Precipitation recycling

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The amount of precipitation falling over a region can be divided into: (1) precipitation associated with water vapor transported into the region (advected precipitation); (2) precipitation associated with water that evaporates from the surface of the region and falls within the same region (locally derived precipitation). The precipitation recycling ratio is defined as Pl/P, where Pl is the precipitation of local origin and P is the total precipitation. The recycling ratio can be thought of as providing a sense of the importance of land-surface processes on the hydrologic budget. The topic has a long history, starting with Mikhail Budyko and associates in the Soviet Union in the

1950s. More recent studies include Brubaker et al. (1993), Eltahir and Bras (1996) and Trenberth (1998). Estimates of the recycling ratio are contingent on the size of the region considered. The ratio is smaller for areas of limited extent and increases for larger regions (Brubaker et al., 1993). Obviously, all precipitation is recycled at the global scale.

Serreze et al. (2003a) examined monthly precipitation recycling for four areas with identical area and shape. The four regions are bounded by 50° N and 70° N latitude. They span longitude bands 60-85° E (west-central Eurasia), 85-110° E (central Eurasia), 110-135° E (east-central Eurasia) and 110-135° W (western North America). The area of each region is 3.08 x 106 km2. These regions were chosen to roughly represent the four major Arctic watersheds. Equal areas were chosen (rather than areas defined by the true basin boundaries) to allow for direct comparisons. The western North American domain (Mackenzie) does include some ocean areas as well as coastal regions where precipitation is very high. Results are based on the period 1960-99 for the Eurasian domains and 1960-89 for the western North American domain.

A number of different formulations of the recycling ratio can be found in the literature. Following the recommendation of Trenberth (1998), Serreze et al. (2003a) employed the formulation of Brubaker et al. (1993). This is given as:

where A is the area of the region. F+ is the advective moisture term. It is calculated as the line integral of the component of the vertically integrated moisture flux directed into the region. It should not be confused with the vapor flux divergence, which is the difference between F+ and the component of the moisture flux directed out of the domain (F—). Calculation of F+ uses the monthly-mean vertically-integrated moisture fluxes at the 2.5 x 2.5 degree grid available from NCEP. ET is averaged over the region and is based on the difference between P and computed P — ET. The formulation assumes equilibrium conditions and a well-mixed atmosphere. This means no changes in atmospheric moisture content, and that the ratio of advected to locally derived precipitation is equal to the ratio of average advected to evaporated moisture in the air. Inspection of Equation (6.5) shows that a large recycling to advection ratio will result when the moisture advection term F+ is small in comparison with ET.

For the Eurasian domains (Figure 6.11), the ratio is largest during July. This is primarily due to the peak in ET as the term F+ still tends to be fairly large in summer. By contrast, the peak for the western North American (Mackenzie) domain is one month earlier, in June. Peak values range from 0.22 (central and eastern Eurasia, or Lena) to 0.28 (central Eurasia, or Yenisey). This points to a significant effect of the land surface on the summer hydrologic regime. Winter values range from 0.0 to 0.11, largest for western North America.

Figure 6.11 Mean monthly precipitation recycling ratio (Pl/P) for four domains chosen to approximately represent the Ob, Yenisey, Lena and Mackenzie basins (from Serreze et al., 2003a, by permission of AGU).

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