Extreme precipitation

Increased temperatures lead to an increase in the water-holding capacity of the atmosphere, such that with each 1 °C increase we have an increase of about 7% in the water-holding capacity (Trenberth et al., 2003). Furthermore, observations suggest that as this capacity increases with increased temperatures, relative humidity remains more or less constant, resulting in an increase in water vapor in the atmosphere owing to enhanced drying of the surface. Figure 2.5 shows

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10 20 30 40 50 60 70 80 90 100 100 Daily precipitation (mm)

Figure 2.5. Percentage of annual precipitation from various daily rainfall rates for a cold climate (black), temperate climate (light gray), and warm climate (dark gray). Cold climates receive more precipitation from lighter amounts, and warm climates from heavier amounts.

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10 20 30 40 50 60 70 80 90 100 100 Daily precipitation (mm)

Figure 2.5. Percentage of annual precipitation from various daily rainfall rates for a cold climate (black), temperate climate (light gray), and warm climate (dark gray). Cold climates receive more precipitation from lighter amounts, and warm climates from heavier amounts.

typical rainfall rates for three climates: a warm climate, a temperate climate, and a cold climate. For the warm climate, more of the rainfall distribution falls in heavier amounts, compared with the temperate and cold climates, suggesting that, as the climate warms, more rainfall would be expected to occur in heavier events.

Trends in one-day and multiday extreme precipitation events in the United States and other countries show a tendency to more days with extreme 24 h precipitation totals (Karl and Knight, 1998). The number of days annually exceeding 50.8 mm (2 inches) of precipitation has been increasing in the United States (Karl et al., 1996). Also, the frequency of 1- to 7-day precipitation totals exceeding station-specific thresholds for one in 1 year and one in 5 year recurrences, as well as for the upper 5 percentiles, have been increasing (Karl and Knight, 1998; Kunkel et al., 1999). Increases are largest for the Southwest, Midwest, and Great Lakes regions of the United States, and increases in extreme events are responsible for a disproportionate share of the observed increases in total annual precipitation (Groisman et al., 2005).

The tendency in most countries that have experienced an increase in monthly or seasonal precipitation has been for this increase to be directly related to an increase in the amount of precipitation falling during the heavy and extreme precipitation events. On the other hand, Akinremi et al. (1999) found that, although the Canadian prairie has experienced increased precipitation over the past 40 years, this increase appears to be mainly due to an increase in the number of lighter (<5 mm) daily rainfall totals.

Recent work by Groisman et al. (2005) provides more evidence that heavy precipitation events did, indeed, increase over the United States during the twentieth century. This increase was accompanied by increases in streamflow, and in the eastern United States there is evidence for increases in heavy streamflow as well. In a similar analysis, Kunkel et al. (2003) found increases in heavy precipitation, defined as those days exceeding the 1-year recurrence threshold; however, they also found evidence of a period of increased heavy rainfall events in the late 1890s, particularly in the western and central United States (Figure 2.6). However, in Canada, Zhang et al. (2001) found no evidence for increases in heavy precipitation for the country as a whole. Only in eastern Canada, during the spring, is there an identifiable trend toward increasing heavy precipitation events.

Alexander et al. (2006) also examined global changes in heavy daily precipitation events by using gridded daily data. The globally averaged trends in the percent contribution of heavy precipitation days (amounts above the 95th percentile) to the annual total have shown small, but statistically significant, increases since 1951.

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Figure 2.6. Time series of heavy precipitation events as defined by the Extreme Precipitation Index (EPI) from Kunkel et al. (2003) for the contiguous United States; anomalies from the long-term mean for 1-day, 5-day, 10-day, and 30-day totals.

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