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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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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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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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R. McTaggart-Cowan, J. R. Gyakum, and M. K. Yau
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R. McTaggart-Cowan, J. R. Gyakum, and M. K. Yau

Abstract

The role that atmospheric water, in both its liquid and vapor phases, plays in cyclogenesis is difficult to determine because of the complex interactions between dynamic and thermodynamic forcings. From a potential vorticity (PV) perspective, it is possible to decompose the atmospheric state into a set of superposed PV anomalies. The modification of these anomalies allows for sensitivity testing using numerical models. Although this approach allows for the determination of cyclogenetic contributions from individual PV features, its application has not accounted for the dynamically consistent modification of the moisture field. This paper develops a PV-based variable that describes the effects of water vapor, cloud, and rainwater on balanced dynamics. A special-case analytic form of this “moist component” PV is developed and interpreted using an idealized model of the atmosphere. The application of the moist component methodology developed here provides the basis for future work, which includes sensitivity tests designed to separate the impacts of dynamics and thermodynamics on cyclogenesis.

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R. McTaggart-Cowan, J. R. Gyakum, and M. K. Yau

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 model's 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 storm's reintensification, while the presence of the low-level circulation associated with the decaying hurricane plays only a secondary role.

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R. McTaggart-Cowan, J. R. Gyakum, and M. K. Yau

Abstract

The importance of remnant tropical cyclone (TC) circulation and moisture structures is investigated for a simultaneous extratropical transition (ET) event involving ex-Hurricanes Danielle and Earl (September 1998). Although both storms undergo prolonged periods of reintensification following ET, the forcings involved in each of their redevelopment processes differ fundamentally. A review of the tropical and baroclinic ET modes in the North Atlantic stresses the importance of jet/front structures to the nature of the reintensification process. Ex-Hurricane Danielle begins to redevelop in the eastern half of the basin in the downstream, poleward sector of an intensifying polar jet. The system undergoes a tropical mode of reintensification, resulting in a troposphere-deep warm environment surrounding the storm, devoid of near-surface fronts and maintained by strong tropopause folds at its periphery. Ex-Hurricane Earl reintensifies near the eastern seaboard according to a baroclinic mode, under the influence of an upshear upper-level trough. A rapid cyclonic rollup of upper-level potential vorticity over the reintensifying low-level center results in a strong baroclinic system with well-defined frontal boundaries.

The two elements of the remnant TCs considered here are circulation and moisture. Potential vorticity-based modifications are made to the initial atmospheric state of the Mesoscale Compressible Community model in order to remove either one or both of these possible cyclogenetic forcings. The resulting set of sensitivity tests is analyzed in terms of system intensity and structure. It is found that the tropical-mode reintensification (ex-Hurricane Danielle) process requires the presence of the remnant's circulation and moisture for rapid redevelopment. However, the baroclinic-mode transition studied (ex-Hurricane Earl) is remarkably insensitive to the removal of the ex-tropical vorticity and moisture structures of the TC remnant.

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R. McTaggart-Cowan, J. R. Gyakum, and M. K. Yau

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The multitude of tropical–extratropical interactions that occur during an extratropical transition (ET) complicate the prediction and diagnosis of these extreme events. This study focuses on the analysis of a double ET and reintensification event that took place between 5 and 7 September 1998. Ex-Hurricanes Earl and Danielle reintensified rapidly over the western and eastern North Atlantic, respectively. A set of simulations designed to test the sensitivity of Earl's ET to features in the downstream state was run using a set of idealizations for a numerical model's initial and boundary conditions downstream of ex-Hurricane Earl. Dynamic tropopause analyses and the “PV thinking” paradigm applied under the Eady model highlight important developmental and structural differences between the tests.

