WMOD00315 ‎(2)‎

• Title: Colorado River Augmentation Demonstration Program - Technical Report January 1983 - March 1985
• Date: 4/1/1986
• Weather Modification - Doc Type: Report

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178 Days of Precipitation
51.3 Inches of Precipitation

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.05 .07.1 .2.3 .5.7 1.0
DAILY PRECIPITATION [INCHES]

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Figure 3-2. - Cumulative distributions for daily precipitation.

or days with small precipitation amounts. The bottom curves
show cumulative seasonal contributions of snowfalls in water
equivalent amounts. Hourly snowfalls less than the median
amount (0.028 inch) contributed less than 21 percent of total
snowfall, as shown by lower curve on figure 3-1. However,
half the total hourly precipitation came from the 21 percent
of precipitation hours with highest snowfall rates. Similarly
in figure 3-2, daily snowfalls less than the daily median (0.145
inch) contributed less than 10 percent to the season total,
and half the total daily precipitation came from the upper
15 percent of daily accumulations.

The duration of precipitation episodes is of considerable in-
terest in planning future weather modification experiments.
An episode was defined as any period having detectable
snowfall until a gap of more than 2 hours with no snowfall
was encountered (the threshold of detectability varied with
the gauge). Figure 3-3 plots the cumulative distributions of
episode durations for 224 episodes spanning 2,326 hours
and totaling 51.4 inches of precipitation. The upper curve
on this figure shows that 25 percent of the episodes had
durations of only 1 hour, and 50 percent had about a 4-hour

duration or less. The middle curve shows that 50 percent of
the precipitating hours were in episodes of 21 hours or less.
The bottom curve shows that half the precipitation fell in
episodes of 24 hours or more which, according to the upper
curve, were only 11 percent of the episodes. The frequent
occurrence (73 percent) 'of episodes of half a day or less
contributed only 16 percent to total snowfall, and therefore
were of limited importance. Over the historical record, the
major winter droughts have occurred when those few large
storms have failed to pass through the area.

The diurnal variation of hourly precipitation is displayed on
figure 3-4. The lower curve shows detectable hourly precip-
itation about 28 percent of the time, and a possible maximum
in frequency in the hours before sunrise. The diurnal distri-
bution of total precipitation, upper curve, shows a broad peak
in accumulations from midnight to sunrise following a mini-
mum during previous hours after sunset. However, these
diurnal variations were not very pronounced. It seems likely
that the random passage of surface fronts and upper air
disturbances was chiefly responsible for thE! timing of precip-
itation episodes and amounts.

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