WMOD00546

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

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of freedom are permitted to independently express the various ch,aracter-
istics of a surface raincell, namely shape, area, location of precipitation
maximum, and,in particular, precipitation gradient.

A model isohyetal pattern for a convective raincell is shown in Fig 2. The
spatial distribution of precipitation for such patterns is given by:

P(x) = PM [0.5 + 0.5 cos (~rx/ro)J G

(1 )

where P(x) is the precipitation amount at point x; PM is the maximum
precipitation amount at point M (not necessarily at the geometrical center
of the pattern); rx is the distance from point M to point x; ro "is the
distance from point M to point 0 (on the zero isopleth) through point x;
and G is the spatial gradient index. The location of M is externally
defi ned by the parameters X Offset and Y Offset, the coord"i nate offset
distance ratios from the geomE~trical CE~nter of the pattern to thE~ zero
isopleth. The shape of the pattern is determined by the parametE~r E (labelled
Ellipse in Fig. 2) the ratio of major to minor axes of the elliptical
pattern. To faci 1 itate general ity of the results, the major axi s of the
isohyetal pattern (labelled LENGTH in Fig. 2) is arbitrarily scaled to always
be 16 km (10 mi) long.

Fig. 3 shows the relationship between G and ASO, the percentage of raincell
area containing 50 percent of the total precipitation volume (labelled half
area in Fig. 2). It can be seen that A50 is a nonlinear function of G. A
value of G equal to 0.6 (A50 = 25) corresponds to an approximately linear
spatial gradient of precipitation. Both parameters are carried throughout
the paper because A50 is an easily derived value from isohyetal maps and,

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