WMOD00399

• Title: WMO Training Workshop on Weather Modification for Meteorologists - Lecture Notes
• Date: 12/1/1979
• Weather Modification - Doc Type: Report

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Huff curves for 1 hour storms from Figure 1 superimposed. The resul~s of
Figure 2 suggest that the gaging requirements are more stringent in ,
Florida than in Illinois. However, this is a false impression. The,
Florida measurements were made for an area of 220 mi2 (595 km2) whil~ the
Illinois measurements were made over an area of 400 mi2 (1080 km2). ~From
Table 2 we have seen that the gaging requirements increase as the samp-
ling area is decreased, so a portion of the discrepancy is explained 'by
the difference in sampling areas. The differing bases for comparison
probably explain the rest of the discrepancy. In Illinois the "true"
rainfalls were obtained with gage densities of 8 to 11 mi2/gage (22 to 30
km2/ gage) while in Florida the "true" rainfalls were obtained with gage
densities of 1 mi2/gage (3 km2/gage). The more stringent basis for com-
parison in Florida is the likely explanation for the greater apparent
gaging requirements in Florida. '

A useful presentation of measurement errors as a function of
gage density is a factor-of-difference (FD) representation (Woodley ~t aI,
1975). The FD is defined as:

Gi/Gf when Gi ~ Gf or Gf/Gi when Gi < Gf

(3)

where Gf represents the mean rainfall as measured by the full networR
density and G. represents the mean rainfall as determined by a subnetwork
with a lesser1density of raingages. FD distributions as a function qf
gage density are presented in Figure 3 from which it can be determin~d
once again that the accuracy of rainfall measurement degrades with de-
creasing gage density. Note that convective rain measurements with ~
I
gage density of about 12 mi2/gage (32 km2/gage), the approximate standard
in Illinois, are in error by a factor of 1.50 relative to the full den-
sity measurement about 30% of the time. This is additional evidence:that
the difference in Figure 2 between Illinois and Florida are due primarily
to the differing standards for comparison and not due to meteorological
differences. .

If the sole interest were the measurement of convective events
on the scales of 200 to 600 mi2 (540 to 1620 km2), the sampling require-
ments would now be determined. In most weather modification endeavors,
however, the interest is in larger areas. In the Florida Area CumulJs
Experiment (FACE) (Woodley et aI, 1977) as an example, the target area
for experimentation covers 4800 mi2 (~1.3 x 10~ km2). The results bf
Huff for Illinois show that the gaging requirements change as a function
of sampling area. This is certainly the case in Florida as well, so the
results of Figure 3 are not valid for the FACE target. Consequently; it
is necessary to estimate the gaging requirements for the larger area:
directly.

If one assumes that the equation (2) is valid for Florida ~s
well, one can substitute 4800 for A in the equation and calculate gage
density (G) as a function of rain amount and duration. This is a highly
questionable procedure because Huff did not work with areas of this ~ize.
The exercise is instructive nevertheless. Letting P = l.O in (25 rnm) and
T = 6 hrs and the sampling error (E) = 0.05 in (1.3 rnm), one finds that
G = 300 mi2/gage (810 km2Jgage) for the FACE target. This represent~ a
factor of four to six relaxation of the gaging requirements for sampling
areas of 400 to 200 mi2 (1080 to 540 km2), respectively. This resul~
appears reasonable based on that which follows.

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