21st Conference on Climate Variability and Change


Comparison of precipitation change estimates across the U.S. - Canadian border

Pavel Ya. Groisman, NOAA/NESDIS/NCDC, Asheville, NC; and R. W. Knight and D. R. Easterling

We assessed the changes in characteristics of precipitation within 2.5º latitude north and south of the United States – Canadian border during the past century using the long instrumentally homogenous time series from national networks (827 and 885 stations in Canada and the U.S., respectively). Within each national network, we compared the time series for total precipitation, seasonal counts of days with precipitation, and intense and extreme precipitation. Significant inhomogeneities were found on the lower end of the precipitation distribution for both countries. After encountering this problem, we determined the thresholds above which we can reliably analyze time series of ‘days with sizable precipitation' (i.e., days with precipitation above these thresholds) as well as precipitation totals. Only a threshold of 2.3 mm is sufficient to eliminate spurious trends in precipitation frequency in southern Canada while the elimination of the lowest non-zero bin in the precipitation reports (0.254 mm) is sufficient to assess precipitation frequency trends over the northern U.S. after the 1940s. For heavy precipitation characteristics, no homogeneity issues were found. Here, increases of the annual number of days in the upper 10%-ile of the daily events vary from 12% (Canada) to 13% (U.S.) per century.

extended abstract  Extended Abstract (1.5M)

Poster Session 5, Climate trends and extremes
Wednesday, 14 January 2009, 2:30 PM-4:00 PM, Hall 5

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