WMOD00541

• Title: Adaptation of the Rhea Winter Orographic Precipitation Model to Morocco
• Date: 4/1/1989
• Weather Modification - Doc Type: Report

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5

c. the large scale vertical motion precipitation component

(Rls),
5,0 that we can write

RT = Ro + Rc + Rls

(1-1)

These three components have been discussed by Elliott and Schaffer
(1962), Hjermstad (1970), Chappell (1970), and others.
In (1-1) the three components may have different magnitudes (when
compared to each other) depending on the degree of stability of the
atmosphere, the underlying topography, and large scale weather condi-
tions. However, in most complex terrain areas, the topography exerts
the most dominant effect due mainly to a more persistent orographic
vertical motion field and convection resulting from forced lifting over
the barrier. Previous observations of the ridge-to-valley winter
precipitation ratios range between 2/1 and 1011 and make evident this
topographic dominance (Hjermstad, 1970; Rogers, 1970; Rhea, 1978; and
others).
In many hydrologic studies (Peck and Brown, 1962; Schermerhorn,
1967), it has been found, based on observations, that precipitation in
mountainous areas increases with elevation, which helped in developing
local linear regression relationships between these two variables
(precipitation versus elevation). These studies, however, did not try
tCI directly relate these factors to any causal meteorological flow
features (wind, temperature, moisture, etc.).
In the last 25 years, a number of orographic precipitation models
have been developed. A few are three-dimensional (3-D) (Colton, 1975).