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Louis W. Uccellini

Abstract

An analysis of the QE II storm of 9–11 September 1978 presents evidence for the existence of upper-level baroclinic processes upstream of the rapidly developing cyclone. The analysis shows that a deepening shortwave trough was located 400 to 500 km upstream of the site of the storm 12 h prior to rapid cyclogenesis. The trough was associated with 1) a polar jet marked by 65 m s−1 winds in its core and significant vertical and horizontal wind shear, 2) positive vorticity advection and divergence at the 300 mb level, and 3) an intense frontal zone that extended from 300 mb down to the surface. It also appears that a tropopause fold likely extruded stratospheric air down to the 700–800 mb level, 400–500 km upstream of the surface low and 12 h prior to the explosive development phase of the cyclone. These findings raise questions about Gyakum's assertion that the QE II storm developed in an area in which the baroclinic support was confined to the lower troposphere and the related assertion by Anthes et al. that upper-level forcing upstream of the area of rapid cyclogenesis was weak and apparently not important in this case.

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Louis W. Uccellini

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Louis W. Uccellini

Abstract

A review of 15 cases of low-level jets (LLJ's) which developed in the Great Plains and which have been previously discussed in the literature is presented. The review notes that boundary-layer processes have been emphasized as causative factors in the development of the LLJ while upper tropospheric features were not considered and the importance of synoptic-scale processes was generally minimized. For 12 out of the 15 cases, a systematic upper level flow pattern is isolated which includes the existence of a trough over the southwest United States and the propagation of upper level jet streaks from the Rocky Mountains toward the Great Plains. This flow pattern is responsible for leeside cyclogenesis or leeside troughing that produces the pressure gradients needed for the development of LLJ'S. For the other three cases, a blocking ridge exists over the Great Plains and the upper level flow is relatively weak. It is during these situations that the “classic,” diurnally oscillating LLJ is observed. A more detailed review of four case studies indicates that the subsynoptic-scale processes associated with the upper level jet streak's forcing of leeside cyclogenesis could, at times, be an important factor in the development of LLJ's in the Great Plains. The review questions 1) the notion that a retrogression of the subtropical high provides the increased pressure gradient force needed for the development of a LLJ in the Great Plains region, and 2) the assumption of using a constant or diurnally oscillating pressure gradient force which has been used for studying the total evolution of the LLJ. Changes in the pressure gradient force related to leeside cyclogenesis and leeside troughing and the isallobaric wind response to these changes seem to be an integral part of the process that leads to the development of LLJ's observed in the Great Plains.

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Jeffrey Homan and Louis W. Uccellini

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Two cases of light and moderate snow events over the Washington, DC area are presented for 14 and 22 February 1986. These cases were selected since the upper-tropospheric and surface features were quite similar, yet significantly different snowfall patterns occurred in the mid-Atlantic status. The numerical guidance and local forecasters displayed minimal skill in forecasting the amount of snow for each case, especially in the Washington, DC area. Analyses in isobaric and isentropic coordinates are presented which focus on the structure and evolution of the thermal fields and associated temperature advection patterns in the lower troposphere. These analyses reveal that the low-level upslope flow (as derived from vertical motion computations in isentropic coordinates) and the associated warm-air advection patterns (as viewed in isobaric coordinates) were important factors in determining the intensity and spatial distribution of precipitation for each case. Three-hourly soundings from the Genesis of Atlantic Lows Experiment (GALE) are presented for the 22 February 1986 case to demonstrate the effects of evaporative cooling on the temperature changes in the lower troposphere as the precipitation rapidly changed from rain and sleet to moderate snow in the Washington, DC area. The GALE soundings are also used to illustrate the importance of 3-h upper-air data to resolve properly the evolution of the wind and temperature fields during significant weather events.

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Ralph A. Petersen and Louis W. Uccellini

Abstract

An explicit technique for computing atmospheric trajectories, based on Greenspan's discrete model formulation, is presented as an alternative to the commonly used implicit scheme. The method provides an economical means of objectively obtaining computer-generated trajectories and accounts for the variable accelerations and local ψtendencies along the entire trajectory path. The initial results presented show that the explicit computations are stable and very nearly energy-conservative. An application of the discrete model approach to a real data base and comparisons with trajectories determined by the implicit method yield favorable results, illustrating the utility of the explicit technique as a diagnostic tool.

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Louis W. Uccellini and Donald R. Johnson

