WMOD00546

• Project Name: High Plains Cooperative Program
• Title: On the Sampling Variance of Raingage Networks
• Weather Modification - Doc Type: Report

Table 4. - Statistical characteristics of Montana
convective storm precipitation patterns

Number of rain-
cell s per
storm

Standard Log-normal Log-norma 1
Mean deviation Median mean standard deviation
4.18 x 105 10.32 x 105 0.86 x 105 11.40 1.80
444.2 73:3.1 167.0 5.18 1.40
117.3 66.4 100.0 4.62 0.54
2.03 1.98 1.0

Storm
Parameter

Volume (m3)
Area (km2)
Duration (min)

Raincell volume
(m3)

2.11 x 105 5.24 x 105 .397 x 105

10.75

1.73

Raincell area
(km2)

223.3

350.7

90.0

4.57

1.23

The sample sizes Nnv, required to detect a 25-percent change in mean storm
precipitation due to seeding whl~n only natural variability is considered was

calculated using the log-normal version of Eq. (3). For a = 0.05 and f3 =
0.10, it was found that Nnv = 2,237. Table 5 gives the percentage of the
total sample size (resulting from sampling variance plus natural variability)

that is due to the sampling variance contribution by various gage densities,
that is 100.Nsv / (Nsv + Nnv). The median area of all raincells of the
Montana storms is 90 km2. However, the median area of the primary (largest
contributor to the total rain volume) raincells is 160 km2. The pdmary
raincells contribute an average of 88 percent of the storm's total rainfall.

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