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John R. Gyakum

Abstract

On Friday, 10 December 1982, a modest, yet unforecasted snowfall occurred in a band extending from the Ohio valley states eastward to western New York State. Aside from this case study representing a crucial forecasting problem, the scientific issues suggested by the results of this examination are especially intriguing. The precipitation was associated with neither a surface cyclone nor an obvious surface front. Although the precipitation began in the vicinity of quasi-geostrophic ascent, the details of the precipitation pattern are better explained by the atmosphere's susceptibility to moist slantwise convection. Additionally, the ascent associated with this precipitation event during its later stages in Illinois was pan of an elevated thermally direct frontal circulation. The relatively strong ascent on the warm side of this frontal circulation was likely assisted by the low moist symmetric instability in the same region.

The synoptic-scale flow pattern played a role in the evolution of this precipitation through quasi-geostrophic ascent, weakened environmental moist symmetric, stability, and geostrophic frontogenetic flow. However, in the western part of the precipitation band, the moisture responsible for the precipitation onset is shown to have been transported from the Texas Gulf Coast into the Midwest by a low-level wind maximum. The depth of this moist layer ranged from 20 to less than 150 mb. and its horizontal extent was about 200 km—dimensions which are substantially smaller than synoptic scale. The limited depth of the moist layer may have contributed to this precipitation event being missed by the operational Limited-Area Fine-Mesh Model (LFM), which has only six tropospheric layers, averaging 150 mb in depth.

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John R. Gyakum

Abstract

The objective of this research is to define the meteorological conditions prior to the explosive development of the QE II storm. By using conventional data and detailed McIDAS satellite imagery we document the genesis of this storm along a preexisting line of active surface Frontogenesis, 12 h before the onset of its extraordinarily rapid 24-b central pressure fall of nearly 60 mb. This particular surface cyclone, having formed on the western edge of a convective complex, is shown to be a lower-tropospheric warm-core phenomenon at the time of its birth. During the first 12 h of existence, the cyclone deepened 7 mb and its surface relative vorticity increased to 17 × 10−5 s−1. The cyclone had intensified sufficiently, after only 6 h, to have developed the characteristic pattern of strong cold and warm frontogenesis regions. During the 24-h period of explosive intensification, a strong midtropospheric trough interacted with the already well-developed surface cyclone. This period corresponds to the cyclone’s transformation into a larger and deeper system.

The particular midtropospheric trough is traced on its southeastward path from the Canadian Northwest Territories until it interacts with the QE II storm during its explosive intensification. This upper-tropospheric trough is also found to be associated with another distinct and intensifying surface cyclone, whose identity is maintained until after the initiation of the QE II storm’s explosive intensification. We demonstrate that this particular surface cyclone has a deep, cold-core structure during the initial 12 h of the separate and shallow QE II storm.

This documentation of a separate, independent origin and development for each of the surface and upper-tropospheric cyclonic disturbances involved in this explosive cyclone intensification motivates us to suggest a two-stage process of cyclone development that may be unique to the explosively developing cyclone. The first stage involves the genesis and development of the surface cyclone. For this particular case, the surface cyclogenesis occurs as a shallow frontal wave that develops independently of an upper-tropospheric trough. This frontal wave develops strong winds of 18 m s−1, extending to 300 km south of its center, and a sufficient amount of cyclonic vorticity (17 × 10−1 s−1 in this case) to dramatically enhance the surface response to the approach of the upper-tropospheric trough. The interaction of the upper-tropospheric trough and the strong surface cyclone constitutes the onset of stage two of the development process that corresponds to the cyclone's explosive intensification period.

This research suggests that not all cyclogenesis can be regarded as a classical type “B” development in which the surface cyclone forms in response to an approaching upper-tropospheric trough. Rather, we suggest that a surface cyclone’s explosive intensification may typically involve the interaction of separate surface and upper-tropospheric cyclonic disturbances, each of whose development may be substantial enough, before their interaction, to warrant their individual examination.

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John R. Gyakum

Abstract

The hurricane-force winds and heavy seas which battered the liner Queen Elizabeth II on 10 and 11 September 1978 were associated with an extreme example of a meteorological “bomb” as defined by Sanders and Gyakum. Despite the existence of surface buoys, and the relatively high density of mobile ships in the North Atlantic, real-time weather analyses, subjective forecasts, and numerical prognoses all erred in the intensity and track of this storm. In this study, deficiencies in the real-time surface analysis were compensated for by the addition of Seasat-A surface wind fields and previously-discarded conventional ship reports. This paper examines the synoptic aspects of this case with emphasis on physical mechanisms most likely responsible for the development.

The cyclone originated as a shallow barocline disturbance west of Atlantic City, New Jersey, and explosive deepening (∼60 mb/24 h) commenced once the storm moved offshore, and in association with cumulus convection adjacent to the storm center. The hurricane-force winds, a deep tropospheric warm core, and a clear eye-like center, all characteristics of a tropical cycline, were associated with this storm at 1200 GMT 10 September.

