latent cooling of the ambient air associated with melting snow. The contribution of the combined processes, and therefore the additional precipitation gained through the latent cooling of melting snow within the melting layer, may contribute as much as 10% to the precipitation rate in stratiform precipitation.

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