About climate trends and variations bulletin
Since the 2010-2011 Winter, the Climate Trends and Variations Bulletin (CTVB) uses gridded temperature and precipitation data product CANGRD to produce maps, graphs, and ranked summary tables in CTVB reporting. The following is a brief description of the data and procedures used in the CTVB reports:
- The CANGRD is based on the Adjusted and Homogenized Canadian Climate Data (AHCCD) for historical climate observations and the near real-time data in the national climate archives for the current year.
- The seasonal and annual mean daily maximum and minimum temperature departures are computed at each observing station and for each year by subtracting the relevant baseline average (defined as average over 1961-1990 reference period) from the relevant seasonal and annual values. The seasonal and annual total precipitation departures are computed similarly. They are then normalized by dividing by the mean reference period and expressed in percentage to produce normalized precipitation departures.
- The station values of temperature departures from the baseline average or the normalized precipitation departures from unevenly distributed stations are interpolated to the evenly spaced (50 km) grid boxes using the Gandin’s Optimal Interpolation (OI). Values for grid boxes over large bodies of water such as Hudson Bay are excluded. The grid box values of mean temperature departures are the average of those for daily minimum and maximum. The grid box values are presented as coloured maps in the CTVB.
- The national and regional time series of temperature departures or normalized precipitation departures are computed by averaging grid box values within the boundaries of the entire country and the climate regions, respectively. They are presented in the ranked summary tables.
- Linear trends in temperature departures are estimated based on the Kendall-tau method. A 9-year running mean is applied to normalized precipitation departures to show changes in precipitation. The national averages and fitted lines are plotted as graphs of annual time series.
The CTVB released before the 2010-2011 Winter were produced differently:
- There were only 131 stations for both temperature and precipitation.
- The mean reference period was computed from 1951-1980 data.
- Station data were quality controlled to remove obvious errors, but were not homogenized.
- Regional series were simple averages of data from available stations within the region. They were then area-weighted to produce national time series.
- Temperature trends were estimated using the linear least-square method.
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