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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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Mark T. Stoelinga, John D. Locatelli, and Peter V. Hobbs

The classical definitions of warm and cold occlusions are updated in light of recent observations and numerical modeling of occluded fronts within midlatitude cyclones. Mesoscale model simulations of occluded fronts show that they are first-order discontinuities in potential temperature, with broad zones of fairly uniform horizontal potential temperature gradient and static stability on either side of the frontal surface. Under this assumption, an analysis of the relationship between the slope of an occluded front and the potential temperature distribution on either side of it yields a “static stability rule,” namely, that an occluded front slopes over the statically more stable air, not the colder air.

The static stability rule leads to modifications of the classical definitions of warm and cold occlusions, in which the term “colder air” is replaced by “statically more stable air.” Examples are given of occluded fronts in which this distinction is important, and the static stability rule is used to examine possible reasons why warm occlusions are the predominant type of occlusion in midlatitude cyclones. The static stability rule also leads to a better understanding of forward-tilting cold fronts, including cold fronts aloft in the central United States.

The static stability rule, and its implications for occluded and other frontal structures, suggests that greater emphasis be placed on the effects of horizontally nonuniform static stability in theoretical and modeling studies of frontogenesis, frontal interactions, and the occlusion process—an emphasis that has been largely absent from such studies in the past.

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John D. Locatelli, Mark T. Stoelinga, Peter V. Hobbs, and Jim Johnson

On 18 September 1992 a series of thunderstorms in Nebraska and eastern Colorado, which formed south of a synopticscale cold front and north of a Rocky Mountain lee trough, produced a cold outflow gust front that moved southeastward into Kansas, southeastern Colorado, and Oklahoma around sunset. When this cold outflow reached the vicinity of the lee trough, an undular bore developed on a nocturnally produced stable layer and moved through the range of the Dodge City WSR-88D Doppler radar. The radar data revealed that the undular bore, in the leading portion of a region of northwesterly winds about 45 km wide by 4 km high directly abutting the cold outflow, developed five undulations over the course of 3 h. Contrary to laboratory tank experiments, observations indicated that the solitary waves that composed the bore probably did not form from the enveloping of the head of the cold air outflow by the stable layer and the breaking off of the head of the cold air outflow. The synoptic-scale cold front subsequently intruded on the surface layer of air produced by the cold outflow, but there was no evidence for the formation of another bore.

Profiler winds, in the region affected by the cold air outflow and the undular bore, contained signals from nocturnally, southward-migrating birds (most likely waterfowl) that took off in nonfavorable southerly winds and remained aloft for several hours longer than usual, thereby staying ahead of the turbulence associated with the undular bore.

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Mark T. Stoelinga, Peter V. Hobbs, Clifford F. Mass, John D. Locatelli, Brian A. Colle, Robert A. Houze Jr., Arthur L. Rangno, Nicholas A. Bond, Bradley F. Smull, Roy M. Rasmussen, Gregory Thompson, and Bradley R. Colman

Despite continual increases in numerical model resolution and significant improvements in the forecasting of many meteorological parameters, progress in quantitative precipitation forecasting (QPF) has been slow. This is attributable in part to deficiencies in the bulk microphysical parameterization (BMP) schemes used in mesoscale models to simulate cloud and precipitation processes. These deficiencies have become more apparent as model resolution has increased. To address these problems requires comprehensive data that can be used to isolate errors in QPF due to BMP schemes from those due to other sources. These same data can then be used to evaluate and improve the microphysical processes and hydrometeor fields simulated by BMP schemes. In response to the need for such data, a group of researchers is collaborating on a study titled the Improvement of Microphysical Parameterization through Observational Verification Experiment (IMPROVE). IMPROVE has included two field campaigns carried out in the Pacific Northwest: an offshore frontal precipitation study off the Washington coast in January–February 2001, and an orographic precipitation study in the Oregon Cascade Mountains in November–December 2001. Twenty-eight intensive observation periods yielded a uniquely comprehensive dataset that includes in situ airborne observations of cloud and precipitation microphysical parameters; remotely sensed reflectivity, dual-Doppler, and polarimetric quantities; upper-air wind, temperature, and humidity data; and a wide variety of surface-based meteorological, precipitation, and microphysical data. These data are being used to test mesoscale model simulations of the observed storm systems and, in particular, to evaluate and improve the BMP schemes used in such models. These studies should lead to improved QPF in operational forecast models.

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