WMOD00291

• Title: Bulk Parameterization of the Snow Field in a Cloud Model
• Date: 6/6/1983
• Weather Modification - Doc Type: Report

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JUNE 1983

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T' = 3 sin[(J - 37)/24]A(K - 2), J = 37, . . . , 61,
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Q~ = (2 X 10-3 X sin[(J:- 37)f24]/(K - 2),
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1079

UN , FARLEY A ND ORVILLE

5. Results

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. K= 3,.. J, 6,

where T' is the temperat~re excess, Q~ the water va-
por excess, and J and R, the horizontal and vertical
indices, respectively.

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c. Numerical techniques~
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The equations are SOlY1' ed over a 19.2 km X 19.2
km domain with 200 m ,grid interval in both X and
Z directions. The advect,on technique used is that of
Crowley (1968), which is of first-order accuracy in
time, second-order in sp~ce. Following Marchuk and
Leith (Leith, 1965), a t~o-step advection scheme is
used; vertical advection 1S calculated first, horizontal
advection second. Direct Imethods described in Rogn-
lie and Kopp (I976) fOIl a rectangular grid domain
are used to solve the Pdisson-type equation for the
streamfunction (Swartztrlmber and Sweet, 1975). The
diffusion terms are calculated by substituting the sec-
ond-order approximatiort of the Laplace term and the
nonlinear values for the ieddy coefficients.

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a. General description

The current model has been tested by running
three comparative experiments using the sounding
shown in Fig. 5. This is the sounding of 2100 GMT
21 July 1976, for Miles City, Montana, in the High
Plains region. The sounding is unstable and moist
and the cloud develops very quickly in response to
the initiating perturbations. Condensation first occurs
at 11 min. .
Case I is run with all microphysical processes ac-
tivated. Case 2 is identical to Case I except for the
absence of the snow field. Case 3 is identical to Case
I but with rain autoconversion turned off, consistent
with observations for the region which show the col-
lision-coalescence process is rarely active in the High
Plains region.
Figs. 6a and 6b display portions of the results for
the three cases. The figures depict an outline of the
cloud and precipitation evolution. At 21 min (10 min
after initial condensation), rain (plotted if> 10-3 g
g-l) first appears in the middle level of the cloud in
Case 2 (no snow case) (Fig. 6a). The rain is formed
initially by autoconversion of cloud water. For Case
1, rain has also formed by autoconversion, but is less

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FI(:!;. 5. The 2100 GMT atmospheric sounding for Miles City, Montana, on 21
July 1976. The horizontal winds applied in the model are also shown, with a single
barb representing I m S-1 and the flag 5 m S-I.
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