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Paul J. Roebber, John R. Gyakum, and Diep N. Trat

Abstract

The inverted trough and coastal front that occurred during ERICA IOP 2 were studied in order to assess the ability of an operational mesoscale model, the Canadian Regional Finite Element Model, to provide credible forecast guidance concerning the regional distribution of precipitation associated with such events. The observed distribution of the heaviest precipitation was dependent on the orientation and inland penetration of the coastal front and its associated (geostrophic) frontogenetic forcing, although considerable local variability was evident. The observations indicate that the front penetrated inland along a line west of Boston through east-central Connecticut. The model, which performed exceptionally well in forecasting the eventual development of the main cyclone, correctly forecast the existence and general location of the inverted trough and coastal front and thereby gave indications of the development of heavy snowfall within the coastal region of New York–New England 18–24 h in advance. However, detailed modeling of the coastal front itself is still beyond the scope of current operational models. In the case studied, the position of the regional environment in which the coastal front forms was forecast incorrectly, preventing a precise regional forecast of the precipitation distribution.

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Richard A. Anthes, Ying-Hwa Kuo, and John R. Gyakum

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 storm's evolution are also examined numerically for their relative importance. In a series of seven simulations in which initial condition, horizontal resolution, and physics are varied, the model storm intensity varies considerably. In the weakest, the minimum pressure and maximum boundary-layer wind speeds are 1001 mb and 15.0 m s−1; in the strongest, these parameters are 960 mb and 50.2 m s−1.

The model simulations without the supplementary data set show little improvement over the forecasts of NMC and FNWC. Those simulations with the supplementary data produce improvements in the S 1 score, the intensity of the storm and the track of the storm. The improvement in the model simulations with the introduction of the supplementary data appears due to their more realistic documentation of the shallow cyclonic circulation, the small low-level static stability, and enhanced lower-tropospheric water vapor content.

Physical processes also played a major role in the simulators. The effect of surface fluxes of sensible and latent heat were moderate on the 24 h pressure and wind forecasts. In addition, these fluxes produced large changes in the temperature and moisture structure of the planetary boundary layer over a large area of cold northerly flow to the rear of the cyclone.

Latent heating was important in determining the storm intensity and track. Including latent heating through a cumulus parameterization scheme with a horizontal resolution of 90 km produced an improvement in the simulated intensity and position, with a reduction in minimum pressure of 7 mb and an increase in boundary-layer wind speed of 5 m s−1. With 45 km horizontal resolution, use of explicit condensation heating rather than the cumulus parameterization produced a further reduction in minimum pressure of 12 mb. Although experiments with explicit rather than parameterized latent heating produced more intense storms, in agreement with observations, the model storm motion was slowed considerably during the last 6 h of the simulation, resulting in an increased position error.

The model storm showed a small increase in intensity when the horizontal grid length reduced from 90 km to 45 km, with the minimum pressure decreasing by 3 mb. A further reduction in horizontal resolution to 22.5 km produced only minor differences in storm intensity.

The most intense model storm was simulated when an explicit medium-resolution planetary boundary-layer formulation replaced the bulk formulation used in most of the experiments. With 45 km resolution, explicit latent heating, and the medium-resolution boundary-layer model, a storm with minimum pressure of 960 mb and a maximum wind speed of 50.2 m s−1 was obtained.

This study suggests that baroclinic instability in the weakly stratified lower troposphere is the major mechanism of growth for this cyclone, as discussed by Reed, although latent heat plays an important role in the later stages of development. The development of this strong, yet relatively shallow, storm has three major implications for improving operational forecasts Of similar storms. First, the vertical resolution of the model must be adequate; our estimate is that at least four model layers are required below 700 mb. Second, the lower-tropospheric winds, static stability, water vapor content, and sea-surface temperature must be resolved accurately in the initial analysis because of the sensitivity of the model storm to these fields. Third, continued improvement of modeling planetary boundary-layer and latent heating processes is likely to be important in cases of this type.

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Richard E. Danielson, John R. Gyakum, and David N. Straub

Abstract

The sequential development of a western, and then an eastern, North Pacific cyclone is examined in terms of eddy energy and a phase-independent wave activity. Based on the propagation of both a contiguous wave activity center and eddy energy, the development of the western cyclone appears to influence its downstream neighbor. A quantitative comparison of these two diagnoses is made in terms of group velocity, and only minor differences are found during much of the initial evolution. It is only once the tropopause undulations lose their wavelike appearance (at which point, application of the group-velocity concept itself becomes quite tenuous) that the downstream propagation of eddy energy seems faster than that of wave activity. Conventional methods of tracking this wave packet are also briefly discussed.

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Shawn M. Milrad, Eyad H. Atallah, and John R. Gyakum

Abstract

The issue of quantitative precipitation forecasting continues to be a significant challenge in operational forecasting, particularly in regions susceptible to frequent and extreme precipitation events. St. John’s, Newfoundland, Canada, is one location affected frequently by such events, particularly in the cool season (October–April). These events can include flooding rains, paralyzing snowfall, and damaging winds.

