A New Conceptual Model for Cyclones Generated in the Lee of the Rocky Mountains

Abstract

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. Introductory remarksDespite the fact that the Norwegian Cyclone Model was based on maritime cyclones making landfall in northwestern Europe (Bjerknes and Solberg 1922), it has been applied essentially unchanged to cyclones in the central United States for 70 years (Friedman 1989;Mass 1991). This is not without some justification, since one of the main features of the Norwegian model are fronts demarcating airmasses, which are also a characteristic of cyclones in the central United States. However, in view of the unique geography of the central United States, it would not be surprising if cyclones in this region displayed structural differences from those in Europe.In this paper we present a new conceptual model for the structure and evolution of cyclones in the cen- Locatelli et al. (1989Locatelli et al. ( , 1995, Martin et al. (1990Martin et al. ( , 1995, Hobbs et al. (1990), andWang et al. (1995), as well as on regular observations of cyclones in the central United States. It is important to note that we are not proposing that all cyclones that form in the lee of the Rocky Mountains are described by our model. However, many cyclones that form in the winter, spring, and even into early summer, in the lee of the Rockies, and move eastward into the central United States, appear to be described better by the conceptual model presented here than by the Norwegian model. Our conceptual model highlights a number of important nonclassic features associated with cyclones in the central United States, some of which have been known for many years. These include lee troughs

Fire-effective Low-level Thermal Ridges on the Southern Great Plains

“…9). This illustrates that fireeffective LLTRs on the southern Great Plains closely reflect MSLP trough axes and are consistent with conceptual models for lee surface pressure troughs indicative of downslope flow and adiabatic warming (Steenburgh andMass 1994 andHobbs et al 1996). As such, surface troughs associated with LLTRs generally result from lee cyclogenesis (Pierrehumbert 1986) and differ from those associated with WCTTs, which instead are "inverted" as described in numerous studies (Miller 1946, Baker 1971, Bosart et al 1972, Bosart 1975, Ballentine 1980, Richwien 1980, Bosart 1981, Forbes et al 1987, Keshishian and Bosart 1987, Bell and Bosart 1988, Doyle and Warner 1990, Keshishian et al 1994, Weisman et al 2002, and Schumacher et al 2008.…”

Drop Shapes and Fall Speeds in Rain: Two Contrasting Examples

“…In both cases, the event had high reflectivities, but for case 2 a welldefined thin line of embedded convection can be seen crossing the Huntsville area. The first event was associated with an overrunning rainfall event from a warm front moving north from the Gulf of Mexico, whereas the second event was associated with a prefrontal rainband similar to that which occurs with cold fronts aloft (Hobbs et al 1996). Both events had relatively high rainfall accumulations.…”

Terrain Influences on Synoptic Storm Structure and Mesoscale Precipitation Distribution during IPEX IOP3

“…Thus, the high topography appeared to "strip" or remove the storm system of its low-level baroclinic structure as it traversed the Sierra Nevada. Destruction of the low-level baroclinity may have also been aided by adiabatic warming to the lee of the Sierra Nevada, as suggested by Hobbs et al (1996) for frontal evolution in the lee of the Rocky Mountains.…”