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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Until the advent of a digitized radar in Florida there was
really no hope of solving the large-area sampling problem because we found
it impossible to instrument an area of this size with raingages in suf-
ficient density to constitute a standard. With the radar, Woodley et al
(1975) found the problem quite tractable. There is not space to describe
the procedure but the results are shown in Fi~re 4. The solid lines are
the FD representation for the 220 mi2 (594 km ) area and the dashed lines
are the FD curves as a function of raingage density for the FACE target
of 4800 mi 2 (1.3 x 10" km2). Nott~ that for roughly the same accuracy
there is approximately a factor of seven reduction in the sampling re-
quirements for the larger area relative to the smaller. This result is
remarkably similar to that estimated independently from Huff's equation
(2). The consistency of th,e two results is encouraging.

We are now in a position to estimate the magnitude of the rain-
gaging problem for warm season convective storms. To a first approxima-
tion these estimates should be valid for warm season convection over most
areas of the world, particularly those areas affected by tropical air
masses. Although cold season rainfall has not been discussed extensively,
the results of Table 1 suggest that the gaging requirements are relaxed
considerablyuup to a factor of three in the Illinois environmentupro-
vided the cold season precipitation has predominately a stratiform char-
acter.

For the estimation of gaging requirements, assume that we must
satisfy a requirement that the rainfall measurements be within a factor
of 1.40 of the "true" rainfall in 90 percent of the instances. This
should be satisfactory in weather modification experimentation. Combin-
ing the studies in Florida and Illinois, the inferences of Table 3 are
obtained. These estimates do not take storm mean rainfall into account
explicitly.

Table 3: Estimation of Raingaginl: Requirements for Convective Storms

(Measurements Within a Factor
of 1.40 of the "True" Rainfall 90\ of the Time)

Required Raingage Density (mi2/gage)*
warm season cold season
I hr duration 24 hr duration I hr duration 24 hr duration
M
E 50 2 5 8
It. 10
5
U 200 5 10 15
R 30
E
M 400 7 15 21
E 45
N
T 550 9 17 27
51
It.
R 4800 27 55 81
E 165
It.
*Mu1tip1y values by 2.70 to get vlllues in km2 / gage