Jonathan E. Martin

A new conceptual model for cyclones generated in the lee of the Rocky Mountains

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.

Cold Fronts Aloft and the Forecasting of Precipitation and Severe Weather East of the Rocky Mountains

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 associate...

Quasigeostrophic Forcing of Ascent in the Occluded Sector of Cyclones and the Trowal Airstream

A numerical model-based analysis of the quasigeostrophic forcing for ascent in the occluded quadrant of three cyclones is presented based upon a natural coordinate partitioning of the Q vector into its along- and acrossisentrope components, Qs and Qn, respectively. The Qn component describes the geostrophic contribution to the rate of change of the magnitude of =pu (traditional frontogenesis), whereas the Qs component describes the geostrophic contribution to the rate of change of direction of =pu (rotational frontogenesis). It is shown that convergence of Qs simultaneously creates the isobaric thermal ridge characteristic of the thermal structure of occluded cyclones and provides the predominant dynamical support for ascent within the occluded quadrant. The absence of significant Qn convergence there suggests that quasigeostrophic (Q-G) frontogenesis plays a subordinate role both in forcing vertical motions and in affecting three-dimensional structural changes in the occluded sector of post-mature phase midlatitude cyclones. A cyclonically ascending, cloud- and precipitation-producing airstream that originates in the warm-sector boundary layer and flows through the trowal portion of the occluded structure is supported by the upward vertical motions implied by the identified Q-G forcing. This airstream is referred to as the ‘‘trowal airstream’’ and it is shown to be responsible for the production of the ‘‘wrap around’’ cloud and precipitation commonly associated with occluded systems. The relationship of the trowal airstream to previously identified cloud and precipitation producing airflows in cyclones is discussed.

The Structure and Evolution of a Continental Winter Cyclone. Part II: Frontal Forcing of an Extreme Snow Event

Abstract The production of a narrow, heavy, occasionally convective snowband that fell within a modest surface cyclone on 19 January 1995 is examined using gridded model output from a successful numerical simulation performed using the University of Wisconsin–Nonhydrostatic Modeling System. It is found that the snowband was produced by a thermally direct vertical circulation forced by significant lower-tropospheric warm frontogenesis in the presence of across-front effective static stability differences as measured in terms of the equivalent potential vorticity (PVe). The sometimes convective nature of the snowband resulted from the development of freely convective motions forced by frontal lifting of the environmental stratification. Model trajectories demonstrate that a stream of warm, moist air ascended through the trowal portion of the warm-occluded structure that developed during the cyclone life cycle. The lifting of air in the trowal was, in this case, forced by lower-tropospheric frontogenesis occ...

The Structure and Evolution of a Continental Winter Cyclone. Part I: Frontal Structure and the Occlusion Process

The frontal structure and occlusion process in a cyclone of moderate intensity that affected the central United States in January 1995 is examined. The deep warm-frontal zone associated with this cyclone had a lateral extension to the southwest of the sea level pressure minimum that, although characterized by cold-air advection near the surface, had many of the characteristics of a warm front aloft. In fact, this feature had a structure similar to the so-called bent-back fronts previously documented only in association with explosively deepening maritime cyclones. The development of a warm-occluded structure was investigated with the aid of a numerical simulation of the event performed using the University of Wisconsin—Nonhydrostatic Modeling System. The development of the warm-occluded structure was asynchronous in the vertical; occurring first at midtropospheric levels and later near the surface, in contrast to the classical occlusion process. Near the surface, the warm-occluded front was formed as the warm front was overtaken by the frontogenetically inactive portion of the historical coldfrontal zone. At midtropospheric levels, the warm occluded structure formed as a result of the cold-frontal zone approaching, and subsequently ascending, the warm-frontal zone in accord with a component of the classical occlusion mechanism. The observed asynchronous evolution of the occluded structure is proposed to result from the vertical variation in vortex strength associated with the upper-level potential vorticity (PV) anomaly that controls the cyclogenesis. It is suggested that the occlusion process begins aloft, where the associated vortex strength is greatest, and gradually penetrates downward toward the surface during the cyclone life cycle. Additionally, a characteristic ‘‘treble clef’’ shape to the upper-level PV anomaly is shown to be a sufficient condition for asserting the presence of a warm-occluded structure in the underlying troposphere.

The Effect of Latent Heat Release on the Evolution of a Warm Occluded Thermal Structure

Abstract The effect of latent heat release on the development of the occluded thermal structure in a major winter storm is examined through comparison of full physics (FP) and no-latent-heat-release (NLHR) simulations of the event performed using the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5). Though both simulations possess a well-developed occluded thermal ridge near the surface, the 3D structure of their respective occluded quadrants is quite different. In particular, the FP simulation depicts the canonical, troposphere-deep warm occluded thermal structure, whereas the NLHR simulation produces only a shallow, poorly developed one. Consistent with these differences in tropospheric thermal structure, the FP cyclone displays a robust treble clef potential vorticity (PV) distribution in the upper troposphere in its postmature phase, while a considerably less robust version characterizes the NLHR simulation. The PV minimum of the treble clef overlies a poleward sloping column ...