A diagnostic assessment of batoclinic forcing reveals that, although the cyclone formed on the anticyclonic shear side of the 500 mb flow, a shallow lower tropospheric layer of cyclonic thermal vorticity advection existed over the surface cyclone center. Calculations using a diagnostic, adiabatic, inviscid quasi-geostrophic model, which can approximately replicate the shallow baroclinic structure of this cyclone, yield instantaneous vertical motion and deepening rates far less than those observed. It is suggested that the convection associated with this cyclone during its explosive deepening played a substantial additional role, as in tropical cyclone formation, in this cyclone's evolution.

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John R. Gyakum

Abstract

The existence of convection and the hurricane-like structure in the explosively-developing cyclone studied in Part I motivates us to assess the importance heating had on this cyclogenesis. To accomplish this, a method to evaluate the three-dimensional thermodynamic and dynamic structure of the atmosphere is proposed, so that we may evaluate potential vorticity changes in the vicinity of this cyclone. Results indicate a 24 h lower tropospheric generation of from five to thirteen times the value observed at 1200 GMT 9 September 1978.

An evaluation of physical effects on thickness change following the surface center shows a large mean tropospheric temperature rise to be due to bulk cumulus heating effects, which could be important in the extraordinary potential vorticity generation concurrent with this cyclone's explosive development.

These vertically integrated values of heating motivate us to solve the quasi-geostrophic omega and vorticity equations forced by an idealized heating function with specified horizontal scale, level of maximum heating and total heating. Resulting theoretical omega profiles and height falls during the 24 h period of explosive development for the observed integrated values of heating, vorticity-stability parameter, and over a wide range of levels of maximum heating readily account for the observed explosive cyclogenesis. It is hypothesized that the relatively weak baroclinic forcing operative in this case helped to organize the convective bulk heating effects on a scale comparable to the cyclone itself in an atmosphere which is gravitationally stable for large-scale motions and gravitationally unstable for the convective scale. This CISK-like mechanism, evidently operative in this case, is further hypothesized to be important in other explosively-developing extratropical cyclones, just as it is generally regarded to be crucial in tropical cyclone development.

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John R. Anderson and John R. Gyakum

Abstract

The interannual and intraseasonal track variability of cold season extratropical cyclones in the Pacific basin is examined using an 8 year cyclone track dataset. An EOF technique incorporating VARIMAX rotation in time is used to objectively describe the regime nature of the variations. Based upon this analysis we conclude that the cyclone behavior can be classified into six major regime types, corresponding to the positive and negative amplitude excursions of each of the first three rotated EOFS. Each of these rotated EOFs explains approximately equal fractions of the total variance. A study of the cyclone tracks for individual extreme periods confirms the existence of times where each of these patterns dominate. The average 500 mb height fields for these extreme periods have been examined and are generally consistent with the cyclone track anomalies. The resultant regime description shows strong interannual variability; however, there appears to be little obvious correlation with the ENSO signal, suggesting that a significant fraction of the interannual variability may be generated within the middle and high latitudes.

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Frederick Sanders and John R. Gyakum

Abstract

By defining a “bomb” as an extratropical surface cyclone whose central pressure fall averages at least 1 mb h−1 for 24 h, we have studied this explosive cyclogenesis in the Northern Hemisphere during the period September 1976–May 1979. This predominantly maritime, cold-season event is usually found ∼400 n mi downstream from a mobile 500 mb trough, within or poleward of the maximum westerlies, and within or ahead of the planetary-scale troughs.

A more detailed examination of bombs (using a 12 h development criterion) was performed during the 1978–79 season. A survey of sea surface temperatures (SST's) in and around the cyclone center indicates explosive development occurs over a wide range of SST's, but, preferentially, near the strongest gradients. A quasi-geostrophic diagnosis of a composite incipient bomb indicates instantaneous pressure falls far short of observed rates. A test of current National Meteorological Center models shows these products also fall far short in attempting to capture observed rapid deepening.

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Timothy A. Bullock and John R. Gyakum

Abstract

The phenomenon of explosive cyclogenesis is studied from the perspective of the synoptic-scale framework within which various intensities of maximum 24-h pressure falls are occurring. This study is accomplished with a construction of composite groups of cyclones that have experienced similar maximum intensification rates within a specified 5° latitude-longitude geographical domain over the Kuroshio Current in the western North Pacific Ocean. An examination of diagnostics computed from the composite fields of geopotential height and temperature reveals several trends. As the degree of intensification increases, the downstream surface ridge and attendant warm, moist inflow become more prominent, the cyclonic vorticity of the initial surface circulation is greater, the downstream frontogenesis is stronger and occurs through a deeper layer of the troposphere, and the location and strength of the vertical-motion forcing become more favorable for development. As a consequence of these results, it is concluded that synoptic-scale forcing mechanisms extending over a large domain, in a composite sense, play a role in determining the amount of intensification experienced by a cyclone. These mechanisms supporting cyclogenesis include not only dynamic support in the form of midtropospheric thermal and vorticity advection but also by deep tropospheric frontogenetic processes occurring both upstream and downstream of the surface low.

