Discontinuities due to Joining Precipitation Station Observations in Canada

When a climatological station is relocated or is closing, it is often possible to join the climate observations of a nearby site to create a longer time series. However, joining climate observations can sometimes introduce artificial discontinuity that affects the trend. A procedure to detect discontinuities at the joining dates for precipitation station observations is described. It is based on standardized ratios between a tested station and a neighbor, and the t test is used to determine whether the means before and after the joining dates are statistically significantly different. The procedure is applied to 234 climatological stations across Canada to identify steps in rainfall and snowfall. The results indicate that joining precipitation station observations creates steps of different magnitude for rain and snow. It is concluded that about 35% of the stations need adjustment for rain whereas 58% of the stations need adjustment for snow. The magnitude of the adjustments varies from 0.75 to 1.25 for rain and from 0.65 to 1.60 for snow. The annual and seasonal trends before and after adjustments are also examined for 1930-2007. The results show that the trends computed from the adjusted data present a more consistent regional pattern than do trends computed from the unadjusted observations.