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David Small, Eyad Atallah, and John Gyakum

Abstract

The community of Tuktoyaktuk (Northwest Territories, Canada) along the Beaufort Sea experiences dramatic shoreline erosion during storm surge events that tend to occur during persistent northwesterly wind events in the late summer months (July–September) when the sea ice coverage of the Beaufort Sea reaches its annual minimum. This study compiles the climatology of hourly surface wind, low-level geostrophic wind, and static stability to investigate the physical mechanisms responsible for the high frequency of northwesterly winds observed at Tuktoyaktuk during the late summer. The results link the prevalence of westerly to northwesterly winds at the surface to the high frequency of northwesterly geostrophic winds and a tendency for low static stability. With an environment that favors strong northwesterly geostrophic wind and suggests lower static stability, the high frequency of strong northwesterlies observed at the surface appears to be associated with momentum mixing by turbulent eddies. A composite analysis indicates that persistently strong northwesterly winds are associated with anomalously low pressure northeast of Tuktoyaktuk and high pressure over the Bering Sea and eastern Siberia. The high pressure anomalies over the Bering Sea also extend well to the east along the northern edge of the Brooks Range. An apparent topographic modification of the sea level pressure (SLP) field by cold air trapped to the north of mountains produces the pressure gradient favorable for strong westerly to northwesterly geostrophic winds at Tuktoyaktuk. The results suggest that cold-air damming contributes to the wind regime at Tuktoyaktuk by altering the pressure gradient along the Beaufort coast.

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Eyad H. Atallah and Lance F. Bosart

Abstract

Several recent landfalling tropical cyclones (e.g., Dennis, Floyd, and Irene 1999) have highlighted a need for a refinement in the forecasting paradigms and techniques in the area of quantitative precipitation forecasting. Floyd proved to be a particularly challenging forecast problem as it was accompanied by catastrophic flooding over large regions of the East Coast, in spite of its relatively quick northward movement. The extent and intensity of the precipitation distribution was strongly modulated by the storm's interaction with a midlatitude trough. In an attempt to better understand and quantify the relevant dynamics during this interaction, potential vorticity (PV) and quasigeostrophic perspectives are utilized.

As Floyd approached the East Coast, precipitation shifted to the left of the storm track due to the presence of a deep midlatitude trough in the Ohio valley. The juxtaposition of a cold-core PV anomaly associated with the midlatitude trough and a warm-core PV anomaly associated with Floyd created a strong and tropospheric-deep baroclinic zone along the eastern seaboard. This baroclinic zone provided a region favorable for deep isentropic ascent as the circulation of Floyd approached, resulting in prolific precipitation production. The latent heat release associated with this precipitation in turn acted to enhance outflow ridging north of Floyd, which was underpredicted by current numerical models. The enhanced outflow ridge resulted in enhanced jet-streak dynamics and a restructuring of the tilt of the midlatitude trough in a manner favorable for excessive precipitation production. Furthermore, the uplifting of the dynamic tropopause in southwesterly flow ahead of Floyd in response to ascent and differential diabatic heating resulted in a tropopause fold, a feature usually associated with upper-level fronts and differential subsidence in northwesterly flow.

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Dorothy Durnford, John Gyakum, and Eyad Atallah

Abstract

Satellites are uniquely capable of providing uniform data coverage globally. Motivated by such capability, this study builds on a previously described methodology that generates numerical weather prediction (NWP) model initial conditions (ICs) from satellite total column ozone (TCO) data. The methodology is based on three principal steps: 1) conversion of TCO to mean potential vorticity (MPV) via linear regression, 2) conversion of two-dimensional MPV to three-dimensional potential vorticity (PV) via vertical mapping onto average PV profiles, and 3) inversion of the three-dimensional PV field to obtain model-initializing height, temperature, and wind fields in the mid- and upper troposphere. The overall accuracy of the process has been significantly increased through a substantial reworking of the details of this previous version. For instance, in recognition of the fact that TCO ridges tend to be less reliable than troughs, the authors vertically map an MPV field that is a synthesis of ozone-derived MPV troughs and analysis MPV ridges. The vertical mapping procedure itself produces a more physical three-dimensional PV field by eliminating unrealistically strong features at upper levels.

