Extended Abstract (300K)7.12
Derivation of an improved snow water equivalent adjustment factor map for application on snowfall ruler measurements in Canada
Éva Mekis, MSC, Toronto, ON, Canada; and R. Hopkinson
The depth of freshly fallen snow, measured by ruler, has been the standard climate measurement of snowfall since Canadian climate measurements began. For all stations, prior to the 1960s, and for non-synoptic stations over the entire record, precipitation amount (water equivalent) for snowfall events has been determined by assuming a density for fresh snow of 100 kg m-3. With the introduction of Nipher shielded snow gauges at nearly 300 locations at the synoptic stations in the 1960’s, the direct measure of snow water equivalent (SWE) became available. However, because the process of ruler measurements has undergone fewer changes over time and provides higher spatial coverage for the country, in climate change detection studies ruler data are used for the full period. For computation of snow water equivalent, the use of the 100 kg m-3 standard density of fresh snow was rejected because of known climatological differences in the characteristics of snow between moist coastal regions, the dry interior and the north. Instead, a new fresh snow water equivalent adjustment factor (rswe) map was developed. The adjustment method follows Metcalfe et al. (1994). It is based on the ratio of adjusted Nipher gauge measurements to snowfall ruler depth measurements for events with snow water equivalent greater than or equal to a trace during the period of record when snow ruler and Nipher gauge measurements were made coincidentally. By applying SWE adjustments factors determined in this way to the ruler measurements, it is possible to estimate the true, long-term average, water equivalent of snowfall. The first approach of this map based on 63 stations was presented by Mekis and Hogg, 1999. Since then additional metadata searches and data quality checks – including all possible co-located Nipher and ruler stations, occasionally using Canadian regional climate expert opinions - were completed to obtain the most realistic results. A new interpolated map of the values of (rswe) based on 175 stations was produced and used to obtain estimates of the average SWE adjustment factor of new snowfall for all long-term climate stations in Canada. The SWE adjustment factors were applied to all ruler measurements to generate time series of snow water equivalent at each location. In the conference paper, sensitivity studies of different factors (wind tower heights, exposure code, etc.), the effect of possible outliers and the possible applications and limitations of the finalized map are presented.
Session 7, Data Reliability, Quality Assessment and Usability (Room 619/620)
Thursday, 15 January 2004, 1:30 PM-5:30 PM, Room 619/620
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