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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

Fred Sanders' teaching and research contributions in the area of quasigeostrophic theory are highlighted in this paper. The application of these contributions is made to the topic of extreme cold-season precipitation events in the Saint Lawrence valley in the northeastern United States and southern Quebec.

This research focuses on analyses of Saint Lawrence valley heavy precipitation events. Synoptic- and planetary-scale circulation anomaly precursors are typically identified several days prior to these events. These precursors include transient upper-level troughs, strong moisture transports into the region, and anomalously large precipitable water amounts. The physical insight of Fred Sanders' work is used in the analysis of these composite results. Further details of this insight are provided in analyses of one case of heavy precipitation.

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John R. Gyakum

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Six years of daily temperature and precipitation forecasting are studied for Urbana, Illinois. Minimum temperature forecast skills, measured against a climatological control, are 57%, 48%, 34% and 20% for the respective forecast ranges of one, two, three, and four days. Maximum temperature skills are comparable. Precipitation probability skills of 29%, 19%, 6% and −2% are found for the same respective forecast ranges. However, our skill in predicting precipitation amount, given that a measurable quantity occurs, is only 17% at the first day range and negligible thereafter. An examination of objective National Weather Service (NWS) forecasts shows this guidance to be slightly less skillful than our consensus in forecasting temperature and precipitation. Some temporal improvement is found in both the consensus and guidance temperature forecasts, but none can be found in the more difficult problem of forecasting precipitation.

Significant warm and dry biases are frequently found in both our consensus and NWS guidance forecasts, especially during the summer season. These biases may be associated with the organized convective character of the precipitation in Illinois. Forecasts often miss these key events and, therefore, will often predict excessively warm maximum temperatures.

Finally, the results show that our consensus skill is comparable to the state of the art. Student or faculty individuals usually lose to our consensus, as does the NWS objective forecast guidance. This establishment of the consensus forecast as typically being superior to an individual forecast has been reported by investigators in eastern United States cities.

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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

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

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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

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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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John R. Gyakum and Ming Cai

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The phenomenon of strong vertical wind sheer (SWS) over the North American continent is documented with conventional rawinsonde data for the period 1 October 1983 through 31 March 1984. One of the objectives of this work is to describe the three-dimensional distribution of frontal zones. Since it is not possible to use the coarse horizontal resolution of the sounding network to document fully the existence of frontal zones, we instead use the excellent vertical resolution of the winds to infer the existence of these strong baroclinic zones through use of the thermal wind relation.

SWS, defined as a shear exceeding that associated with a horizontal temperature gradient of 1 5.5°C/500 km, is most frequently observed in the lower and upper troposphere, though secondary frequency peaks are observed in the middle troposphere. The SWS phenomenon is preferentially located in the middle latitudes where the strongest baroclinity is typically found. Monthly fields of SWS show the frequency maxima to he closely associated with the mean jet stream and strong baroclinity.

An examination of an exceptionally active five-day period of SWS activity shows a very strong baroclinic zone throughout the United States. The most active region of SWS in the eastern area is located in a zone of cyclonic wind shear downstream of a time-averaged though. This particular region of SWS is associated with explosive surface cylogenesis. A study of a corresponding period of exceptionally weak SWS activity reveals a markedly different synoptic pattern in which relatively weak wind maxima and baroclinity are shifted poleward into Canada; significantly weaker surface anticylones and no explosive cyclogenesis occurs. These results suggest that the phenomenon of SWS Phenomenon of SWS can be related to synoptic-scale weather patterns.

We demonstrate that SWS phenomenon is preferentially associated with Richardson numbers of less than 1, suggesting a strong association with turbulence. SWS events are associated preferentially with more stable lapse rates than are found otherwise. We also find that SWS events at 700 mb are associated approximately 65% of the time with relative humidities of 75% or less. This result suggests that much of the turbulence observed during these SWS events occurs in cloud-free conditions.

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Philip Marsh and John R. Gyakum
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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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