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

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I

The nature of precipitation
An elusive and perplexing weather variable

Measuring precipitation

Precipitation is ubiquitous in our lives, and can
have profound impacts on a farmer's profit and
loss. A we! period during planting time would
force a farmer to have a late crop, a rain at the
right time can save a farmer's dryland crop.
while an excessive rainfall can make fields
muddy and ruin crops. During the growing
season, it is interesting to listen 10 farmers in a
cafe the day after a rain. The main talk of the
town is the rain; some sections got a 9000
shower while the next section over received no
rain. Precipitation is an elusive and perplexing
weather variable: even with leday's technology.
it remains difficult to measure, model and
predict.

Unlike temperature. which is measurable all
around us all the time. precipitation can be
intermittent: it can rain not at all. a little or
heavily. Rainfall can also be spatially irregular:
at one moment, it can rain on one side of the
street but not !he other, on one stretch of
highway but not another, on one field or on one
watershed but not on an adjacent one. This
variation from one location to another, and
from one lime to another, can generate
significant confusion when collecting data to
analyze rainfall amounts. For example. it is
possible that sufficient rain could fall on a rain
gauge while no rain falls on a farmer's nearby
field. How then can a meteorologist manage to
work with nature if the pattern of rain is so
uncertain?

The rainfall in the SOAR target area is mainly
convective during the operational season and
thus it is more localized and Its spatial and
temporal distribution as well as the intensity of
rainfall is highly variable. Therefore. it is

essential to measure area rainfall on real time
as accurately as possible. Various methods of
rainfall measurements are available_

Conventional technologies. which we can call
rain gauges. are based on simple concepts:
either precipitation falls into a bucket where it is
measured with a ruler. or it falls on a pan of
known but small volume. which tips when full.
The number of times the pan tips is then
counted The mere presence of a rain gauge al
a site alters the flow of the local wind. and this
changes the precipitation measurement from
what could have fallen if the instrument were
not there. Among the many other concerns are
errors from evaporation during measurement.
Second is the question as to whether the
measurements made at one or more locations
within a region fairly reflect the precipitation
over that region. A pilot study conducted by
SOAR has already demonstrated that rain
gauges do not accurately or fairly represent the
rainfall amount received in the immediate area
during the growing season when seeding is
conducted. This changes drastically in the
wintertime when the precipitation
characteristics change from convective
sporadic and spotty nature of precipitation to a
more stratiform and uniform rainfall in the
wintertime. Evidence from local rain gauge
data from 1971.2000 shQ'NS that rain gauge
point data is more representative of the
immediate area rainfall in the wintertime and
not in the summer time. In addition. historical
data from 1916-2002 shows that rainfall
amounts in the growing season have a
sinusoidal variation (alternate increases and
decreases in rainfall above an average value)
with a period of about 80 to 100 years. This

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