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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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Gary M. Lackmann, John R. Gyakum, and Robert Benoit

Abstract

Wintertime precipitation events in the Mackenzie River basin (MRB) play an important role in the hydrology of the region because they contribute substantially to water storage prior to the spring runoff maximum. The Mesoscale Compressible Community (MC2) Model is used to simulate a representative wintertime MRB precipitation event. The MC2 simulation, gridded analyses, and raw observations are used to (i) document meteorological conditions associated with the precipitation event, (ii) assess the ability of the model to reproduce the precipitation event and antecedent large-scale moisture transport, and (iii) identify which planetary- and synoptic-scale features are responsible for the observed moisture transport using piecewise quasigeostrophic potential vorticity (QGPV) inversion.

Precipitation in the MRB develops north of an intense frontal boundary as a southwesterly flow of moisture originating over the Pacific Ocean is lifted over cold, dense arctic air near the surface. A lee cyclone forms along the frontal boundary as an upper-tropospheric disturbance approaches from the west. The MC2 model adequately represents the lee cyclone formation, the observed precipitation event, and large-scale moisture transport, as determined through comparison of the model output with analyses and raw observations. A plume of moisture advances northeastward from the subtropical Pacific Ocean toward the MRB during the 24–36-h period prior to the precipitation event. Piecewise QGPV inversion demonstrates that the background climatological flow and a cyclonic QGPV anomaly located over the eastern Pacific Ocean are associated with the initial moisture transport into the Gulf of Alaska. Later, a second cyclonic QGPV anomaly centered over the Gulf of Alaska is associated with moisture transport from over the Gulf of Alaska into the MRB. The moisture flux is generally largest in the lower troposphere owing to the larger concentration of water vapor there. The Rocky Mountains, located west of the MRB, block much of the eastward moisture transport below the 800-hPa level. Moisture transport in the layer between 700 and 800 hPa is therefore crucial for MRB precipitation in situations where the moisture originates over the Pacific. QGPV inversions based on a vertically partitioned QGPV field indicate that QGPV anomalies located below the dynamic tropopause are associated with larger moisture transport at the 700-hPa level than their tropopause-based counterparts.

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Kevin A. Bowley, John R. Gyakum, and Eyad H. Atallah

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Zonal available potential energy A Z measures the magnitude of meridional temperature gradients and static stability of a domain. Here, the role of Northern Hemisphere dynamic tropopause (2.0-PVU surface) Rossby wave breaking (RWB) in supporting an environment facilitating buildups of A Z on synoptic time scales (3–10 days) is examined. RWB occurs when the phase speed of a Rossby wave slows to the advective speed of the atmosphere, resulting in a cyclonic or anticyclonic RWB event (CWB and AWB, respectively). These events have robust dynamic and thermodynamic feedbacks through the depth of the troposphere that can modulate A Z. Significant synoptic-scale buildups in A Z and RWB events are identified from the National Centers for Environmental Prediction Reanalysis-2 dataset from 1979 to 2011 for 20°–85°N. Anomalies in AWB and CWB are assessed seasonally for buildup periods of A Z. Positive anomalies in AWB and negative anomalies in CWB are found for most A Z buildup periods in the North Pacific and North Atlantic basins and attributed to localized poleward shifts in the jet stream. Less frequent west–east dipoles in wave breaking anomalies for each basin are attributed to elongated and contracted regional jet exit regions. Finally, an analysis of long-duration AWB events for winter A Z buildup periods to an anomalously high A Z state is performed using a quasi-Lagrangian grid-shifting technique. North Pacific AWB events are shown to diabatically intensify the North Pacific jet exit region (increasing Northern Hemisphere A Z) through latent heating equatorward of the jet exit and radiative and evaporative cooling poleward of the jet exit.

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Marco L. Carrera, John R. Gyakum, and Charles A. Lin

Abstract

The presence of orography can lead to thermally and dynamically induced mesoscale wind fields. The phenomenon of channeling refers to the tendency for the winds within a valley to blow more or less parallel to the valley axis for a variety of wind directions above ridge height. Channeling of surface winds has been observed in several regions of the world, including the upper Rhine Valley of Germany, the mountainous terrain near Basel, Switzerland, and the Tennessee and Hudson River Valleys in the United States. The St. Lawrence River valley (SLRV) is a primary topographic feature of eastern Canada, extending in a southwest–northeast direction from Lake Ontario, past Montreal (YUL) and Quebec City (YQB), and terminating in the Gulf of St. Lawrence. In this study the authors examine the long-term surface wind climatology of the SLRV and Lake Champlain Valley (LCV) as represented by hourly surface winds at Montreal, Quebec City, and Burlington, Vermont (BTV). Surface wind channeling is found to be prominent at all three locations with strong bidirectionalities that vary seasonally. To assess the importance of the various channeling mechanisms the authors compared the joint frequency distributions of surface wind directions versus 925-hPa geostrophic wind directions with those obtained from conceptual models. At YUL, downward momentum transport is important for geostrophic wind directions ranging from 240° to 340°. Pressure-driven channeling is the dominant mechanism producing northeasterly surface winds at YUL. These northeasterlies are most prominent in the winter, spring, and autumn seasons. At YQB, pressure-driven channeling is the dominant physical mechanism producing channeling of surface winds throughout all seasons. Of particular importance, both YUL and YQB exhibit countercurrents whereby the velocity component of the wind within the valley is opposite to the component above the valley. Forced channeling was found to be prominent at BTV, with evidence of diurnal thermal forcing during the summer season. Reasons for the predominance of pressure-driven channeling at YUL and YQB and forced channeling at BTV are discussed.

