Search

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.)

A high-resolution Canadian lightning climatology is presented. Generating the high-resolution flash density climatology from a relatively short observation period (approximately 10 years) of the Canadian Lightning Detection Network can be challenging because of the natural variations in the lightning frequency. To address this, an objective methodology was developed with the intended purpose of reducing random variations while still retaining the real spatially significant local variations in the cloud-to-ground lightning flash densities. This technique is applied to the annual 1 km lightning flash density values across most of Canada (south of 60°–70°N) to generate a high-resolution lightning climatology. Lightning flash density maps for selected areas are presented that demonstrate typical patterns resulting from the optimizing methodology. This high-resolution climatology can be used to assess lightning occurrence and risk for many applications including protection measures for buildings and other structures such as electrical transmission lines, insurance purposes, as well as general climatological knowledge and public safety. An example that applies the high-resolution lightning climatology to wind turbine lightning protection is provided because it was the initial motivation for this research. [Traduit par la rédaction] Nous présentons une climatologie canadienne de la foudre à haute résolution. Produire la climatologie de densité d’éclairs à haute résolution à partir d'une période d'observation plutôt courte (approximativement 10 ans) du Réseau canadien d'observation de la foudre peut poser certains problèmes à cause des variations naturelles dans la fréquence de la foudre. C'est pourquoi nous avons mis au point une méthodologie objective qui vise à réduire les variations aléatoires tout en conservant les variations locales réelles spatialement significatives dans les densités d’éclairs nuage–sol. Nous appliquons cette technique aux valeurs annuelles de densité d’éclairs sur 1 km dans la plus grande partie du Canada (au sud de 60°–70°N) pour produire une climatologie de la foudre à haute résolution. Nous présentons des cartes de densité d’éclairs pour des endroits sélectionnés qui révèlent des configurations particulières résultant de la méthodologie d'optimisation. Cette climatologie à haute résolution peut servir à estimer la fréquence et le risque de foudre pour de nombreuses applications, y compris les mesures de protection des bâtiments ou d'autres structures comme les lignes électriques, pour les besoins des assurances, de même que pour la connaissance climatologique générale et la sécurité publique. Nous présentons un exemple qui applique la climatologie de la foudre à haute résolution à la protection des éoliennes contre la foudre, car c’était la motivation première de cette recherche. [ABSTRACT FROM AUTHOR]

Linkages between the terrestrial ecosystem and precipitation play a critical role in regulating regional weather and climate. These linkages can manifest themselves as positive or negative feedback loops, which may either favor or inhibit the triggering and intensity of thunderstorms. Although the Canadian Prairies terrestrial system has been identified as having the potential to exert a detectable influence on convective precipitation during the warm season, little work has been done in this area using in situ observations. The authors present findings from a novel study designed to explore linkages between the normalized difference vegetation index (NDVI) and lightning duration (DUR) from the Canadian Lightning Detection Network for 38 census agricultural regions (CARs) on the Canadian Prairies. Statistics Canada divides the prairie agricultural zone into CARs (polygons of varying size and shape) for the purpose of calculating agricultural statistics. Here, DUR is used as a proxy for thunderstorm activity. Statistical analyses were undertaken for 38 CARs for summers [June--August (JJA)] between 1999 and 2008. Specifically, coefficients of determination were calculated between pairs of standardized anomalies of DUR and NDVI by season and by month. Correlations were also calculated for CARs grouped by size and/or magnitude of the NDVI anomalies. The main findings are as follows: 1) JJA lightning activity is overwhelmingly below average within larger dry areas (i.e., areas with below-average NDVI); that is, the linkages between NDVI and DUR increased significantly as both the area and magnitude of the dry anomaly increased. 2) In contrast, CARs with above-average NDVI did not consistently experience above-average lightning activity, regardless of the CAR size. 3) The lower threshold for the length scale of the dry anomalies required to affect the boundary layer sufficiently to reduce lightning activity was found to be approximately 150 km (~18 000 km2). 4) The authors'' analysis suggests that the surface-convection feedback appears to be a real phenomenon, in which drought tends to perpetuate drought with respect to convective storms and associated rainfall, within the limits found in 1) and 3). [ABSTRACT FROM AUTHOR]

