An Overview of the Second Generation Adjusted Daily Precipitation Dataset for Trend Analysis in Canada

A Second Generation Adjusted Daily Precipitation 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, and hence more useful for trend studies. In this new version, daily adjustments are improved from the previous since they are derived from an extended dataset and enhanced metadata knowledge. Datasets were updated to cover recent years including 2009. The impact of adjustments on rainfall and snowfall total amounts and trends was examined in detail. Compared to the original observations, the rain total amount have increased by 5 to 10% in Southern Canada and more than 20% in the Canadian Arctic as a result of adjustments, while the effect of snow adjustments displayed bigger range and variability throughout the country. The slope of rain trend lines decreased as a result of higher correction applied to the older rain gauge, while the slope of 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 significant decrease was observed in the south-western part of the country for 1950-2009.