In fact, two distinct solution modes are diagnosed both in the control and in the sensitivity tests. Earl's ET proceeds according to a “baroclinic mode” of redevelopment, whereas Danielle displays distinct “tropical mode” ET signatures throughout the period of investigation. The presence of a strong zonal jet immediately downstream of the transitioning cyclone is found to be sufficient to induce a baroclinic mode of redevelopment characterized by cyclonic potential vorticity rollup and strong near-surface frontogenesis. Under the influence of an upstream jet in isolation, the reintensification takes on distinctly tropical characteristics as the enhanced northward intrusion of warm, moist air ahead of the system creates a local environment favorable for a tropical mode of redevelopment. A description of the dynamics associated with these two distinct redevelopment modes may aid in the understanding and prediction of these events.

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Z. Long, W. Perrie, J. Gyakum, D. Caya, and R. Laprise

Abstract

It is well known that large lakes can perturb local weather and climate through mesoscale circulations, for example, lake effects on storms and lake breezes, and the impacts on fluxes of heat, moisture, and momentum. However, for both large and small lakes, the importance of atmosphere–lake interactions in northern Canada is largely unknown. Here, the Canadian Regional Climate Model (CRCM) is used to simulate seasonal time scales for the Mackenzie River basin and northwest region of Canada, coupled to simulations of Great Bear and Great Slave Lakes using the Princeton Ocean Model (POM) to examine the interactions between large northern lakes and the atmosphere. The authors consider the lake impacts on the local water and energy cycles and on regional seasonal climate. Verification of model results is achieved with atmospheric sounding and surface flux data collected during the Canadian Global Energy and Water Cycle Experiment (GEWEX) program. The coupled atmosphere–lake model is shown to be able to successfully simulate the variation of surface heat fluxes and surface water temperatures and to give a good representation of the vertical profiles of water temperatures, the warming and cooling processes, and the lake responses to the seasonal and interannual variation of surface heat fluxes. These northern lakes can significantly influence the local water and energy cycles.

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John R. Gyakum, Paul J. Roebber, and Timothy A. Bullock

Abstract

We examine the idea that antecedent vorticity development, defined as the surface vorticity spinup in the period prior to a cyclone's maximum intensification, is an important dynamical conditioning process for explosive cyclogenesis. Previous suggestions from case study research that subsequent intensification may be proportional to the intensity of the preexisting circulation are supported through the systematic study of a large sample of weakly and explosively developing cyclones in the North Pacific and North Atlantic basins.

Additional support for this concept is found with an examination of composite weakly and strongly developing cyclones at the onset of their most rapid intensification period. At this onset, the strongly developing cyclone composite has substantially stronger surface circulation and vorticity than is found in a composite of the weak cases. Ensembles of successive forecasts of an explosive cyclogenesis case during the Experiment on Rapidly Intensifying Cyclones over the Atlantic (ERICA) suggest similar dynamical behavior, in that small errors in the surface intensity subsequently amplify into larger errors only 12 h later under predominantly similar upper-level conditions.

The temporal evolution of large-scale geostrophic vorticity for 62 cases of cyclogenesis shows that stretching in the presence of relative vorticity is present throughout the life cycle of both the weakly and rapidly developing cases. An examination of 794 cyclones in the North Pacific basin reveals a general trend of increased maximum development as the antecedent deepening increases. Explosively developing cyclones are preferentially characterized by at least 12 h of antecedent development.

We investigate the relationship between the amplitude of the 500-mb quasigeostrophic-ascent forcing and maximum surface cyclone intensification and find a significant positive correlation, as previous studies have shown. However, computations with model-based surface convergence suggest that the response to the upper-level forcing is conditioned by the low-level antecedent vorticity development. Furthermore, variations in successive numerical weather prediction model forecasts of maximum cyclone intensification are well correlated with variations in the initial surface vorticity as well as variations in the 500-mb forcing.

This study suggests that explosive development is typically characterized by a nonlinear interaction between two cyclonic disturbances in the lower and upper troposphere. These disturbances, in some cases, may have formed independently of one another. Thus, the correct simulation of the full life cycle of these cyclones, including the antecedent phase, may be crucial for accurate numerical forecasts.

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