Spatial and temporal characteristics of heavy precipitation events over Canada

Spatial and temporal characteristics of heavy precipitation events over Canada (excluding the high Arctic) are examined for the period 1900-98. Using daily precipitation data, heavy rainfall and snowfall events are defined at each station by identifying a threshold value which is exceeded by an average of three events per year. Annual and seasonal time series of heavy event frequency are then obtained by counting the number of exceedances per year. Characteristics of the intensity of heavy precipitation events are investigated examining the 90th percentiles of daily recipitation, the annual maximum daily value, and the 20-year return values. It was found that decadal variability is the dominant feature in both the frequency and the intensity of extreme precipitation events over the country. For the country as a whole, there appear to be no identifiable trends in extreme precipitation (either frequency or intensity), during the last century. The observed increase in precipitation totals in the 20th century was mainly due to increase in the number of small to moderate events.

Stations with coherent temporal variability in the frequency of heavy precipitation events are grouped by cluster analysis and examined on a regional basis. Results show stations belonging to the same group are generally located in a continuous region, indicating that the temporal distribution of the number of events is spatially coherent. It is also found that heavy snowfall events are more spatially coherent than heavy rainfall events. Indices representing temporal variations of regional heavy precipitation display strong interdecadal variability with limited evidence of long term trends. They vary markedly depending on the precipitation type, season and region. Spring heavy rainfall events over eastern Canada have shown an increasing trend superimposed on the strong decadal variability. However, heavy rainfall events in other seasons/regions are generally not associated with any trends or decadal variability. The number of heavy snowfall events in southern Canada shows an upward trend from the beginning of the 20th century until the late 1950s to 1970s, followed by a downward trend to the present. In the last 50 years, heavy snowfall events in northern Canada have been increasing with marked decadal variation.

The majority of stations have a significant positive correlation between the total amount of snowfall contributed by heavy events versus that contributed from non-heavy events. The relationship is strongest over western Canada. On the other hand, relatively few (<20%) stations had a significant correlation between total rainfall in heavy events and non-heavy events. These results suggest that the amount of precipitation falling in heavy and non-heavy events increase or decrease coherently for snow, but not for rain.