John R. Gyakum

Synoptic-Dynamic Climatology of the “Bomb”

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 (SSTs) in and around the cyclone center indicates explosive development occurs over a wide range of SSTs, 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 ob...

Numerical Simulations of a Case of Explosive Marine Cyclogenesis

Abstract The extratropical cyclone which damaged the liner Queen Elizabeth II in September 1978 is a well-documented example of explosive marine cyclogenesis in which the 24 h surface central pressure fall was 60 mb commencing 1200 GMT 9 September. Operational models of both the National Meteorological Center (NMC) and Fleet Numerical Weather Central (FNWC) predicted virtually none of the observed surface intensification. This study reports on results of simulations performed with a primitive equation model. Emphasis will be placed on discovering why such poor forecasts were made of this storm. The extensive data set compiled by Gyakum (1983a, b) is used both to initialize and verify the model in a series of 24 h simulations, in order to assess the impact of initializing the model with these supplementary data. Physical processes identified observationally by Gyakum as being important in the storms evolution are also examined numerically for their relative importance. In a series of seven simulations in ...

On the Evolution of the QE II Storm. I: Synoptic Aspects

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 cent...

On the Evolution of the QE II Storm. II: Dynamic and Thermodynamic Structure

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 cyclones 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 spe...

North American extreme temperature events and related large scale meteorological patterns: A review of statistical methods, dynamics, modeling, and trends

Abstract The objective of this paper is to review statistical methods, dynamics, modeling efforts, and trends related to temperature extremes, with a focus upon extreme events of short duration that affect parts of North America. These events are associated with large scale meteorological patterns (LSMPs). The statistics, dynamics, and modeling sections of this paper are written to be autonomous and so can be read separately. Methods to define extreme events statistics and to identify and connect LSMPs to extreme temperature events are presented. Recent advances in statistical techniques connect LSMPs to extreme temperatures through appropriately defined covariates that supplement more straightforward analyses. Various LSMPs, ranging from synoptic to planetary scale structures, are associated with extreme temperature events. Current knowledge about the synoptics and the dynamical mechanisms leading to the associated LSMPs is incomplete. Systematic studies of: the physics of LSMP life cycles, comprehensive model assessment of LSMP-extreme temperature event linkages, and LSMP properties are needed. Generally, climate models capture observed properties of heat waves and cold air outbreaks with some fidelity. However they overestimate warm wave frequency and underestimate cold air outbreak frequency, and underestimate the collective influence of low-frequency modes on temperature extremes. Modeling studies have identified the impact of large-scale circulation anomalies and land–atmosphere interactions on changes in extreme temperatures. However, few studies have examined changes in LSMPs to more specifically understand the role of LSMPs on past and future extreme temperature changes. Even though LSMPs are resolvable by global and regional climate models, they are not necessarily well simulated. The paper concludes with unresolved issues and research questions.

Analysis of Hurricane Catarina (2004)

Abstract The development of Hurricane Catarina over the western South Atlantic Ocean in March 2004 marks the first time that the existence of a hurricane has been confirmed by analysis and satellite imagery in the South Atlantic basin. The storm undergoes a complex life cycle, beginning as an extratropical precursor that moves east-southeastward off the Brazilian coast and toward the midlatitudes. Its eastward progress is halted and the system is steered back westward toward the Brazilian coast as it encounters a strengthening dipole-blocking structure east of the South American continent. Entering the large region of weak vertical shear that characterizes this blocking pattern, Catarina begins a tropical transition process over anomalously cool 25°C ocean waters above which an elevated potential intensity is supported by the cold upper-level air associated with the trough component of the block. As the convective outflow from the developing tropical system reinforces the ridge component of the dipole blo...

North Pacific Cold-Season Surface Cyclone Activity: 1975–1983

Abstract An eight year sample of cold-season (1 October through 31 March) extratropical cyclones in the, Pacific Ocean basin is used to study central pressure changes and life cycle characteristics. We find that over 90% of the cyclones passing through the area of the Kuroshio Current intensify in this region. Corresponding percentages in excess of 60% extend from the Kuroshio, south of 45°N, eastward to 130°W. Mean 24-h central pressure falls of all cyclones exceed 9 mb through the entire basin west of 140°W in the latitude band 30° to 50°N. A statistical analysis of 24-h central pressure changes is performed on all cyclones within our domain. A frequency distribution of 1996 cases of 24-h maximum deepening reveals statistically significant departures from a Gaussian distribution, with the coefficient of skewness substantially negative. We also find similarly significant departures from normal in a frequency distribution of all 24-h central pressure changes, in spite of the fact that this distribution wo...

Sensitivity Testing of Extratropical Transitions Using Potential Vorticity Inversions to Modify Initial Conditions: Hurricane Earl Case Study

Abstract This study uses the Mesoscale Compressible Community model to simulate the extratropical transition and reintensification of Hurricane Earl (1998) for the purposes of testing sensitivity to modification of the models initial conditions. Though relatively strong “classical” cyclogenetic forcings were present in this case, operational forecasts seriously underpredicted the severity of the reintensification. Employing a piecewise potential vorticity (PV) inversion, the authors remove localized PV anomaly (PV′) maxima from the initial conditions and rebalance the fields for input to the model. Several PV′ structures in an upstream trough, and the PV′ associated with the hurricane, are removed individually and the model is rerun. Comparison of the resulting output with that of the control integration allows for a quantification of the impact of each PV anomaly on the regeneration of Earl. It is found that the existence of an upstream trough is of primary importance to the storms reintensification, w...

Heavy Cold-Season Precipitation in the Northwestern United States: Synoptic Climatology and an Analysis of the Flood of 17-18 January 1986

Abstract Warm, moist southwesterly airflow into the northwestern United States during the cold season can result in rapid snowmelt and flooding. The objectives of this research are to document characteristic synoptic flow patterns accompanying cold-season (November–March) flooding events, and isolate flow anomalies associated with the moisture transport during a representative event. The first objective is accomplished through a 46-case composite spanning the years 1962–88; the second objective is addressed through diagnosis of a flooding event that occurred on 17–18 January 1986. The 46-case composite is constructed for a 6-day period centered at 1200 UTC on the day of heavy precipitation onset (denoted τ0). Composite 500-hPa geopotential height anomaly fields reveal anomalous ridging over the Bering Sea preceding the precipitation event, a negative anomaly over the Gulf of Alaska throughout the composite evolution, and a positive anomaly over the southwestern Unites States and adjacent eastern Pacific O...

The 1998 Ice Storm—Analysis of a Planetary-Scale Event

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 even...