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Kevin A. Bowley, Eyad H. Atallah, and John R. Gyakum

Abstract

Available potential energy (APE), a measure of the energy available for conversion to kinetic energy, has been previously applied to examine changes in baroclinic instability and seasonal changes in the general circulation. Here, pathways in which the troposphere can build the reservoir of zonal available potential energy A Z on synoptic (3–10 day) time scales are explored. A climatology of A Z and its generation G Z and conversion terms are calculated from the National Centers for Environmental Prediction–Department of Energy Reanalysis 2 dataset from 1979 to 2011 for 20°–85°N. A standardized anomaly-based identification technique identifies 183 A Z buildup events, which are grouped into two event types based upon their final A Z standardized anomaly (σ) value: 1) buildup anomalous (BA) events, which exceed 1.5σ, and 2) buildup neutral (BN) events, which do not exceed 1.5σ. Increases in G Z and reductions in baroclinic conversion C A, source and sink terms for A Z, are shown to equally contribute toward increasing A Z in most seasons. A synoptic analysis of composited mass fields for winter BA events (n = 18 events) and winter BN events (n = 28 events) is performed to identify contributions to anomalously low C A and high G Z. A process of high-latitude cooling near 160°E–120°W is found for both composite event types. The cooling processes are characterized by a period of poleward moisture flux and ascent followed by an isolation of the Arctic from the midlatitude flow, resulting in enhanced G Z. Negative anomalies in C A are also diagnosed, which generally occur in regions with northerly dynamic tropopause wind anomalies and neutral to positive thickness anomalies.

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Kevin M. Grise, Seok-Woo Son, and John R. Gyakum

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Extratropical cyclones play a principal role in wintertime precipitation and severe weather over North America. On average, the greatest number of cyclones track 1) from the lee of the Rocky Mountains eastward across the Great Lakes and 2) over the Gulf Stream along the eastern coastline of North America. However, the cyclone tracks are highly variable within individual winters and between winter seasons. In this study, the authors apply a Lagrangian tracking algorithm to examine variability in extratropical cyclone tracks over North America during winter. A series of methodological criteria is used to isolate cyclone development and decay regions and to account for the elevated topography over western North America. The results confirm the signatures of four climate phenomena in the intraseasonal and interannual variability in North American cyclone tracks: the North Atlantic Oscillation (NAO), the El Niño–Southern Oscillation (ENSO), the Pacific–North American pattern (PNA), and the Madden–Julian oscillation (MJO). Similar signatures are found using Eulerian bandpass-filtered eddy variances. Variability in the number of extratropical cyclones at most locations in North America is linked to fluctuations in Rossby wave trains extending from the central tropical Pacific Ocean. Only over the far northeastern United States and northeastern Canada is cyclone variability strongly linked to the NAO. The results suggest that Pacific sector variability (ENSO, PNA, and MJO) is a key contributor to intraseasonal and interannual variability in the frequency of extratropical cyclones at most locations across North America.

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

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Freezing rain is an especially hazardous winter weather phenomenon that remains particularly challenging to forecast. Here, we identify the salient thermodynamic characteristics distinguishing long-duration (six or more hours) freezing rain events from short-duration (2–4 h) events in three regions of the United States and Canada from 1979 to 2016. In the northeastern United States and southeastern Canada, strong surface cold-air advection is not common during freezing rain events. Colder onset temperatures at the surface and in the near-surface cold layer support longer-duration events there, allowing heating mechanisms (e.g., the release of latent heat of fusion when rain freezes at the surface) to act for longer periods before the surface reaches 0°C and precipitation transitions to rain. In the south-central United States, cold air at the surface is replenished via continuous cold-air advection, reducing the necessity of cold onset surface temperatures for event persistence. Instead, longer-duration events are associated with warmer and deeper >0°C warm layers aloft and stronger advection of warm and moist air into this layer, delaying its erosion via cooling mechanisms such as melting. Finally, in the southeastern United States, colder and especially drier onset conditions in the cold layer are associated with longer-duration events, with evaporative cooling crucial to maintaining the subfreezing surface temperatures necessary for freezing rain. Through an improved understanding of the regional conditions supporting freezing rain event persistence, we hope to provide useful information to forecasters in their attempt to predict these potentially damaging events.

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

Abstract

This study focuses on errors in extreme precipitation in gridded station products incurred during the upscaling of station measurements to a grid, referred to as representativeness errors. Gridded precipitation station analyses are valuable observational data sources with a wide variety of applications, including model validation. The representativeness errors associated with two gridding methods are presented, consistent with either a point or areal average interpretation of model output, and it is shown that they differ significantly (up to 30%). An experiment is conducted to determine the errors associated with station density, through repeated gridding of station data within the United States using subsequently fewer stations. Two distinct error responses to reduced station density are found, which are attributed to differences in the spatial homogeneity of precipitation distributions. The error responses characterize the eastern and western United States, which are respectively more and less homogeneous. As the station density decreases, the influence of stations farther from the analysis point increases, and therefore, if the distributions are inhomogeneous in space, the analysis point is influenced by stations with very different precipitation distributions. Finally, ranges of potential percent representativeness errors of the median and extreme precipitation across the United States are created for high-resolution (0.25°) and low-resolution areal averaged (0.9° lat × 1.25° lon) precipitation fields. For example, the range of the representativeness errors is estimated, for annual extreme precipitation, to be from +16% to −12% in the low-resolution data, when station density is 5 stations per 0.9° lat × 1.25° lon grid box.

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

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

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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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