WMOD00542

• Title: Adaptation of the Rhea Winter Orographic Precipitation Model to Morocco
• Weather Modification - Doc Type: Report

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AOAPTATION OF THE RHEA OROGRAPHIC PRECIPITATION MOOEL TO MOROCCO

Ali E1Majdoub
Harry Orville
South Dakota School of Mines and Technology
Rapid City, South Dakota, U.S.A.

1. INTRODUCTION

After a severe drought which lasted
about four years (1979-1983), Moroccan and
US officials planned a scientific weather
modification program named A1ghait in an
attempt to increase the winter snowpack
over the Atlas Mountains as a part of a
general water resources management policy
(see Bensari et al., these pro-
ceedings; Silverman et al., 1983).
Cloud seeding has been done using aircraft
delivery systems and ground-based silver
iodide generators during the November to
April period each year since 1984. The
target reservoir is Ben E1 Ouidane located
25 km to the south-southeast of the city
of Beni Mel1al and 800 m above sea level.
A mountain ridge of approximately 1 km
average height above the plains is between
Beni Mel1a1 and the reservoir.

Radar and cloud physics aircraft
observations have been made of the clouds
and precipitation systems during the opera-
tions. Rawinsonde soundings are made
routinely twice a day and special soundings
are made every operational day. Extensive
snowfall and rain measurements are being
taken at many ground locations.

Forecasts of the daily weather
conditions are made at the main weather
center in Casablanca. The potential for
seeding effects is assessed and results
transmitted over the radio to the opera-
tions center. Various models have been
used in this assessment. One of them, a
two-dimensional, steady-state, multi-layer
model was developed in the 1970's by
J. Owen Rhea to diagnose the effects of
topography on winter/spring precipitation
in western Colorado, employing rawinsonde
data and a fine mesh topographic grid. We
believe that the Rhea model could be useful
as a forecast tool, and help in determining
the precipitation potential for the day.

It is well known that the terrain in
mountainous regions has an influence on
precipitation. In this paper, we will be
dealing with the effect of the Atlas
Mountain range on winter orographic pre-
cipitation. In the last 25 years, a number

J. Medina
U.S. Department of the Interior
Bureau of Reclamation
Denver, Colorado, U.S.A.

of orographic precipitation numerical
models have been developed, most of them
two-dimensional (in the X-Z plane) and
steady-state (e.g., Myers, 1962; Sarker,
1967; Willis, 1970; Fraser et al., 1973;
Plooster, 1974; Young, 1974; and Rhea,
1977). The subject of this paper is to
discuss the adaptation of the Rhea model
to Morocco.

2. MODEL DESCRIPTION

The physics of the model is based on
the following equation (Rhea, 1977):

R

E
Pwg

ft rP2
Jo Jpl

w dp dt

(1)

dQs
dz

where

R

Precipitation amount [cm]

E

Precipitation efficiency
(proportion of the condensate
which precipitates)
(dimensionless)

Vertical motion (em S-I)

Density of water (1 g/cm3)

w

Pw

g

Gravitational acceleration
(cm/s2)

dqs
dz

J P2 dp =

PI

Specific humidity change with
height [cm-1]

Pressure depth of saturated
air column [g cm-1 S-2]

J: dt = Duration of the process [s]

Figure 1 shows a symbolic
two-dimensional flow across a barrier;
based on this picture, the atmospheric
water balance equation for one grid
interval (xQ, xd can be written as:

To be published In the proceedings of the .
Fifth WMO Conference on Weather Modification and Appllecl Cloud Physics
Beijing, China