A Pacific Moisture Conveyor Belt and Its Relationship to a Significant Precipitation Event in the Semiarid Southwestern United States

Abstract In this study the term moisture conveyor belt (MCB) is defined as an elongated band of enhanced poleward water vapor fluxes (WVFs) above the PBL that is rooted in the Tropics. This new terminology is illustrated through an exemplary detailed case study of an MCB over the northeastern Pacific during 9–13 November 2003 that provides the moisture for a significant precipitation event in the dry southwestern United States. The analysis of the involved moisture transports and dynamics comprises both Eulerian and Lagrangian approaches, and is based upon output from a simulation with the University of Wisconsin-Nonhydrostatic Modeling System, as well as analysis data, surface observations, and satellite images. The formation of the MCB is related to a quasi-stationary upper-level cutoff low (COL) resulting from a wave-breaking event over the North Pacific. A pronounced upper-tropospheric baroclinic zone and a strong, inertially unstable subtropical jet (STJ) are found to the east of the COL, where at la...

An Operational Ingredients-Based Methodology for Forecasting Midlatitude Winter Season Precipitation

Abstract An ingredients-based methodology (IM) for the operational analysis and prediction of midlatitude winter season precipitation is developed. Diagnostics for five fundamental physical ingredients involved in the production of precipitation—forcing for ascent, moisture, instability, precipitation efficiency, and temperature—are incorporated into the IM. The forcing ingredient is combined with the instability ingredient to form a new parameter, PVQ, that serves as an indicator of heavy precipitation potential by identifying regions where these two ingredients coexist. The diagnostics and PVQ are incorporated into ingredients maps that facilitate a systematic approach to forecasting the duration, intensity, and type of winter precipitation.

A Synoptic Climatology of the Subtropical Kona Storm

Ten years of surface and upper-air analyses from the ECMWF Tropical Ocean Global Atmosphere (TOGA) dataset were used to construct a synoptic climatology of kona storms in the subtropical central and eastern Pacific Ocean. Within a sample of 115 cyclones that predominantly occurred during the Northern Hemisphere cool season, three distinct types of kona storms were identified: cold-frontal cyclogenesis (CFC) cyclones, coldfrontal cyclogenesis/trade wind easterlies (CT) cyclones, and trade wind easterlies (TWE) cyclones. Of the three types, CFC cyclones were found to be the most common type of kona storm, while CT and TWE cyclones occur much less frequently. The geographical distribution, propagation characteristics, and the monthly and interannual variability in the number of kona storms are presented. Kona storms initially develop across a large portion of the subtropical Pacific, with the greatest concentration of kona storms found within a southwest-to-northeast-oriented band from west of Hawaii to 408N, 1408W. A distinct latitudinal stratification was evident for each type of kona storm, with CFC, CT, and TWE cyclones each more likely to initially develop at successively lower latitudes. The analysis reveals that kona storms can propagate in any direction but exhibit a clear preference to propagate toward the northeast. Use of the multivariate ENSO index indicates that the number of kona storms that develop during each cool season is not correlated to the phase of ENSO. An analysis of the composite structure and evolution of each type of kona storm revealed some common and some unique characteristics. Development of the surface cyclone in all types results from the intrusion of an upper-level disturbance of extratropical origin into the subtropics, although differences in the initial structure and subsequent evolution of the 300-hPa trough were noted for each type of kona storm. The analysis also revealed that relatively weak 300-hPa winds are present throughout the evolution of each type of kona storm and that the composite kona storm tends to be nestled along the southern boundary of a region of higher surface pressure during the mature stage of its evolution. The development of robust ridges in the 300-hPa geopotential and 1000‐500-hPa thickness fields downstream of the composite surface cyclone were noteworthy features that characterized the evolution of all kona storms, the latter feature strongly suggesting that these disturbances are fundamentally baroclinic in nature.

Structure and Evolution of Winter Cyclones in the Central United States and Their Effects on the Distribution of Precipitation. Part III: The Development of a Squall Line Associated with Weak Cold Frontogenesis Aloft

Abstract From 8 to 9 March 1992 cold frontogenesis aloft (CFA), which was associated with the development of a vigorous baroclinic wave, triggered a series of squall lines that produced large hail and several tornadoes as they moved across the central United States. The air lifted by the CFA, which produced the squall lines, was made potentially unstable as a result of the circulation associated with a surface drytrough. This study provides further support for the view that in winter and early spring CFA plays an important role in triggering severe weather in the central United States.