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Copyright of Atmosphere -- Ocean (Taylor & Francis Ltd) is the property of Taylor & Francis Ltd and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Atmosphere -- Ocean (Taylor & Francis Ltd) is the property of Taylor & Francis Ltd and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The 0 ∘ C temperature threshold is critical for many meteorological and hydrological processes driven by melting and freezing in the atmosphere, surface, and sub-surface and by the associated precipitation varying between rain, freezing rain, wet snow, and snow. This threshold is especially important in cold regions such as Canada, because it is linked with freeze–thaw, snowmelt, and permafrost. This study develops a Canada-wide perspective on near-0 ∘ C conditions using hourly surface temperature and precipitation type observations from 92 climate stations for the period from 1981 to 2011. In addition, nine stations from various climatic regions are selected for further analysis. Near-0 ∘ C conditions are defined as periods when the surface temperature is between -2 and 2 ∘ C. Near-0 ∘ C conditions occur often across all regions of the country, although the annual number of days and hours and the duration of these events varies dramatically. Various types of precipitation (e.g., rain, freezing rain, wet snow, and ice pellets) sometimes occur with these temperatures. Near-0 ∘ C conditions and the reported precipitation type occurrences tend to be higher in Atlantic Canada, although high values also occur in other regions. Trends of most temperature-based and precipitation-based indicators show little or no change despite a systematic warming in annual surface temperatures over Canada. Over the annual cycle, near-0 ∘ C temperatures and precipitation often exhibit a pattern: short durations occur around summer, driven by the diurnal cycle, and a tendency toward longer durations around winter, associated with storms. There is also a tendency for near-0 ∘ C surface temperatures to occur more often than expected relative to other temperature windows at some stations due, at least in part, to diabatic cooling and heating that take place with melting and freezing, respectively, in the atmosphere and at the surface. [ABSTRACT FROM AUTHOR]

The interior of western Canada, up to and including the Arctic, has experienced rapid change in its climate, hydrology, cryosphere, and ecosystems, and this is expected to continue. Although there is general consensus that warming will occur in the future, many critical issues remain. In this first of two articles, attention is placed on atmospheric-related issues that range from large scales down to individual precipitation events. Each of these is considered in terms of expected change organized by season and utilizing mainly "business-as-usual" climate scenario information. Large-scale atmospheric circulations affecting this region are projected to shift differently in each season, with conditions that are conducive to the development of hydroclimate extremes in the domain becoming substantially more intense and frequent after the mid-century. When coupled with warming temperatures, changes in the large-scale atmospheric drivers lead to enhancements of numerous water-related and temperature-related extremes. These include winter snowstorms, freezing rain, drought, forest fires, as well as atmospheric forcing of spring floods, although not necessarily summer convection. Collective insights of these atmospheric findings are summarized in a consistent, connected physical framework. [ABSTRACT FROM AUTHOR]

The 0 °C temperature threshold is critical to many meteorological and hydrological processes driven by melting and freezing in the atmosphere, surface and sub-surface and by the associated precipitation varying between rain, freezing rain, wet snow and snow. This threshold, linked with freeze-thaw, is especially important in cold regions such as Canada. This study develops a Canada-wide perspective on near 0 °C conditions with a particular focus on the occurrence of its associated precipitation. Since this analysis requires hourly values of surface temperature and precipitation type observations, it was limited to 92 stations over the 1981–2011 period. In addition, nine stations representative of various climatic regions are selected for further analysis. Near 0 °C conditions are defined as periods when the surface temperature is between −2 °C and 2 °C. Near 0 °C conditions occur often across all regions of the country although the annual number of days and hours and the duration of these events varies dramatically. Various forms of precipitation (including rain, freezing rain, wet snow and ice pellets) are sometimes linked with these temperatures with highest fractions tending to occur in Atlantic Canada. Trends of most temperature-based and precipitation-based indicators show little or no change despite a systematic warming in annual temperatures. Over the annual cycle, near 0 °C temperatures and precipitation often exhibit a pattern with short durations near summer driven by the diurnal cycle, while longer durations tend to occur more towards winter associated with storms. There is also a tendency for near 0 °C temperatures to occur more often than expected relative to other temperature windows; due at least in part to diabatic cooling and heating occurring with melting and freezing, respectively, in the atmosphere and at the surface. [ABSTRACT FROM AUTHOR]