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Transverse circulations in the exit and entrance regions of jet streaks are investigated through numerical simulation, a case study, and an application of the isallobaric wind equation in isentropic coordinates, to study the interaction between upper and lower tropospheric jets and the development of severe convective storms. A hybrid isentropic-sigma coordinate numerical model is used to simulate the mass and momentum adjustments associated with a jet streak propagating in a zonal channel. The numerical results depict a two-layer mass adjustment in the exit and entrance region of the jet streak. The results also verify that the isallobaric wind on lower isentropic surfaces is a primary component of the return branches of transverse circulations and is foxed by the two-layer mass adjustment accompanying the propagating jet streak. Results from the case study of a severe weather out- break show that 1) a low-level jet (LLJ) beneath the exit region of an upper tropospheric jet streak is embedded in the lower branch of an indirect circulation, 2) intensification of the lower branch and development of the LLJ is largely a result of an increased isallobaric wind component, and 3) the development of the LLJ is coupled to the upper tropospheric jet streak by the two-layer mass adjustment within the exit region of the streak. The isallobaric wind component of the LLJ is the primary reason for the axis of the LLJ being at a significant angle to the upper jet's axis and the resulting veering of the wind with height. In the exit region, the geometry of this adjustment, combined with warm, moist, lower tropospheric air to the right and ahead of the jet streak and cool, dry air at the jet streak level, produced the differential advections that convectively destabilized the atmosphere. Results of the case study support the concept that the development of conditions favorable for severe convective storms can be forced by mass and momentum adjustments which accompany the propagation of an upper tropospheric jet streak.

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Louis W. Uccellini and Steven E. Koch

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Thirteen case studies of mesoscale wave disturbances (characterized by either a singular wave of depression or wave packets with periods of 1–4 h, horizontal wavelengths of 50–500 km, and surface pressure perturbation amplitudes of 0.2–7.0 mb) are reviewed to isolate common synoptic features for these cases and to shed light on possible energy sources for the waves. A strong thermal inversion in the lower troposphere (north of a frontal boundary) and a jet streak propagating toward a ridge axis in the upper troposphere are commonly observed in all the cases. In general, the area of wave activity is bounded by the jet axis to the west or northwest, a surface front to the southeast, an inflection axis (between the trough and ridge axes) to the southwest and the ridge axis to the northeast.

The conditions specified by Lindzen and Tung as being necessary to form a wave duct, which include the existence of the lower-tropospheric inversion, seem to be met in many of these cases. This suggests that a ducting mechanism contributes to the long duration of these wave events by preventing the vertical propagation of wave energy.

Questions are raised concerning the role of either convection or shear instability as source mechanisms for the generation of these mesoscale wave disturbances. The observed development of the waves within the exit region of a jet streak propagating toward an upper-level ridge axis is shown to be consistent with the hypothesis that the actual energy source needed to initiate and sustain thew wave events may be related to a geostrophic adjustment process associated with upper-tropospheric jet streaks.

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Donald R. Johnson and Louis W. Uccellini

Abstract

Five methods for computing the pressure gradient force within a sigma domain of a hybrid model are compared for flow over a steeply sloped terrain. The comparison includes pressure gradient calculations determined from a direct transformation to sigma coordinates, an application of an atmospheric deviation state to the direct transformation (Phillips, 1974), an evaluation within isobaric coordinates (Janjić, 1977), a flux form (Johnson, 1980), and a flux prime form that applies the Phillips' deviation state to the flux form. The results from a numerical simulation establish that the Janjić, Phillips and flux prime methods reduce truncation errors substantially, and successfully predict the surface anticyclonic circulation that develops within vertically and horizontally sheared baroclinic flow over elevated terrain. A discussion of the generation of kinetic energy and elimination of a spurious kinetic energy source through reduction of the truncation error by the flux prime method is presented.

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Gregory J. Hakim and Louis W. Uccellini

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On 17 March 1989, moderate to heavy snow developed in a 100- to 200-km-wide band extending from South Dakota to northern Michigan. The 4- to 8-inch snowfall within this band was not associated with major cyclogenesis, and developed 500 to 600 km north of a stationary surface front. A diagnostic analysis based on an application of the General Meteorological Package (GEMPAK 5.0) to a numerical simulation from the operational nested-grid model (NGM) is utilized to show that the development of this snow band is related to the interaction of two upper-tropospheric jet streaks and their associated transverse circulation patterns. The eastward propagation of a jet streak from the West Coast toward the middle United States and to the south of a slower-moving jet along the U.S.-Canadian border led to a merger of the ascent maxima associated with the direct and indirect circulations of the northern and southern jets, respectively. The snow band developed as the ascending branches of the jet-streak circulation patterns merged, with the eastward propagation of the heaviest snow linked to the motion of the coupled circulation pattern. The study also demonstrates the usefulness of the operational NGM for providing the higher-resolution datasets required to relate the evolution of jet-streak circulation patterns to the development of mesoscale precipitation bands.

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Daniel Keyser and Louis W. Uccellini

Although the development of limited-area, regional-scale numerical–weather prediction models has been driven largely by the practical need to improve quantitative precipitation forecasts, a related motivation has been scientific interest in investigating mesoscale phenomena and processes under “controlled” conditions. The establishment of mesoscale meteorology as a recognized discipline of the atmospheric sciences provides a context for exploring how regional models can be used effectively as research tools. It is argued that these models are sufficiently advanced and refined to provide investigators with four-dimensional, dynamically consistent data sets to supplement and extend those available from observations. Consequently, regional models combined with observations potentially are of considerable utility in conducting case studies of mesoscale phenomena. These contentions are illustrated with examples from the literature and with suggestions for future research involving the application of regional models to case studies of midlatitude mesoscale phenomena associated with significant precipitation or severe weather. Perceptions of how the advent of regional models is contributing to fundamental changes in traditional synoptic meteorology are discussed.

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