WMOD00402

• Project Name: Sierra Cooperative Pilot Project
• Title: Sierra Cooperative Pilot Project - Final Report for Period November 1984 - September 1986
• Date: 6/1/1987
• Weather Modification - Doc Type: Report

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1.15

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Figure 1.8 showe the various calculated quahtities from the model when
the depth of the convective field is 250 m, the wind velocit~r is 0.7 m s-l,
the liquid water content at the top of the convective layer is 0.1 g m-3
and there is no temperature difference between the air and tile sn~~ surface.
For a temperature at the top, chosen to be -50C, the model rE!sults show that
the snow surface temperature would be -2.80C and the depth oj: the :surface
layer is about 42 m. The cloud base in the plume is about 178 m above the
base of the convective layer. Since the moisture content inc]~eases with time,
this results in the cloud base in the surrounding downdraft being i!lbout 2 m
higher than in the plume. The latent hea,t released due to tile condensation
of water vapor within this, 2 m interval in the plume, remembering that the
vapor is still undersaturated within this interval in the downdraft, leads
to a sudden increase in the virtual temperature excess betwel!n the updraft
and the downdraft. This then leads to an increase in buoyancy for,ce and
hence an increase in updraft velocity and, due to the continuity oondition,
a decrease in plume radius in the cloud layer. At all heights the turbu-
lent intensities are almost constant and the same inside and outside the
plumes.

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In another example the convective Held is 450 m deep, the wind velo-
city is again 0.7 m s-l, the liquid watex~ content at the top bound.ary is
0.05 g m-3, and the cloud-top temperature is chosen to be -l!SoC. The tem-
perature difference between the bottom of the convective layer and the snow
surface is again zero. The model in Figt~e 1.9 gives a snow surface tem-
perature as -10.70C and the depth of the surface layer as 76 m. The plume
cloud base is at about 392 m above the bottom of the convective field or
468 m above the surface, which is 1.7 m lower than in the downdraft. The
structures of the physical properties shclW the similar charalcteristics as
in Figure 1,8.

The calculated vapor flux is about 2.3 g m-2 hr-l for the first case
(Fig. 1.8) and 2.2 g m-2 hr-l for the second case (Fig. 1.9). This vapor
flux is a function of the vapor mixing ratio difference between the satu-
rated air at the snow surface and the air at the bottom of the convective
layer. The vapor flux is positive when t:he saturation vapor mixing ratio
over snow is higher than in the circulating air.

The average temperature in the 250 Dl deep convective layer in the first
illustration is about -3.90C. It takes about 32 hours, with 2.3 g' m-2 hr-l
vapor flux from the snowy surface, to raise the average relative humidity
of the convective layer from 80% to 90%. It takes about 31 hours in the
second case, where the average temperatw~e is about -12.90C, to moisten the
450 m deep convective layer air from 80% to 90% in average relative humid-
ity. Thus while this process does slowly change the air mass, these changes
are very slow, and it is the circula.ting updrafts and their ability to
carry airborne particles that are likely to be of importance in most cases
of interest.