Copyright of Atmosphere -- Ocean (Taylor & Francis Ltd) is the property of Taylor & Francis Ltd and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The Interior of Western Canada, up to and including the Arctic, has experienced rapid change in its climate, hydrology, cryosphere and ecosystems and this is expected to continue. Although there is general consensus that warming will occur in the future, many critical issues remain. In this first of two articles, attention is placed on atmospheric-related issues that range from large scales down to individual precipitation events. Each of these is considered in terms of expected change organized by season and utilizing climate scenario information as well as thermodynamically-driven future climatic forcing simulations. Large scale atmospheric circulations affecting this region are generally projected to become stronger in each season and, coupled with warming temperatures, lead to enhancements of numerous water-related and temperature-related extremes. These include winter snowstorms, freezing rain, drought as well as atmospheric forcing of spring floods although not necessarily summer convection. Collective insights of these atmospheric findings are summarized in a consistent, connected physical framework. [ABSTRACT FROM AUTHOR]

Freezing precipitation and ice pellet events on the Canadian Prairies and Arctic territories of Canada often lead to major disruptions to air and ground transportation, damage power grids and prevent arctic caribou and other animals from accessing the plants and lichen they depend on for survival. In a warming climate, these hazards and associated impacts will continue to happen, although their spatial and temporal characteristics may vary. In order to address these issues, the occurrence of freezing rain, freezing drizzle, and ice pellets from 1964 to 2005 is examined using hourly weather observations at 27 manned 24 h weather stations across the different climatic regions of the Prairie Provinces and Arctic Territories of Canada. Because of the enormous size of the area and its diverse climatic regions, many temporal and spatial differences in freezing precipitation and ice pellet characteristics occur. The 12 most widespread freezing rain events over the study area are associated with only two atmospheric patterns with one linked to strong warm advection between low and high pressure centres and the other pattern associated with chinooks occurring east of the Rocky Mountains. Given the annual patterns of freezing rain occurrence found in this study, it is proposed that a maximum of five regimes exist and three occur within the Prairies and Arctic. [ABSTRACT FROM AUTHOR]

Lightning is a major cause of wildland fires in Canada. During an average year in the province of Alberta, 330 000 cloud-to-ground lightning strikes occur. These strikes are responsible for igniting 45% of reported wildfires (,450 fires) and 71% of area burned (,105 000 ha). Lightning-caused wildland fires in remote areas have large suppression costs and a greater chance of escaping initial attack when compared with human-caused fires, which are often located close to infrastructure and suppression resources. In this study, geographic and temporal covariates were paired with meteorological reanalysis and radiosonde observations to generate a series of 6-h and 24-h lightning prediction models valid from April to October. These models, based on cloud-to-ground lightning from the Canadian Lightning Detection Network, were developed and validated for the province of Alberta, Canada. The ensemble forecasts produced from these models were most accurate in the Rocky Mountain and Foothills Natural Regions, achieving hits rates of 85%. The Showalter index, latitude, elevation, longitude, Julian day and convective available potential energy were found to be highly important predictors. Random forest classification is introduced as a viable modelling method to generate lightning forecasts. [ABSTRACT FROM AUTHOR]