Cloud-to-ground (CG) lightning data have been analyzed for the years 2001--09 for North America, which includes Alaska, Canada, and the lower 48 U.S. states. Flashes recorded within the North American Lightning Detection Network (NALDN) are examined. No corrections for detection efficiency variability are made over the 9 yr of the dataset or over the large geographical area comprising North America. There were network changes in the NALDN during the 9 yr, but these changes have not been corrected for nor have the recorded data been altered in any way with the exception that all positive lightning reports with peak currents less than 15 kA have been deleted. Thus, the reader should be aware that secular changes are not just climatological in nature. All data were analyzed with a spatial resolution of 20 km. The analyses presented in this work provide a synoptic view of the interannual variability of lightning observations in North America, including the impacts of physical changes in the network during the 9 yr of study. These data complement and extend previous analyses that evaluate the U.S. NLDN during periods of upgrade. The total (negative and positive) flashes for ground flash density, the percentage of positive lightning, and the positive flash density have been analyzed. Furthermore, the negative and positive first stroke peak currents and the flash multiplicity have been examined. The highest flash densities in Canada are along the U.S.--Canadian border (1--2 flashes per square kilometer) and in the United States along the Gulf of Mexico coast from Texas through Florida (exceeding 14 flashes per square kilometer in Florida). The Gulf Stream is ''outlined'' by higher flash densities off the east coast of the United States. Maximum annual positive flash densities in Canada range primarily from 0.01 to 0.3 flashes per square kilometer, and in the United States to over 0.5 flashes per square kilometer in the Midwest and in the states of Louisiana and Mississippi. The annual percentage of positive lightning to ground varies from less than 2%% over Florida to values exceeding 25%% off the West Coast, Alaska, and the Yukon. A localized maximum in the percentage of positive lightning in the NALDN occurs in Manitoba and western Ontario, just north of North Dakota and Minnesota. When averaged over North America, first stroke negative median peak currents range from 19.8 kA in 2001 to 16.0 kA in 2009 and for all years, average 16.1 kA. First stroke positive median peak currents range from a high of 29.0 kA in 2008 and 2009 to a low of 23.3 kA in 2003 with a median of 25.7 kA for all years. There is a relatively sharp transition from low to high median negative peak currents along the Gulf and Atlantic coasts of the United States. No sharp transitions are observed for the median positive peak currents. Relatively lower positive peak currents occur throughout the southeastern United States. The highest values of mean negative multiplicity exceed 3.0 strokes per flash in the NALDN with some variation over the 9 yr. Lower values of mean negative multiplicity occur in the western United States. Positive flash mean multiplicity is slightly higher than 1.1, with the highest values of 1.7 observed in the southwestern states. As has been noted in prior research, CG lightning has significant variations from storm to storm as well as between geographical regions and/or seasons and, consequently, a single distribution for any lightning parameter, such as multiplicity or peak current, may not be sufficient to represent or describe the parameter. [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]

Statistical models valid May–September were developed to predict the probability of lightning in 3-h intervals using observations from the North American Lightning Detection Network and predictors derived from Global Environmental Multiscale (GEM) model output at the Canadian Meteorological Centre. Models were built with pooled data from the years 2000–01 using tree-structured regression. Error reduction by most models was about 0.4–0.7 of initial predictand variance. Many predictors were required to model lightning occurrence for this large area. Highest ranked overall were the Showalter index, mean sea level pressure, and troposphere precipitable water. Three-hour changes of 500-hPa geopotential height, 500–1000-hPa thickness, and MSL pressure were highly ranked in most areas. The 3-h average of most predictors was more important than the mean or maximum (minimum where appropriate). Several predictors outranked CAPE, indicating it must appear with other predictors for successful statistical lightning prediction models. Results presented herein demonstrate that tree-structured regression is a viable method for building statistical models to forecast lightning probability. Real-time forecasts in 3-h intervals to 45–48 h were made in 2003 and 2004. The 2003 verification suggests a hybrid forecast based on a mixture of maximum and mean forecast probabilities in a radius around a grid point and on monthly climatology will improve accuracy. The 2004 verification shows that the hybrid forecasts had positive skill with respect to a reference forecast and performed better than forecasts defined by either the mean or maximum probability at most times. This was achieved even though an increase of resolution and change of convective parameterization scheme were made to the GEM model in May 2004. [ABSTRACT FROM AUTHOR]

Cloud-to-ground lightning data have been analyzed for the years 1998–2000 for North America (Canada plus the contiguous United States) for all ground flashes, positive flashes, the percentage of positive lightning, peak currents for negative and positive lightning, and for negative and positive multiplicity. The authors examined a total of 88.7 million flashes divided among the three years: 31.1 million (1998), 29.5 million (1999), and 28.2 million (2000). Annual flash densities are derived from 245–424 km[sup 2] regions and are uncorrected for flash detection efficiency. The highest flash densities in Canada are along the U.S.–Canadian border (1–3 flashes km[sup -2] ), and in the United States along the Gulf of Mexico coast and Florida (exceeding 9 flashes km[sup -2] ). Maximum annual positive flash densities in Canada generally range primarily from 0.1 to 0.3 flashes km[sup -2] , and in the United States to over 0.7 flashes km[sup -2] (areas in the Midwest, the Gulf Coast, and Florida). Areas of greater than 20% positive lightning occur throughout British Columbia and the midwest United States extending into Manitoba and Ontario. High percent positive also occurs in Quebec and much of eastern Canada. The median negative peak current is 16.5 kA. The median positive peak current, with the peak currents less than 10 kA removed from the calculation, is 19.8 kA. Median positive peak currents exceed 35 kA in the Midwest from west Texas to Nebraska to the Canadian border. The area of maximum mean negative multiplicity, exceeding 2.6 strokes, occurs in western Canada from just east of the British Columbia–Alberta border to and including Saskatchewan. Mean negative multiplicity also peaks in the southeastern United States. Mean positive multiplicity is observed to have maximum values in Alberta, Saskatchewan, and in a region centered on Tennessee. The authors examined the time of maximum flash rate in North America and find it is... [ABSTRACT FROM AUTHOR]

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.)