WMOD00385

• Title: Augmentation Potential through Weather Modification - Working Document
• Date: 2/1/1975
• Weather Modification - Doc Type: Report

The modified area of effect model yields quantitative predictions of pre-
cipitation rates along a profile across the San Juan Mountains us~ng
upwind rawinsonde data and assuming either artificial or natural nuclea-
tion. An additional modification was the development of a different
model for determining the height at the top of a lifted cloud, given the
height at the top of a cloud in the valley and other sounding information.

The approach used in the Twelve Basin Investigation identified the major
mountain massifs which contribute runoff to each major basin. An indi-
vidual massif may be related to several study watershed seeding units
within a major basin, and also may be related to more than one major
basin. Figure 2 shows the massifs identified for the study and the basic
orientation of each massif. Table 4 lists the massifs for each major
basin and the stream gages used in the analysis.

~

To make optimum use of the methods available for analyzing seeding poten-
tial, it is desirable to have high elevation recording gage data and
special storm sounding data for each massif. The actual data available
for analysis, however, are somewhat limited due to the small number of
recording rain gage sites within or near the individual massifs. A pre-
cipitation episode was specified as the occurrence of 0.10 inch of pre-
cipitation in 24 hours at one or more of the rain gage sites within the
massif. The beginning and ending time of the episodes were tabulated
from the Climatological Data publications of hourly precipitation data.
The meteorologically determined time lag between sounding time and pre-
cipitation station makes it possible to relate a I-hour precipitation
record (zero hour) to the aerologically determined cloud top temperature.

The network of rawinsonde stations operated by the National Weather Ser-
vice (NWS), either currently, or during some portion of the study period,
are shown on figure 1. Table 5 lists those stations for which data were
not available for the entire study period.

Soundings are taken, as a rule, every 12 hours without reference to storm
conditions. Prior to June 1957, the regular sounding times were 0300 and
l500Z, subsequently the sounding times were 000 and l200Z. In certain
cases, therefore, it was not possible to relate a sounding with a precipi-
tation episode, especially if it was a short duration.

A basic assumption is that the airmass characteristics depicted by a
sounding from a site upwind of a massif will be applicable to the air-
mass characteristics over the massif at a later time. The Northern
Hemisphere series of daily surface and 500 mbar charts were used to
determine the time lag from the sounding station to the massif. Those
charts are available only through 1966, at present, and subsequent to
this, the NWS daily weather map series were used to determine the time
lags.

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