Adjustments for trace measurements in Canada

Precipitation measured as trace amount (T) may increase the annual total precipitation up to 20% in the Arctic. Unfortunately observations of trace measurements were not consistent neither in space nor in time. In the first part of the presentation the evolution of standard procedures of the trace measurement in observing manuals (MANOBS - Manual of Surface Weather Observation) and practices will be summarized. The possible reasons for discontinuities will also be outlined. In the present status of Adjusted Historical Canadian Climate Database (AHCCD) the trace adjustments for each recorded Trace flag may cause artificial inhomogeneities at the time when the MANOBS definition of trace was modified. This study presents a preliminary assessment of small measurable precipitation amount including the suggested corrections for trace flags in AHCCD datasets. The question to be answered is weather the observer would record small amounts of rain or snow instead of using the trace flag before the proper definition of the Trace. If it happened, then the discontinuity due to trace correction would be small. Time series will be evaluated for possible changes in general behavior, trends and steps using all precipitation observations less than 0.3, 0.5, 1 and 5 mm. Case studies using synoptic, climate and mixed stations; at same location or relocated stations will be examined. The results will be displayed together will all relevant information including metadata, frequency of trace measurements, annual time-series and maps.