A precipitation climatology is developed at St. John’s for 1979–2005, based on discrete precipitation events occurring over a time period of up to 48 h. Threshold amounts for three categories of precipitation events (extreme, moderate, and light) are statistically derived and utilized to categorize such events. Anomaly plots of sea level pressure (SLP), 500-hPa height, and precipitable water are produced for up to 3 days prior to the event. Results show that extreme events originate along the Gulf Coast of the United States, with the location of anomaly origin being farther to the north and west for consecutively weaker events, culminating in light events that originate from the upper Midwest of the United States and south-central Canada. In addition, upper-level precursor features are identified up to 3 days prior to the events and are mainly located over the west coast of North America.

Finally, results of a wind climatology produced for St. John’s depict a gradual shift in the predominant wind direction (from easterly to southwesterly) of both the 925-hPa geostrophic wind and 10-m observed wind from extreme to light events, inclusively. In addition, extreme events are characterized by almost exclusively easterly winds.

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Zonghui Huo, Da-Lin Zhang, and John R. Gyakum

Abstract

The relative importance of various potential vorticity (PV) perturbations and their mutual interactions associated with the superstorm of 12–14 March 1993 are investigated by applying a piecewise PV inversion diagnostic system to a 36-h simulation of the storm. It is shown that the contributions from all PV anomalies to the surface development increase with time, although their relative significance varies during the rapid deepening stage. In general, the upper-level dry PV anomalies contribute the most to the rapid deepening of the storm, followed, in order, by the lower-level thermal anomaly and latent heat release.

Comparing the PV anomalies and their inverted circulations reveals that there exists a favorable phase tilt between the upper- and lower-level anomalies that allows lower- and upper-level mutual interactions, in which the circulations associated with the upper-level PV anomalies enhance the lower-level anomalies and vice versa. In addition to the vertical interactions, lateral interactions are also present among the upper-level PV anomalies and the background flow. It is also found that the background flow advection dominates the vortex–vortex and vortex–background flow interactions in the deepening of the storm. The vortex–vortex interactions of the two upper-level positive PV anomalies cause the negative tilt of the main upper-level trough during the rapid deepening period.

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Marco L. Carrera, John R. Gyakum, and Da-Lin Zhang

Abstract

Secondary cyclogenesis has been identified as a difficult forecast challenge. In this paper, the authors examine the dominant physical processes associated with the predictability of a case of explosive secondary marine cyclogenesis and provide a better understanding of the large variability in the recent model-intercomparison simulations of the case. A series of sensitivity experiments, involving changes to the model initial conditions and physical parameterizations, is performed using the Canadian Mesoscale Compressible Community Model with a grid size of 50 km.

It is found that errors in the model initial conditions tend to decay with time, and more rapidly so in “dry” simulations. The model fails to produce the secondary cyclogenesis in the absence of latent heating. Water vapor budget calculations from the control experiment show that the surface moisture flux from 6 to 12 h is the largest contributor of water vapor to the budget area in the vicinity of the cyclone center, and remains an important moisture supply throughout the integration period. During the first 12 h, these fluxes are crucial in inducing grid-scale diabatic heating and destabilizing the lower troposphere, thereby facilitating the subsequent rapid deepening of the storm. A secondary maximum in surface latent heat flux to the north and east of the primary maximum acts to force the cyclogenesis event to the south and east of a coastal circulation center. When the surface evaporation is not allowed, much less precipitation is produced and the secondary cyclone fails to develop. Calculations of the potential temperature on the dynamic tropopause (i.e., 2-PVU surface) in the absence of surface evaporation indicate a significantly damped thermal wave when compared with the control integration.

This result for a case of secondary cyclogenesis differs from those generally found for large-scale extratropical cyclogenesis where upper-level baroclinic forcings tend to dominate, and motivates the need for better physical parameterizations, including the condensation and boundary layer processes, in operational models. The authors speculate that the different treatment of condensation and boundary layer processes may have been partly responsible for the enhanced variability in the simulation of this case in a recently completed international mesoscale model intercomparison experiment.

