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Recent studies that have classified ice particles from airborne imaging probe data have concluded that the vast majority of ice particles in stratiform precipitation systems are of an “irregular shape.” This conclusion stands in contrast to the findings from microscope observations of snow particles at the ground during the Improvement of Microphysical Parameterization through Observational Verification Experiment from November to December 2001 in the Oregon Cascade Mountains (IMPROVE-2), which show that most snow crystals (either single crystals or the component crystals of snow aggregates) are readily identified as regular types within established crystal classification systems. This apparent contradiction is rectified by examining the definition of the term irregular as applied to ice particles and by considering limitations of different methods for observing ice particles. It is concluded that the finding of the airborne probe-based studies is a consequence of both limitations of the observing technology and an overly broad definition of irregular shape that is not consistent with the more restrictive definition established in well-known snow crystal classification schemes. When detailed microscope analysis of snow crystals is performed at the ground, and all regular types are included in the classification, the vast majority of snow crystals are of an identifiable regular type, rather than an irregular type. The classification of the vast majority of particles as irregular implies that there is little hope to describe the important properties of these particles (such as their scattering properties, fall speeds, and temperature and humidity conditions in which they grew), when in fact, many of these particles are of known types with known properties. Instead of using the term irregular, classification studies should use a term that focuses on the limitation of the observation method as being the defining characteristic of the category, such as “unidentified” or “undetermined.” [ABSTRACT FROM AUTHOR]

This paper compares airborne in situ observations of cloud microphysical parameters with the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) simulations, using the Reisner-2 bulk microphysical parameterization, for a heavy precipitation event over the Oregon Cascades on 13–14 December 2001. The MM5 correctly replicated the extent of the snow field and the growth of snow particles by vapor deposition measured along aircraft flight tracks between altitudes of 4.9 and 6 km, but overpredicted the mass concentrations of snow. The model produced a broader number distribution of snow particles than observed, overpredicting the number of moderate-to-large-sized snow particles and underpredicting the number of small particles observed along the aircraft flight track. Over the mountain crest, the model overpredicted depositional growth of snow and mass concentrations of snow, but underpredicted the amount of cloud liquid water and conversion of snow to graupel. The misclassification of graupel as snow and excessive amounts of snow resulted in the model overpredicting precipitation on the lee slopes and in localized areas along the foothills of the Cascades. The model overpredicted cloud liquid water over the lower windward slopes and foothills, where accretion of cloud liquid water by rain was the primary precipitation-producing mechanism. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]