WMOD00541

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

2. MODEL DESCRIPTION
Since the model was designed to develop an operationally or'iented
computation scheme for orographic precipitation, simplicity, fast
running time, and usage of routinely available data as model input were
key considerations. The Rhea model was developed for the western
Colorado Mountains and tested using a data base of upper air soundings
covering 13 years, at 12 hourly intervals. It has been validated over a
computation area (model domain) of about 153,600 km2 (60,000 mi2).
Observed precipitation data were available for a number of stations, and
snow courses and stream flow data for watersheds were also available for
model validation.
The model was designed to be run with realistic topography. It has
the flexibility of using topographic grid meshes ranging from 10 km to
2.5 km on a side, but tests have been made only on a 10 km grid interval
(Fig. 1-2) in this study. The model treats the interactions of air
layers with the underlying topography by allowing forced vertical dis-
placement of the air column. Figure 2-1 shows a symbolic two-dimen-
sional flow across a barrier. The model is constructed of a series of
parallel x-z 2-D cross sections. As many as 45 of these are used to
predict the total precipitation over the model domain. In order to
compute the precipitation amounts, the model keeps track of the conden-
sate or evaporation resulting from these vertical displacements. As the
layers flow over a barrier, part of the condensate precipitates and the
remaining part (which does not precipitate) is allowed to move down-
stream to the next grid point where a fraction of it and the newly
formed condensate (generated as a result of topographic lift) will again
partly precipitate, and so on. It should be noted, however, that

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