Bias in Minimum Temperature Introduced by a Redefinition of the Climatological Day at the Canadian Synoptic Stations

Abstract

On 1 July 1961, the climatological day was redefined to end at 0600 UTC (coordinated universal time) at all synoptic (airport) stations in Canada. Prior to that, the climatological day ended at 1200 UTC for maximum temperature and 0000 UTC for minimum temperature. This study shows that the redefinition of the climatological day in 1961 has created a cold bias in the annual and seasonal means of daily minimum temperatures across the country while the means of daily maximum temperatures were not affected. Hourly temperatures taken at 121 stations for 1953-2007 are used to determine the magnitude of the bias and its spatial variation. It was found that the bias is more pronounced in the eastern regions; its annual mean varies from 20.28 in the west to 20.88C in the east. Not all days are affected by this change in observing time, and the annual percentage of affected days ranges from 15% for locations in the west to 38% for locations in the east. An approach based on hourly values is proposed for adjusting the affected daily minimum temperatures over 1961-2007. The adjustment on any individual day varies from 0.58 to 12.58C. The impact of the adjustment is assessed by examining the trends in the annual mean of the daily minimum temperatures for 1950-2007. Overall, with the adjustment, the trends are becoming either more positive or are reversing from negative to positive, and they have changed by as much as 18C in numerous locations in the eastern regions.

Impact of Aligning Climatological Day on Gridding Daily Maximum–Minimum Temperature and Precipitation over Canada

“…This has not been a significant issue for the gridding of monthly, annual, or 30-yr normals of temperature or precipitation for which differences in the climatological day are averaged over many days. Vincent et al (2009) have also demonstrated that the difference in climatological day can produce a bias from 20.28C in the west to 20.88C in the east in the annual mean of the daily minimum temperature. This influence was more apparent in eastern Canada where, as noted above, 0600 UTC is close to the time at which the actual minimum temperature occurs.…”

“…At a couple of stations, including Edmonton International Airport in Alberta, the small differences were positive, but that may be caused by the nearby heat island effect known to occur at Edmonton City Centre station. The change in ME, MAE, and SD appears to be greater in eastern Canada, probably because these stations were more frequently affected by the redefinition of climatological day for minimum temperature (Vincent et al 2009). Except for Swift Current CDA and Kapuskasing CDA, the changes in the difference statistics at ordinary climate stations were slight.…”

“…The approach differs from Vincent et al (2009) in four ways: 1) adjustment to the precise climate day of local ordinary climate stations, 2) adjustment to both maximum and minimum temperature, 3) computation of new minimum and maximum temperatures for all days, and 4) use of a variable monthly correction factor.…”

Potential and limitations of using satellite data to evaluate the spatial detail in climatological air temperature maps

“…It should be mentioned that the maximum temperature computed from discrete hourly observations underestimates the true maximum temperature from continuous observations by about 0.5°C. This bias is uniform across the country and the seasons (Vincent et al, 2009) and results from using hourly temperature measurements (on or near the hour as in the max T air computations) instead of maximum thermometer readings which show the daily maximum as it occurs to the closest hour and minute of the day. In fact, with data in Fig.…”

A hybrid-domain approach for modeling climate data time series

“…There are only three missing values in the Tmn series, which are replaced by the corresponding long‐term means for the respective month for the OGP test. From July 1961 onwards, the daily minimum temperature data have been adjusted by Vincent et al [2009] to account for the change in the definition of the climatological day, using hourly observations at nearby airport station.…”