WMOD00542

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

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. Watershed(s) contours for
precipitation depth and volume
estimates.

The model outputs:

. Precipitation estimates for each
grid point (Fig. 3).

. Precipitation estimates for each
station by interpolation of the
surrounding grid points.

. Average precipitation depth and
total water volume for watershed.

The outputs can be summed over periods of
time to be specified by the user.

3. HODEL ADAPTATION AND SOME RESULTS

The model requires topography and
upper air soundings. The direction of the
wind and its speed over the mountains are
crucial for the prediction of precipita-
tion. Consequently, 36 elevation grids
have been developed from Moroccan topo-
graphic maps for the area, for each 100
wind direction class as determined from
the Beni Mellal sounding where rawinsonde
launches are made regularly every 12 hl:>urs
(OOZ and 12Z). The model then selects the
appropriate grid. The large scale vertical
motion (w) is not taken into account for

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II

II . . .
PAECIP IN INCtES

Fig. 3: Example of model produced
precipitation pattern.

our case because at least three soundin~
points are needed to evaluate this param-
eter over the area of interest. This
par'ameter (w), computed using the Bellamy
(1949) technique, would be added linearly
to the topographically induced vertical
motion (see Fig. 1). Rhea found an average
relative difference of -23% between a pro-
duction run with w>O and a test run with
w=O. This problem of a realistic w for
initial conditions should be addressed
in future studies.

Some of the parameters for which
sensitivity tests have been made are:

1. The vertic,al di splacement factor
is a parameter which depends on the degree
of stability of the atmosphere. It
involves a few specifications:

a) If there is an inversion above
the cloud top, then the displace-
ment of the "cloud top" streamline
is set to zero.

b) If the environmental lapse rate,
as depicted by the 500 mb to 700 m
temperature difference is stable,
but with no inversion above the
cloud top, then the factor is set
to (O.4) llho.

c) If the layer 500-700 mb is
approximately in a neutral
stability condition, then the
factor is set to (O.7) Aho.
Prelimina~y results show that
simulating convection over the
Atlas Mountains by increasing the
vertical displacement improves
significantly the correlation
between computed (model) precipi-
tation versus observed precipita-
tion. This is consistent with the
dominant character of convective
band structure of precipitation in
the model domain (Baddour et al.,
1988).

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2. The efficiency factor (E), as set
in the model, depends only on the tem-
perature of the highest layer with RH>65%
(E = -kTc), but should not exceed 0.25;
when Tc>OoC, E=O (k is a positive constant
of order .01). Some runs have been per-
formed with the efficiency as initially set
(E = -kTc) and with increased efficiency
(E = -k'Tc, k')k). The correlation did not
improve but the summed differences of pre-
cipitation [LObs - Lmod] decreased. This
can be easily seen when referring to (I),
where E impacts linearly on the total
aml:>unt of precipitation computed by the
model. Other test runs should be made in
the future involving other parameters, such
as the relative humidity thresholds, the
"dead layers" or blocked layers deter-
mined from the initial winds, the
inclusion of the large scale vertical