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

Abstract

A mesoscale model simulation of a wide cold-frontal rainband observed in the Pacific Northwest during the Improvement of Microphysical Parameterization through Observational Verification Experiment (IMPROVE-1) field study was used to test the sensitivity of the model-produced precipitation to varied representations of snow particles in a bulk microphysical scheme. Tests of sensitivity to snow habit type, by using empirical relationships for mass and velocity versus diameter, demonstrated the defectiveness of the conventional assumption of snow particles as constant density spheres. More realistic empirical mass–diameter relationships result in increased numbers of particles and shift the snow size distribution toward larger particles, leading to increased depositional growth of snow and decreased cloud water production. Use of realistic empirical mass–diameter relationships generally increased precipitation at the surface as the rainband interacted with the orography, with more limited increases occurring offshore. Changes in both the mass–diameter and velocity–diameter relationships significantly redistributed precipitation either windward or leeward when the rainband interacted with the mountain barrier.

A method of predicting snow particle habit in a bulk microphysical scheme, and using predicted habit to dynamically determine snow properties in the scheme, was developed and tested. The scheme performed well at predicting the habits present (or not present) in aircraft observations of the rainband. Use of the scheme resulted in little change in the precipitation rate at the ground for the rainband offshore, but significantly increased precipitation when the rainband interacted with the windward slope of the Olympic Mountains. The study demonstrates the promise of the habit prediction approach to treating snow in bulk microphysical schemes.

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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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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

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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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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