Homogenized daily temperatures for trend analyses in extremes over Canada

A database of long-term and homogenized monthly temperatures has been created for climate change analyses over Canada. A technique based on regression models was developed to identify "inhomogeneities" in individual annual maximum and minimum temperature time series using surrounding stations. The inhomogeneities were often step changes caused by factors such as changes in the station exposure, location, instrumentation, and observing program. Using this technique, it was possible to divide the tested series into homogeneous segments at the identified steps. Monthly adjustment factors, derived from the regression models, were applied to bring each segment into agreement with the most recent homogeneous part of the series.

Recent work has involved the adjustments to daily maximum and minimum temperatures. Reliable daily datasets are essential for the accurate description of spatial and temporal characteristics of extreme temperature related variables. Adjustment of daily temperatures has raised several concerns mainly due to the large variability in day to day temperature, particularly at high latitude stations. An approach was investigated using linear interpolation between "target values" derived from the monthly adjustment factors. This approach has two main advantages. First, it doesn't require the use of a daily reference series (which is often difficult to produce), and second, the average of the daily adjustments over a particular month is equal to the monthly adjustment factor. Therefore, homogenized daily temperatures compatible with the homogenized monthly datasets can be obtained.

To assess the impact of these daily adjustments on extreme temperatures, trends in the percentage of days above (below) the 95th (5th) percentile have been computed for maximum and minimum daily temperatures both before and after adjustment. Seasonal trend maps were then examined for various periods of time. Preliminary results show that the inhomogeneity having the greatest impact on the trends during the past five decades is associated with the 1961 change to observing window at principal stations. An overall assessment of Canadian temperature extremes indicates no consistent trends in the number of extreme hot summer days, but significant trends toward fewer days with extreme low minimum temperatures during the last century.