We summarize the temporal and spatial characteristics of polarity, multiplicity and first-stroke peak current of approximately 23.5 million cloud-to-ground (CG) lightning flashes detected by the Canadian Lightning Detection Network for the period 1999-2008. Regional differences in these parameters reflect the complex nature and structure of thunderstorms across the country.The annual mean percentage of positive CG flashes was found to be lowest in eastern Canada (11%) and highest in northern Canada (17%). The data do not show any trends over the years in any region. The monthly distribution of positive CG flashes reflects a strong seasonality in all regions, with higher values in winter than in summer. Areas of more than 25% positive flashes are observed along the west coast of British Columbia, in Yukon extending southeast into central British Columbia, in southern Manitoba, northern Quebec, Newfoundland and off the coast of Nova Scotia. The percentages of single-stroke positive and negative flashes for northern (western, eastern) Canada are 93% and 63%, (89% and 48%, 90% and 50%), respectively. The monthly distribution of multiplicity for negative CG flashes peaks between 2 and 2.4 strokes per flash in the summer and early fall in all regions. The multiplicity of positive flashes (slightly higher than 1 stroke per flash) shows little variation throughout the year in all regions. The annual variation of median negative and positive first-stroke peak currents reflects a latitudinal dependence over the past decade. The lowest values for each polarity are observed in southern Canada and the highest values occur in the North. The data do not show any trends in peak currents over the years in the eastern or western regions of Canada. The monthly median first-stroke peak currents for both polarities are strongest in winter and reach a minimum during summer in all regions. Large current flashes ≥⃒100 kA are usually detected in summer and comprise less than 1% of the average annual CG flashes detected in Canada. Large current flashes with stroke multiplicity ≥⃒10 are usually associated with negative polarity. These CG flashes are mostly detected in western Canada.[Traduit par la rédaction] Nous résumons les caractéristiques temporelles et spatiales de polarité, de multiplicité et de courant de pointe de la première décharge d'approximativement 23,5 millions d'éclairs nuage-sol détectés par le Réseau canadien de détection de la foudre (RCDF) entre 1999 et 2008. Les différences régionales dans ces paramètres reflètent la nature et la structure complexes des orages se produisant au pays. Nous avons trouvé que le pourcentage annuel moyen d'éclairs nuage-sol positifs affiche une valeur minimale dans l'est du Canada (11 %) et une valeur maximale dans le nord du Canada (17 %). Les données ne montrent aucune tendance au fil des années dans quelque région que ce soit. La distribution mensuelle des éclairs nuage-sol positifs présente une forte saisonnalité dans toutes les régions, les valeurs élevées s'observant davantage en hiver qu'en été. On observe des zones dans lesquelles plus de 25 % des éclairs sont positifs le long de la côte ouest de la Colombie Britannique, au Yukon en s'étendant vers le sud-est jusque dans le centre de la Colombie Britannique, dans le sud du Manitoba, dans le nord du Québec, à Terre Neuve et au large de la côte de la Nouvelle Écosse. Les pourcentages d'éclairs positifs et négatifs à décharge unique pour le nord (l'ouest, l'est) du Canada sont 93 % et 63 % (89 % et 48 %, 90 % et 50 %), respectivement. La distribution mensuelle de la multiplicité pour les éclairs négatifs nuage-sol touche un sommet entre 2 et 2,4 décharges par éclair en été et au début de l'automne dans toutes les régions. La multiplicité des éclairs positifs (légèrement supérieure à 1 décharge par éclair) affiche peu de variation durant l'année dans toutes les régions.La variation annuelle des courants de pointe médians des premières décharges négatives et positives révèle une dépendance par rapport à la latitude au cours de la dernière décennie. Les valeurs les plus basses pour chaque polarité s'observent dans le sud du Canada et les valeurs les plus élevées, dans le nord. En ce qui concerne les courants de pointe, les données ne montrent aucune tendance au fil des années dans les régions de l'est ou de l'ouest du Canada. Les courants de pointe médians mensuels des premières décharges pour les deux polarités sont plus forts en hiver et atteignent un minimum en été dans toutes les régions. Les éclairs de courant élevé (≥⃒ 100 kA) sont habituellement détectés en été et comptent pour moins de 1 % du nombre annuel moyen d'éclairs nuage-sol détectés au Canada. Les éclairs de courant élevé ayant une multiplicité de décharges ≥⃒ 10 sont généralement associés à une polarité négative. Ces éclairs nuage-sol sont principalement détectés dans l'ouest du Canada. [ABSTRACT FROM AUTHOR]