Since these mechanisms are well resolved by contemporary numerical models and routinely available data, the aforementioned trends might be used operationally to evaluate the potential for cyclone intensification.

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Richard H. Grumm and John R. Gyakum

Abstract

An examination is made of the current National Meteorological Center (NMC) operational models’ ability to forecast surface anticyclones. A study of the 1981–82 cold season reveals systematic underprediction of the phenomenon on the part of both the Limited Area Fine Mesh (LFM) and spectral models. However, the LFM forecasts weaker anticyclones than does the spectral model. This difference is apparent in the region of eastern North America and the western Atlantic Ocean. The systematic underprediction found in this study is as great as Colucci and Bosart found for NMC's six-layer primitive equation model.

No overall systematic forecast bias is found for the 1000–500 mb mean temperatures over the surface anti-cyclones. However, excessively warm temperatures are forecast in the Pacific northwest region of both models, and the LFM forecasts erroneously cold temperatures in the western Atlantic basin south of 40°N. The spectral model shows a significant improvement over the LFM in this latter region.

The mean anticyclone displacement error for both models at 48-h range is about 500 km. There is also a tendency for both models to place anticyclones erroneously to the south and east of their observed positions, suggesting the models' translation of these anticyclones to be too fast. Colucci and Bosart also found a fast bias, but this study suggests an overall improvement in anticyclone placement.

Finally, a case of a recent poorly forecasted anticyclone-cyclone complex illustrates the deleterious effects those forecasts can have in the attempt to correctly forecast significant precipitation events. Our study shows an unforecasted precipitation event to have occurred in a lower troposphere warm advection region associated with a poorly forecasted surface anticyclone.

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John R. Gyakum and Paul J. Roebber

Abstract

The ice storm of 5–9 January 1998, affecting the northeastern United States and the eastern Canadian provinces, was characterized by freezing rain amounts greater than 100 mm in some areas. The event was associated with a 1000–500-hPa positive (warm) thickness anomaly, whose 5-day mean exceeded +30 dam (+15°C) over much of New York and Pennsylvania. The region of maximum precipitation occurred in a deformation zone between an anomalously cold surface anticyclone to the north and a surface trough axis extending from the Gulf of Mexico into the Great Lakes. The thermodynamic impact of this unprecedented event was studied with the use of a four-dimensional data assimilation spanning an 18-day period ending at 0000 UTC 9 January 1998. A moisture budget for the precipitation region reveals the bulk of the precipitation to be associated with the convergence of water vapor transport throughout the precipitation period. The ice storm consisted of two primary synoptic-scale cyclonic events. The first event was characterized by trajectories arriving in the precipitation zone that had been warmed and moistened by fluxes over the Gulf Stream Current and the Gulf of Mexico. The second and more significant event was associated with air parcels arriving in the precipitation zone that had been warmed and moistened for a period of several days in the planetary boundary layer (PBL) of the subtropical Atlantic Ocean. These parcels had equivalent potential temperatures of approximately 330 K at 800 hPa as they traveled into the ice storm's precipitation zone.

Analogs to this unprecedented meteorological event were sought by examining anomaly correlations (ACs) of sea level pressure, and 1000–925 and 1000–500-hPa thicknesses. Five analogs to the ice storm were found, four of which are characterized by extensive freezing rain. The best analog, that of 22–27 January 1967, is characterized by freezing rain extending from the northeastern United States into central Ontario. However, the maximum amounts are less than 50% of the 1998 case. An examination of air parcel trajectories for the 1967 case reveals a similar-appearing horizontal spatial structure of trajectories, with several traveling anticyclonically from the subtropical regions of the eastern Atlantic. However, a crucial distinguishing characteristic of these trajectories in the 1967 case is that the air parcels arriving in the precipitation zone had equivalent potential temperature values of only 310 K, as compared with 330 K for the 1998 ice storm trajectories. It was found that these air parcels had traveled above the PBL and, therefore, had not been warmed and moistened by fluxes from the subtropical oceans.

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John R. Gyakum and Earl S. Barker

Abstract

The continental cyclone of 28–29 March 1984 was noteworthy for its explosive intensification within six hours. Surface and upper-air data are analyzed for this storm throughout its 18-h life cycle of growth and decay over the eastern United States. The short time scale of this low's explosive development motivates us to place particular emphasis upon the hourly surface observations and their relationship to the rapid cyclogenesis.

While the synoptic-scale environment of the cyclogenesis consisted of quasi-geostrophic ascent, surface data reveal that the intersection of a heated moist tongue of air and an intensifying cold front was the approximate location of the initial vortex. The surface-based lifted index over the incipient cyclone was about −8°C. Static stability had decreased steadily prior to the cyclogenesis. Frontogenctic forcing and deep convection followed the surface cyclone for the next four hours of explosive deepening.

Filling of the cyclone occurred as the surface frontogenetic forcing eased, and the convection was displaced from the cyclone's center. The second pulse of explosive deepening ensued as an axis of maximum 500-mb cyclonic vorticity advection passed over the surface low.

This study shows that several physical processes, of differing scales, combine synergistically to effect this rare case of explosive land cyclogenesis.

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