WMOD00176

• Project Name: Annual Report
• Title: Seeding Operations & Atmospheric Research, 2005 Annual Report
• Prepared By: Duncan Axisia
• Date: 12/31/2005
• Weather Modification - Doc Type: Report

Evaluation uncertainties using rainfall
from rain gauges

Severa! attempts have been made to use rain
gauges to investigate a cloud seeding signature.
Silverman (2003) explains that even if one was
able to locate four rain gauges per convective
storm, statistically the rain gauge density "was
responsible for no more than 10% of the total
sample size requirement to detect a 25% change
in mean precipitation amount due to seeding."
Therefore, by mathematical proportion, to detect
a 25% change in mean precipitation amount, a
100% sample size would amount to 40 rain
gauges per convective storm. It is disbelieving,
therefore, that any scientist would chose to
perform an analysis comparing a seeded area to
an unseeded area using only rain gauges to
evaluate the delicate rainfall changes caused by
cloud seeding.

The National Academy of Sciences (NAS, 2003)
has been very specific about the problems in
using rain gauges to analyze a cloud seeding
program. The NAS states that the objective of a
scientific evaluation is to establish whether the
total rain in a target area under treatment, is
different than it would have been without
treatment. In order to be successful in such an
endeavor, one must be able to measure changes
in precipitation with sufficient accuracy to
separate the effects of treatment from natural
variability. Specifically, with reference to the rain
gauge vs. radar estimated rainfall analysis
methods, the NAS states that:

Rain gauges give a fairly accurate measurement of rain at
lhe point of lhe gauge, but rain is highly variable in space and
time, especially in convective wealher situations. The
frequency distribution of stann rainfall amounts is highly
skewed, wilh a large number of small events interspersed
wilh a small number of large events lhat account for most of
lhe total rain. Wilh lhe density of rain gauges nonnally
attainable, and integration over periods of hours, area-
average rain amounts have large errors, especially in
convective situations. Radar is being used more frequently
for measuring rain, wilh lhe advantage of much better spatial
coverage and temporal resolution. But lhis introduces
anolher variable, namely, lhe relation between lhe measured
radar parameter and rainfall at lhe surface, which depends
on lhe drop size distribution, which may be affected by
seeding.

The scientific community has therefore
established that radar should be the tool of
choice for cloud seeding evaluations, with the
limitation that seeding may alter the size
distribution of dropieis ihai are 'seen' by radar.
This limitation has been addressed by Silverman
and Sukarnjanaset and discussed in the
following section.

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Evaluation uncertainties using radar
estimated rainfall

Radar rainfall estimates provide detailed
information on precipitation events, unsurpassed
by other operationally used sensors. The benefits
of having quantitative rainfall information over
large areas with high temporal and spatial
resolution have applications in all aspects of
hydrology and water resources management.
Over the last two decades a substantial body of
research works on radar quantification of storm
rainfall has been developed. This work has
shown that, given an appropriate processing of
radar information, these data can be used to
estimate rainfall at the ground.

Some studies have argued that since glaciogenic
seeding alters the drop size distribution at the
supercooled levels, this may affect the Z-R
relationship and the radar estimated rainfall.
However, this argument may be unfounded since
by the time these drops fall to the levels scanned
by radar at cloud base for measuring rain mass,
the drop size distribution will have been
readjusted by natural process. Glaciogenic
seeding would be increasing the number of
raindrops at base and not the rain droplet size. A
major development in this aspect was when
Silverman and Sukarnjanaset (1996) quoted in
Silverman (2003) made a point of measuring the
rain spectra below seeded and unseeded clouds
using airborne in-situ measurements in the
Thailand experiment. Aircraft equipped with
state-of-the-art cloud droplet size spectrometers
are able to measure cloud droplet size and the
number of droplets per unit volume at different
size ranges. "The rain mass per unit volume in
the seeded clouds was greater than that from the
unseeded clouds and, when the larger area of
the seeded cloud rain shafts were taken into
account, the total rain mass produced by the
seeded clouds was considerably greater. These
findings tended to provide assurance that the
radar-estimated increases in rain volume are
real." (Silverman, 2003)

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