Flash density and occurrence features for more than 23.5 million cloud-to-ground (CG) lightning flashes detected by the Canadian Lightning Detection Network (CLDN) from 1999 to 2008 are analyzed on 20 × 20 km equal area squares over Canada. This study was done to update an analysis performed in 2002 with just three years of data. Flashes were detected throughout the year, and distinct geographic differences in flash density and lightning occurrence were observed. The shape and locations of large scale patterns of lightning occurrence remained almost the same, although some details were different. Flash density maxima occurred at the same locations as found previously: the Swan Hills and Foothills of Alberta, southeastern Saskatchewan, southwestern Manitoba and southwestern Ontario. A region of greater lightning occurrence but relatively low flash density south of Nova Scotia occurred at the same location as reported previously. New areas of higher flash density occurred along the US border with northwestern Ontario and southern Quebec. These appear to be northward extensions of higher flash density seen in the previous study. The greatest average CG flash density was 2.8 flash km-2 y-1 in southwestern Ontario, where the greatest single-year flash density (10.3 flash km-2 y-1) also occurred. Prominent flash density minima occurred east of the Continental Divide in Alberta and over the Niagara Escarpment in southern Ontario.Lightning activity is seen to be highly influenced by the length of the season, proximity to cold water bodies and elevation. The diurnal heating and cooling cycle exerted the main control over lightning occurrence over most land areas; however, storm translation and transient dynamic features complicated the time pattern of lightning production. A large portion of the southern Prairie Provinces experienced more than 50% of flashes between 22:30 and 10:30 local solar time. The duration of lightning over a 20 × 20 km square at most locations in Canada is 5-10 h y-1, although the duration exceeded 15 h y-1 over extreme southwestern Ontario. Lightning occurred on 15-30 days each year, on average, over most of the interior of the country. The greatest number of days with lightning in a single year was 47 in the Alberta foothills and 50 in southwestern Ontario. Beginning and ending dates of the lightning season show that the season length decreases from north to south; however, there are considerable east-west differences between regions. The season is nearly year-round in the Pacific coastal region, southern Nova Scotia, southern Newfoundland and offshore.[Traduit par la rédaction] Nous analysons les caractéristiques de densité et d'occurrence de la foudre à partir de plus de 23,5 millions d'éclairs nuage-sol détectés par le Réseau canadien de détection de la foudre (RCDF) entre 1999 et 2008 dans des cellules de surface uniforme de 20 km de côté au Canada. Cette étude a été réalisée pour mettre à jour une analyse effectuée en 2002 avec seulement trois années de données. Les éclairs ont été détectés tout au long de l'année et nous avons observé des variations liées à la géographie dans la densité des éclairs et l'occurrence de la foudre. La forme des configurations de foudre à grande échelle et les endroits où ces configurations se sont produites sont demeurés à peu près les mêmes, sauf pour quelques détails. Les maximums de densité d'éclairs se sont produits aux mêmes endroits que ceux trouvés précédemment: les collines Swan et les contreforts des Rocheuses en Alberta, le sud-est de la Saskatchewan, le sud-ouest du Manitoba et le Sud-ouest de l'Ontario.Nous avons observé une région de plus grande occurrence de foudre mais de densité d'éclairs relativement faible au sud de la Nouvelle Écosse, au même endroit que là où elle avait été observée auparavant. De nouvelles zones de densité d'éclairs plus forte s'observent le long de la frontière séparant les États Unis du nord-ouest de l'Ontario et du sud du Québec. Ces zones semblent être des extensions vers le nord de zones de densité d'éclairs plus forte observées dans l'étude précédente. La densité moyenne la plus élevée d'éclairs nuage-sol était de 2,8 éclairs km-2 a-1 dans le Sud-ouest de l'Ontario, où l'on observe aussi la densité d'éclairs la plus élevée pour une année donnée (10,3 éclairs km-2 a-1). Des minimums marqués de densité d'éclairs se sont produits à l'est de la ligne continentale de partage des eaux en Alberta et sur l'escarpement de Niagara dans le sud de l'Ontario. Nous constatons que l'activité de la foudre est fortement influencée par la durée de la saison, la proximité de masses d'eau froide et l'élévation. Le cycle de réchauffement et de refroidissement journalier a été le facteur le plus déterminant dans l'occurrence de la foudre dans la majeure partie des régions continentales; cependant, le déplacement des perturbations et les caractéristiques dynamiques transitoires ont compliqué le calendrier de la production de foudre. Dans une grande partie du sud des provinces des Prairies, plus de 50 % des éclairs se sont produits entre 22 h 30 et 10 h 30, temps solaire local. La durée des éclairs dans une cellule de 20 km de côté dans la majorité des endroits au Canada est de 5 à 10 heures par année, bien que cette durée ait excédé 15 heures par année dans l'extrême Sud ouest de l'Ontario. À chaque année, la foudre s'est produite entre 15 et 30 jours, en moyenne, dans la majeure partie de l'intérieur du pays. Le plus grand nombre de jours avec foudre au cours d'une année donnée était de 47 dans les contreforts albertains des Rocheuses et de 50 dans le Sud-ouest de l'Ontario. Les dates de commencement et de fin de la saison de la foudre montrent que la durée de la saison diminue en allant du nord au sud; cependant, il y a des différences est-ouest condidérables entre les régions. La saison dure presque toute l'année dans la région de la côte du Pacifique, dans le sud de la Nouvelle Écosse, dans le sud de Terre Neuve et au large des côtes. [ABSTRACT FROM AUTHOR]

The article evaluates the present and future climate in the Mackenzie Delta and near-shore Beaufort Sea region of Canada. It notes the integration of the output from seven international Global Climate Models (GCMs) to measure 18 future (2010-2039) temperature and precipitation projections over the area. It also offers insights in addressing climate-change impact assessments for the Arctic regions in the future.

Copyright of Atmosphere - Ocean (Canadian Meteorological & Oceanographic Society) is the property of Canadian Meteorological & Oceanographic Society and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Abstract: A major freezing rain storm causing catastrophic losses occurred in early January 1998 over eastern Canada and the northeastern United States. The various types of precipitation and associated flow and precipitation features of this storm are discussed with a particular emphasis on the effects due to precipitation phase changes in the Montréal area. Hourly surface observations, Doppler radar data and model reanalysis information are used to describe the hydrometeor phase and production. There were two periods of significant ice accumulation at the ground. They were characterized by more organized precipitation compared to other times but there were also distinct differences between the two periods in terms of wind fields. This hydrometeor development was linked in part to the flow fields in the St. Lawrence River Valley and its topographic features. The results from a time dependent microphysical model suggest that there was considerable horizontal temperature advection that balanced the warming/cooling of the atmosphere due to precipitation phase changes aloft and at the surface. Overall, local-scale factors significantly affected the nature of this major event. [Copyright &y& Elsevier]