It is found that the ozone-influenced upper-level initializing fields improve the quantitative precipitation forecast (QPF) of the 24–25 January 2000 East Coast snowstorm for two of the three (re)analyses. Furthermore, the best QPF involves ozone-influenced upper-level initializing fields. Its high threat scores reflect a superior placement, amplitude, and structure. This best QPF is apparently superior to a forecast of the same case where TCO data were assimilated using four-dimensional variational data assimilation.

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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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Alain Roberge, John R. Gyakum, and Eyad H. Atallah

Abstract

Significant cool season precipitation along the western coast of North America is often associated with intense water vapor transport (IWVT) from the Pacific Ocean during favorable synoptic-scale flow regimes. These relatively narrow and intense regions of water vapor transport can originate in either the tropical or subtropical oceans, and sometimes have been referred to as Pineapple Express events in previous literature when originating near Hawaii. However, the focus of this paper will be on diagnosing the synoptic-scale signatures of all significant water vapor transport events associated with poleward moisture transport impacting the western coast of Canada, regardless of the exact points of origin of the associated atmospheric river. A trajectory analysis is used to partition the events as a means of creating coherent and meaningful synoptic-scale composites. The results indicate that these IWVT events can be clustered by the general area of origin of the majority of the saturated parcels impacting British Columbia and the Yukon Territories. IWVT events associated with more zonal trajectories are characterized by a strong and mature Aleutian low, whereas IWVT events associated with more meridional trajectories are often characterized by an anticyclone situated along the California or Oregon coastline, and a relatively mature poleward-traveling cyclone, commonly originating in the central North Pacific.

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

Abstract

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, Eyad H. Atallah, and John R. Gyakum

Abstract

St. John’s, Newfoundland, Canada (CYYT), is frequently affected by extreme precipitation events, particularly in the cool season (October–April). Previous work classified precipitation events at CYYT into categories by precipitation amount and a manual synoptic typing was performed on the 50 median extreme precipitation events, using two separate methods. Here, consecutive extreme precipitation events in December 2008 are analyzed. These events occurred over a 6-day period and produced over 125 mm of precipitation at CYYT. The first manual typing method, using a backward-trajectory analysis, results in both events being classified as “southwest,” which were previously defined as the majority of the backward trajectories originating in the Gulf of Mexico. The second method of manual synoptic typing finds that the first event is classified as a “cyclone,” while the second is a “frontal” event. A synoptic analysis of both events is conducted, highlighting important dynamic and thermodynamic structures. The first event was characterized by strong quasigeostrophic forcing for ascent in a weakly stable atmosphere in association with a rapidly intensifying extratropical cyclone off the coast of North America and transient high values of subtropical moisture. The second event was characterized by primarily frontogenetical forcing for ascent in a weakly stable atmosphere in the presence of quasi-stationary high values of subtropical moisture, in association with a northeast–southwest-oriented baroclinic zone situated near CYYT. In sum, the synoptic structures responsible for the two events highlight rather disparate means to produce an extreme precipitation event at CYYT.

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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

Abstract

Quantitative precipitation forecasting (QPF) continues to be a significant challenge in operational forecasting, particularly in regions susceptible to extreme precipitation events. St. John’s, Newfoundland, Canada (CYYT), is affected frequently by such events, particularly in the cool season (October–April).

The 50 median events in the extreme (>33.78 mm during a 48-h period) precipitation event category are selected for further analysis. A manual synoptic typing is performed on these 50 events, using two separate methodologies to partition events. The first method utilizes a Lagrangian backward air parcel trajectory analysis and the second method utilizes the evolution of dynamically relevant variables, including 1000–700-hPa horizontal temperature advection, 1000–700-hPa (vector) geostrophic frontogenesis, and 700–400-hPa absolute vorticity advection.

Utilizing the first partitioning method, it is found that south cases are characterized by a strong anticyclone downstream of St. John’s, southwest events are synoptically similar to the overall extreme composite and are marked by a strong cyclone that develops in the Gulf of Mexico, while west events are characterized by a weak Alberta clipper system that intensifies rapidly upon reaching the Atlantic Ocean. The second partitioning method suggests that while cyclone events are dominated by the presence of a rapidly developing cyclone moving northeastward toward St. John’s, frontal events are characterized by the presence of a strong downstream anticyclone and deformation zone at St. John’s.

It is the hope of the authors that the unique methodology and results of the synoptic typing in this paper will aid forecasters in identifying certain characteristics of future precipitation events at St. John’s and similar stations.

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