WMOD00593

• Contract/Permit #: 14-06-D-7052
• Title: Ecological Impacts of Snowpack Augmentation in the San Juan Mountains, Colorado
• Date: 3/1/1976
• State: CO
• Weather Modification - Doc Type: Report

excluding the period January-April 1972 for which
no synoptic type classification is presently
available (Table 11). Eighty-seven percent of the
days on which seeding might have occurred comprised
four synoptic types - types Ao' A. Bn_c' and Em
(for discussion of the types, see Elliott 1949).
In the period considered, these types accounted for
75 percent of days.

An analysis of these types for the months when seed-
ing has occurred (October-May) was therefore carried
out to answer the following questions:

1. How frequently do these types generally occur?

2. On how many days on which the type occurs does
precipitation ~0.5 tmll (0.02 inches). (water
equivalent) fall at Durango and Silverton?

3. How much does each type generally contribute to
the monthly precipitation total?

The classification for 1928-1957 was used to answer
these questions and therefore the resulting values
are the means for this 30 year period.

Table 12 shows the percentage frequencies of the four
main "seedable" types for winter months. They occur
most frequently at the beginning and end of the
winter and least frequently in December, with a mean
frequency of 47 percent of days for all months when
seeding is carried out (i.e. approximately 14 days
per month).

An examination of daily precipitation data for
Durango and Silverton indicates that not all days of
these synoptic types receive precipitation. Table 13
gives a "precip:ftation index" for each type, such
that:

Index

R
T x 100 %

where T total number of days of the type per
month and
R total number of rain days ;C 0.5 mm
(0.02 inches) in that type per month.

Thus, in Table 12, type B occurred on average on
22 percent of days in October. Of these days, only
23 percent results in precipitation> 0.5 mm
(0.02 inches) at Silverton, and similarly only
18 percent resulted in precipitation at Durango
(Table 13).

It is now possible to determine how important each
rain day (of each type) is to the monthly precipi-
tation totals. Table 14 shows the percentage con-
tribution of each "precipitation day type" to the
monthly total precipitation. Thus, at Durango, when
precipitation occurs on a type A day in October,
the average contribution to the total monthly pre-
cipitation is 18 percent. Similarly, the figure is
14 percent at Silverton. It might be expected that
if cloud seeding in the San Juans becomes opera-
tional in all zones of the project area, the per-
centage contribution of each type to the monthly
total would increase. However, an assessment of

Table 11. North American synoptic weather types on "seedable days"

Type Winter (1970-71)
A/A * 14
0
B 7
B 4
n-c
D
EL 2
E 1
n
E 6
m

(Oct-Dec only)
Winter (1971-72)

Total

6
1

1
2

20

8

5
2

3

1
8

2

47

*Types.A and A can be considered together.
0
Table 12. Percentage frequency of occurrence of principal "seedable types", 1928-1957
Type Oct Nov Dec Jan Feb March April May ~
A 16 14 9 14 15 21 16 22 I
B 22 11 7 6 10 11 18 15 .1
B 11 6 5 6 10 8 10 14
n-c
E 5 7 9 '\
m 13 8 13 14 9
54 38 30 39 43 53 58 60

64