Trends in Canada's climate are analyzed using recently updated data to provide a comprehensive view of climate variability and long-term changes over the period of instrumental record. Trends in surface air tem perature, precipitation, snow cover, and streamflow indices are examined along with the potential impact of lowfrequency variability related to large-scale atmospheric and oceanic oscillations on these trends. The results show that temperature has increased significantly in most regions of Canada over the period 1948-2012, with the largest warming occurring in winter and spring. Precipitation has also increased, especially in the north.Changes in other climate and hydroclimatic variables, including a decrease in the amount of precipitation falling as snow in the south, fewer days with snow cover, an earlier start of the spring high-flow season, and an increase in April streamflow, are consistent with the observed warming and precipitation trends. For the period 1900-2012, there are sufficient temperature and precipitation data for trend analysis for southern Canada (south of 60°N) only. During this period, temperature has increased significantly across the region, precipitation has increased, and the amount of precipitation falling as snow has decreased in many areas south of 55°N. The results also show that modes of low-frequency variability modulate the spatial distribution and strength of the trends; however, they alone cannot explain the observed long-term trends in these climate variables.

Abstract. Although precipitation has been measured for many centuries, precipitation measurements are still beset with significant inaccuracies. Solid precipitation is particularly difficult to measure accurately, and wintertime precipitation measurement biases between different observing networks or different regions can exceed 100 %. Using precipitation gauge results from the World Meteorological Organization Solid Precipitation Intercomparison Experiment (WMO-SPICE), errors in precipitation measurement caused by gauge uncertainty, spatial variability in precipitation, hydrometeor type, crystal habit, and wind were quantified. The methods used to calculate gauge catch efficiency and correct known biases are described. Adjustments, in the form of "transfer functions" that describe catch efficiency as a function of air temperature and wind speed, were derived using measurements from eight separate WMO-SPICE sites for both unshielded and single-Alter-shielded precipitationweighing gauges. For the unshielded gauges, the average undercatch for all eight sites was 0.50 mm h −1 (34 %), and for the single-Alter-shielded gauges it was 0.35 mm h −1 (24 %). After adjustment, the mean bias for both the unshielded and single-Alter measurements was within 0.03 mm h −1 (2 %) of zero. The use of multiple sites to derive such adjustments makes these results unique and more broadly applicable to other sites with various climatic conditions. In addition, errors associated with the use of a single transfer function to correct gauge undercatch at multiple sites were estimated.

This study examines the trends and variations in several indices of daily and extreme temperature and precipitation in Canada for the periods 1950-2003 and 1900-2003 respectively. The indices are based on homogenized daily temperature and adjusted daily precipitation measurements which are special datasets that include adjustments for site relocation, changes in observing programs and corrections for known instrument changes or measurement program deficiencies. For 1950-2003, the analysis of the temperature indices indicates the occurrence of fewer cold nights, cold days and frost days, and conversely more warm nights, warm days and summer days across the country. The results are generally similar for 1900-2003 but they also include a decrease in the diurnal temperature range in southern Canada and a decrease in the standard deviation of the daily mean temperatures for many stations in western Canada. The analysis of the precipitation indices for 1950-2003 reveals more days with precipitation, a decrease in daily intensity and a decrease in the maximum number of consecutive dry days. The annual total snowfall significantly decreased in the south and increased in the north and northeast during the second half of the twentieth century. The results are generally similar for 1900-2003. The national series for the century shows an increase in annual snowfall from 1900 to the 1970s followed by a considerable decrease until the 1980s which also corresponds to a pronounced downward trend in the frequency of frost days. No consistent changes were found in most of the indices of extreme precipitation for both periods. RESUMÉ [Traduit par la rédaction] Cette étude examine les tendances et les variations dans plusieurs indices de température et de précipitation quotidiennes et extrêmes au Canada durant les périodes 1950-2003 et 1900-2003, respectivement. Ces indices sont basés sur des mesures de températures quotidiennes homogénéisées et de précipitations quotidiennes ajustées, qui sont des données spéciales comportant des ajustements pour tenir compte de la relocalisation de sites, de changements dans les programmes d'observation et de corrections relatives à des changements d'instruments ou à des lacunes des programmes de mesure connues. Pour la période 1950-2003, l'analyse des indices de température indique une fréquence plus faible de nuits froides, de jours froids et de jours de gel et, réciproquement, plus de nuits chaudes, de jours chauds et de jours d'été dans le pays. Les résultats sont à peu près semblables pour la période 1900-2003 mais ils incluent également une diminution dans l'amplitude quotidienne de la température dans le sud du Canada et une diminution dans l'écart type des températures quotidiennes moyennes pour plusieurs stations de l'ouest du Canada. L'analyse des indices de précipitations pour la période 1950-2003 révèle un plus grand nombre de jours avec précipitation, une diminution dans l'intensité quotidienne et une diminution dans le nombre maximum de jours secs consécutifs. La chute de neige total...

