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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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Paul A. Sisson and John R. Gyakum

Abstract

Several classes of significant cold-season precipitation events occurring in Burlington, Vermont (KBTV), during the 33-yr period from 1963 to 1995, are studied with the objective of identifying large-scale circulation precursors to the more extreme events. Several physically interesting and unique features that correspond to 24-h totals of 25 to 50 mm of precipitation are found. Preferential southerly and more maritime surface geostrophic flow occur in the heavier cases, in association with a strong cyclone (anticyclone) to the west (east) of KBTV. The 1000–500-hPa positive thickness anomaly corresponds to a depth-mean virtual temperature anomaly of +10.5°C in the heavy events. Additionally, statistically significant negative thickness anomalies, responsible for triggering these significant precipitation events, can be traced westward to a position in the Pacific Ocean at least 6 days prior to the event. Significantly heavier precipitable water amounts and preferentially strong water vapor transports from maritime regions are also associated with the heavier cold-season precipitation events.

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Gary M. Lackmann and John R. Gyakum

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 τ 0). 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 Ocean during and after the event. The gulf trough and southwestern ridge lead to enhanced southwesterly geostrophic flow into the northwestern United States at τ 0. A positive temperature anomaly at the 850-hPa level advances northeastward into the northwestern United States by τ 0, and expands over much of the United States by τ +48.

Piecewise geostrophic moisture transport computations for 17–18 January 1986, based on quasigeostrophic potential vorticity inversion, demonstrate that the transport of moisture into the northwestern United States is largely associated with a duo of mobile cyclones that track from the subtropical Pacific Ocean toward British Columbia. There is also a smaller contribution from a stationary anticyclone over the southwestern United States. These results indicate that the role of the planetary-scale flow, as depicted in the composite analyses, is to provide a persistent storm track, while the moisture flow within this storm track is modulated by cyclone-scale dynamics.

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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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Jessica K. Turner and John R. Gyakum

Abstract

Surface observations, soundings, and a thermodynamic budget are used to investigate the formation process of 93 arctic airmass events. The events involve very cold surface temperatures—an average of −42.8°C at Norman Wells, a centrally located station in the formation region—and cooling in the 1000–500-hPa layer. A multistage process for their formation in northwestern Canada is proposed. This process is contrary to the classical conceptualization of extremely shallow, surface formations.

In the first stage of formation, snow falls into a layer of unsaturated air in the lee of the Rocky Mountains, causing sublimational cooling and moistening the subcloud layer. Simultaneously, the midtroposphere is cooled by cloud-top radiation emissions. In the second stage, snowfall abates, the air column dries, and clear-sky surface radiational cooling predominates, augmented by the high emissivity of fresh snow cover. The surface temperature falls very rapidly, up to a maximum of 18°C day−1 in one event. In the final stage, after near-surface temperatures fall below the frost point, ice crystals and, nearer the surface, ice fog form. At the end of formation, there is cold-air damming, with a cold pool and anticyclone in the lee of the Rockies, lower pressure in the Gulf of Alaska, and an intense baroclinic zone oriented northwest to southeast along the mountains.

There have been secular changes in the characteristics of the arctic air masses over the period 1948–2008. The surface temperature during the events has become warmer, and the air masses are deeper and moister. The 1000-hPa diabatic cooling during events, which includes latent heat and radiative processes, has decreased by 2.2°C day−1.

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John R. Gyakum and Katherine J. Samuels

Abstract

Objective precipitation guidance has been evaluated for specific regions within the continental United States during the period 1984–85. Cold-season precipitation probability skills for seven locations range from 52% at 12–24 h forecast range to about 21% at 36–48 h range. While these skills show the probability forecasts to be generally useful, an examination of forecasts with an absolute error of greater than 0.5 reveals this smaller sample to contain a disproportionately large number of observed precipitating events. This suggests that large-error-precipitation-probability forecasts have an unexpectedly large number of essentially unforecasted precipitation events, rather than false alarms. Warm-season precipitation probability skills are generally lower and show more variability within a given forecast range, with values ranging from 38 to 6% at 24–36 h range.

Limited-area Fine-Mesh (LFM model, cold-season, quantitative precipitation forecasts (QPFs) for specific cities show no skill beyond a 12-h forecast range. This loss of skill is associated with statistically significant overprediction of precipitation. However, to account for a coding error in the LFM model, we recomputed our statistics by halving all QPFs. The skills of these forecasts rose to respectable overall levels of 18.2, 14.8, 13.1 and 4.0% for the respective forecast ranges of 0–12, 12–24, 24–36 and 36–48 h. These revised forecasts have eliminated all suggestion of precipitation overprediction, and instead show a systematic underprediction of precipitation.

Cold-season, area-averaged QPFs taken directly from the LFM show a loss of skill against the climatological control forecast beyond 24 h. When we halved all forecasts, our area-averaged results showed, generally, more respectable overall skills of 9.3, 20.8, 16.9 and 5.2% for the respective forecast ranges.

Warm-season point and area-averaged QPFs show no skill against the climatological control forecast for any of the four forecast ranges out to 48 h. Statistically significant precipitation underprediction is found for the raw warm season QPFs. When the forecasts are halved, the skills deteriorate to even lower values and systematic underprediction of precipitation is more prevalent.

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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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Paul J. Roebber and John R. Gyakum

Abstract

The ice storm of 5–9 January 1998, affecting parts of the northeastern United States and the eastern Canadian provinces, was characterized by freezing rain amounts greater than 100 mm in some areas. 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. Mesoscale processes were examined to understand their role in regulating the persistence, phase, and intensity of the event. The persistently cold near-surface air in the precipitating region was linked to orographic channeling of winds from the cold anticyclone to the north. The position of the surface-based freezing line was strongly tied to pressure-driven channeling of surface winds by orography during the first freezing rain episode (4–7 January), while this channeling contributed to the depth of the cold air north of the U.S. border and governed details of the position of the freezing line in the Lake Champlain valley during the second episode (7–10 January). For example, in the absence of the orographic channeling, model sensitivity simulations suggest that little or no freezing precipitation would have occurred at Burlington, Vermont (BTV), during the ice storm. A frontogenetical focus within the St. Lawrence, Ottawa, and Lake Champlain valleys was provided by orographic channeling of the cold air in combination with geostrophic southerlies in the warm air. The frontogenesis was an important contributor to the higher precipitation amounts during the ice storm, with model sensitivity estimates indicating that in the absence of the valleys, total freezing rain volumes would have been reduced by 12.1% and 16.5% in the first and second episodes, respectively. A discussion of the expected predictability of such events is provided.

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David Small, Eyad Atallah, and John R. Gyakum

Abstract

A modified blocking index is defined based on vertically integrated potential vorticity. The application of this index identifies blocking activity over the Northern Hemisphere during all seasons. The index is developed by systematically identifying the magnitude and spatial scale that best characterizes persistent anticyclonic circulation anomalies in different seasons. By applying a systematic approach to the detection of blocking, the interannual, seasonal, and intraseasonal patterns of blocking frequency across the Northern Hemisphere are able to be characterized. The results are consistent with previous studies in finding that blocking is more frequent in the cold season months than in the warm season, although the results suggest that blocking occurs much more frequently in the summer and fall than many studies have previously reported. By examining blocking frequency monthly, interesting patterns of intraseasonal variability are found, especially over the central Pacific in August and the eastern Pacific in September and October, where blocking is nearly as frequent as in the winter. Possible explanations for this intraseasonal variability are discussed.

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