Rehabilitation and analysis of Canadian daily precipitation time series

Abstract

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, not practical with monthly data, could be implemented. For each of three rain gauge types, corrections to account for wind undercatch and evaporation were implemented. Gauge specific wetting loss corrections were applied for each rainfall event. For snowfall, ruler measurements were used throughout the time series, to minimize potential discontinuities introduced by the adoption of Nipher shielded snow gauge measurements in the mid‐1960s. Density corrections based upon coincident ruler and Nipher measurements were applied to all ruler measurements. Where necessary, records from neighbouring stations were joined employing a technique based on a simple ratio of observations. The adjustment procedures used remove systematic biases due to changes in the measurement program but do not account for inhomogeneities related to local site changes etc. It is assumed that such local changes introduce random inhomogeneities which are smoothed by combining the results from numerous stations. Work to adjust data from about 500 stations and generate monthly grids or maps is well underway but preliminary trend results were examined for this project by grouping stations by region. Regional time series of normalized anomalies are computed as the arithmetic mean of stations within the region. Annual and seasonal graphs of national and regional time series are presented. The national time series shows an increase in precipitation of 1.7% of mean/decade over 1948–95. The greatest increase is in the autumn. For the same period, Canada north of 55°N showed an increase of 2.3% of mean/decade, more than the south, but much less than the 4–5% of mean/decade suggested by Groisman and Easterling (1994). The ratio of liquid to solid precipitation for Canada has declined slightly over the 1948–95 period.