A second generation adjusted precipitation daily dataset has been prepared for trend analysis in Canada. Daily rainfall and snowfall amounts have been adjusted for 464 stations for known measurement issues such as wind undercatch, evaporation and wetting losses for each type of rain-gauge, snow water equivalent from ruler measurements, trace observations and accumulated amounts from several days. Observations from nearby stations were sometimes combined to create time series that are longer; hence, making them more useful for trend studies. In this new version, daily adjustments are an improvement over the previous version because they are derived from an extended dataset and enhanced metadata knowledge. Datasets were updated to cover recent years, including 2009. The impact of the adjustments on rainfall and snowfall total amounts and trends was examined in detail. As a result of adjustments, total rainfall amounts have increased by 5 to 10% in southern Canada and by more than 20% in the Canadian Arctic, compared to the original observations, while the effect of the adjustments on snowfall were larger and more variable throughout the country. The slope of the rain trend lines decreased as a result of the larger correction applied to the older rain-gauges while the slope of the snow trend lines increased, mainly along the west coast and in the Arctic. Finally, annual and seasonal rainfall and snowfall trends based on the adjusted series were computed for 1950-2009 and 1900-2009. Overall, rainfall has increased across the country while a mix of non-significant increasing and decreasing trends was found during the summer in the Canadian Prairies. Snowfall has increased mainly in the north while a significant decrease was observed in the southwestern part of the country for 1950-2009. RÉSUMÉ [Traduit par la rédaction] Un ensemble de données quotidiennes de précipitations ajustées de deuxième génération a été préparé pour l'analyse des tendances au Canada. Les hauteurs quotidiennes des chutes de pluie et des chutes de neige ont été ajustées pour 464 stations en fonction de problèmes connus comme la sous-capture due au vent, l'évaporation et les pertes par mouillage pour chaque type de pluviomètre, l'équivalent en eau de la neige selon des mesures avec une règle, les observations de traces et les hauteurs accumulées de plusieurs jours. Les observations de stations situées à proximité ont parfois été combinées pour créer des séries chronologiques plus longues et donc plus utiles pour les études de tendance. Dans cette nouvelle version, les ajustements quotidiens constituent une amélioration par rapport à la version précédente parce qu'ils sont dérivés d'un ensemble de données étendu et d'une meilleure connaissance des métadonnées. Les ensembles de données ont été mis à jour pour inclure les années récentes, y compris 2009. L'effet des ajustements sur les hauteurs totales et les tendances des chutes de pluie et des chutes de neige a été examiné en détail. En raison des ajustements, les hauteurs totales de plui...

The goal of this project was to develop adjustment procedures to use daily resolution data to generate high quality time series of precipitation and to perform regional trend analyses on the resulting datasets. A total of 69 locations, most with data covering the period 1900-96 were used. Data availability in much of the Canadian Arctic was restricted to 1948-96. By using daily data, improved corrections to precipitation data