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

means that decades of data are required for a
scientifically credible rainfall analysis using rain
gauges. We can all entertain to write articles or
stories about rainfall comparisons, but none of
these entertaining pieces will survive scientific
scrutiny and would only serve the purpose of
confusing the public.

One new perspective in rainfall measurement
is to move up the geographic scale, from site-
specific to measurements over larger regions.
A more radical perspective is to leave the
earth-bound view and explore new
measurement technologies. Estimations of
precipitation based on all available
measurements over a wide area will inspire
more confidence.

Conventional precipitation instruments are
"touched" by atmospheric water when it falls to
the surface and therefore provide direct
measurements of the falling water. A few
technologies in use nowadays only "look" at
atmospheric water, where there is no contact
between the measuring instrument and the
water. These fall in a category called indirect
measurements, using, for example,
microwaves and infrared energy. Surface-
based weather radar directs a focused beam of
microwave energy at wavelengths that are
related to the size of water droplets in air. If
there are airborne water droplets, some of the
radar beam is reflected back to the instrument.
The amount of reflection is an indirect
measurement of the amount of airborne water.
The newer radars can also detect the direction
and speed of moving droplets, discriminate
between water and ice in clouds, detect hail,
identify the particles present, and attain more
accurate estimates of rainfall rates.

The US government is operating a network of
surface-based weather radar called NEXRAD
that provides much data, including rainfall data.
The NEXRAD network is an array of beams
aimed skywards illuminating the atmosphere
over the US in a canopy of invisible "light."
Atmospheric water droplets under the canopy
are illuminated and "seen." A single radar can
illuminate an area of more than 4,500 square
miles while a rain gauge only covers the area
of about 3 square inches.

While it is easy to think of the conventional
direct measurements as more accurate than
indirect measurements, for our needs this may
not be so. If all measurements are combined,
the sum of their individual strengths can
enhance the precipitation database. A more
robust database reduces regional rainfall
uncertainty .

.
.
.
.
.
.-.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
:1
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

Southern Ogallala rain gauges

For rainfall measurements, SOAR uses an in-
ground version of the cylindrical funnel rain
gauge in an attempt to minimize evaporation.
Advantages if using rain gauges are that it is
cheap and easy to maintain and read; gauge
data are available for many years and hence it
is possible to do climatological analysis using
long period gauge data; and comparison
between gauge data from different regions is
possible.

However, the disadvantages are much more
numerous:

a) It measures point data. Thus, even having
a high density of gauge network area
estimate of rainfall with reasonable
accuracy is difficult to achieve. Moreover, it
is impractical and not-cost effective to have
a dense network that can measure the
highly variable spatial distribution of
rainfall. Density of gauge network in the
target area is currently at one rain gauge
every 33 square miles.

b) Since gauges are read once every month
to get real time information they should be
located in places where there is easy
access for an observer. However, there
are remote areas of interest that are
difficult to be accessed. Therefore, gauge
networks cannot cover all area of interest
to make a real time observation.

c) Source of errors in the reading can easily
occur due to: airflow; unrepresentative
orientation and exposure of the gauge;
human observation and transmission;
evaporation from within the cylinder; gauge
leaks and overflow.

d) Some times there might be time delay in
getting the gauge data at the end of the
month.

23