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Christopher D. McCray, Eyad H. Atallah, and John R. Gyakum

Abstract

Freezing rain can cause severe impacts, particularly when it persists for many hours. In this paper, we present the climatology of long-duration (6 or more hours) freezing rain events in the United States and Canada from 1979 to 2016. We identify three focus regions from this climatology and examine the archetypal thermodynamic evolution of events in each region using surface and radiosonde observations. Long-duration events occur most frequently in the northeastern United States and southeastern Canada, where freezing rain typically begins as lower-tropospheric warm-air advection develops the warm layer aloft. This warm-air advection and the latent heat of fusion released when rain freezes at the surface erode the cold layer, and freezing rain transitions to rain once the surface temperature reaches 0°C. In the southeastern United States, a larger percentage of events are of long duration than elsewhere in North America. Weak surface cold-air advection and evaporative cooling in the particularly dry onset cold layers there prevent surface temperatures from rising substantially during events. Finally, the south-central United States has a regional maximum in the occurrence of the top 1% of events by duration (18 or more hours), despite the relative rarity of freezing rain there. These events are associated with particularly warm/deep onset warm layers, with persistent low-level cold-air advection maintaining the cold layer. The thermodynamic evolutions we have identified highlight characteristics that are key to supporting persistent freezing rain in each region and may warrant particular attention from forecasters tasked with predicting these events.

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Melissa Gervais, Eyad Atallah, John R. Gyakum, and L. Bruno Tremblay

Abstract

An important aspect of understanding the impacts of climate change on society is determining how the distribution of weather regimes will change. Arctic amplification results in greater warming over the Arctic compared to the midlatitudes, and this study examines how patterns of Arctic air masses will be affected. The authors employ the Community Earth System Model Large Ensemble (CESM-LE) RCP 8.5, consisting of 30 ensemble members run through the twenty-first century. Self-organizing maps are used to define archetypes of 850-hPa equivalent potential temperature anomalies with respect to a changing climate and assess changes in their frequency of occurrence. In the model, a pattern with negative anomalies over the central Arctic becomes less frequent in the future. There is also an increase in the frequency of patterns associated with an amplified ridge (trough) with positive (negative) anomalies over western (eastern) North America. It is hypothesized that the increase in frequency of such patterns is the result of enhanced forcing of baroclinic waves owing to reduced sea ice over the western Arctic. There is also a decline in patterns that have anomalously high over the North Atlantic, a pattern that is associated with intense ridging in the 500-hPa flow over the North Atlantic and colder over Europe. The authors relate the decrease of these patterns to an enhancement of the North Atlantic jet induced by a warming deficit in the North Atlantic Ocean.

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Shawn M. Milrad, Eyad H. Atallah, and John R. Gyakum

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Tropical cyclones in the western North Atlantic basin are a persistent threat to human interests along the east coast of North America. Occurring mainly during the late summer and early autumn, these storms often cause strong winds and extreme rainfall and can have a large impact on the weather of eastern Canada. From 1979 to 2005, 40 named (by the National Hurricane Center) tropical cyclones tracked over eastern Canada. Based on the time tendency of the low-level (850–700 hPa) vorticity, the storms are partitioned into two groups: “intensifying” and “decaying.” The 16 intensifying and 12 decaying cases are then analyzed using data from both the National Centers for Environmental Prediction (NCEP) North American Regional Reanalysis (NARR) and the NCEP global reanalysis. Composite dynamical structures are presented for both partitioned groups, utilizing both quasigeostrophic (QG) and potential vorticity (PV) perspectives. It is found that the proximity to the tropical cyclone and subsequent negative tilt (or lack thereof) of a precursor trough over the Great Lakes region is crucial to whether a storm “intensifies” or “decays.” Heavy precipitation is often the main concern when tropical cyclones move northward into the midlatitudes. Therefore, analyses of storm-relative precipitation distributions show that storms intensifying (decaying) as they move into the midlatitudes often exhibit a counterclockwise (clockwise) rotation of precipitation around the storm center.

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Shawn M. Milrad, John R. Gyakum, and Eyad H. Atallah

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The 19–21 June 2013 Alberta flood was the costliest (CAD $6 billion) natural disaster in Canadian history. The flood was caused by a combination of above-normal spring snowmelt in the Canadian Rockies, large antecedent precipitation, and an extreme rainfall event on 19–21 June that produced rainfall totals of 76 mm in Calgary and 91 mm in the foothills. As is typical of flash floods along the Front Range of the Rocky Mountains, rapidly rising streamflow proceeded to move downhill (eastward) into Calgary.

A meteorological analysis traces an antecedent Rossby wave train across the North Pacific Ocean, starting with intense baroclinic development over East Asia on 11 June. Subsequently, downstream Rossby wave development occurred across the North Pacific; a 1032-hPa subtropical anticyclone located northeast of Hawaii initiated a southerly atmospheric river into Alaska, which contributed to the development of a cutoff anticyclone over Alaska and a Rex block (ridge to the north, cyclone to the south) in the northeastern North Pacific. Upon breakdown of the Rex block, lee cyclogenesis occurred in Montana and strong easterly upslope flow was initiated in southern Alberta.

The extreme rainfall event was produced in association with a combination of quasigeostrophically and orographically forced ascent, which acted to release conditional and convective instability. As in past Front Range flash floods, moisture flux convergence and positive θ e advection were collocated with the heavy rainfall. Backward trajectories show that air parcels originated in the northern U.S. plains, suggesting that evapotranspiration from the local land surface may have acted as a moisture source.

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