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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1.17

2. The Effect: of the Lower Saturation Vapor Pressure OVer
Ice than Over Water on the Vapor-Driven Convectiolt.

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The moist plume model showed that evaporation of snow CClln dri1i7e con-
vection and maintain a cloud layer without the assistance of heat nux.
However, a peculiar condition is possible because of the different satura-
tion vapor pressures associated with ice and supercooled watE!r. The vapor
pressure is just saturated at cloud base. Since we are dealJ.ng with a
super-cooled water cloud when the cloud first forms, the vapor preuure at
cloud base is the saturation vapor pressure at cloud-base temperatl~e with
respect to water. The turbulence is weak and the vapor flux is mNlll, as
shown in the illustrations in the previous section. The downdraft will
thus be nearly adiabatic and the vapor mixing ratio constant with height
beneath the cloud. Therefore, the water vapor mixing ratio In the cir-
culating air at the bottom of the convective field is approximatel~( equal
to the saturation vapor mixing ratio with respect to water at clow! base.

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The saturation vapor pressure and vapor mixing ratio over ice, are
lower than that over water, and the air at the snow surface is satll1rated
with respect to ice. Thus, under sub-zero temperatures, the liquid. water
at cloud base must be cooler than the surface, and so these c;:ondit:Lons can-
not occur with the cloud base lower than a certain height. ~~en ~~e cloud
base is lowered to this limiting height, the mixing ratio of the s,ilturated
cloud base air is equal to the saturation mixing ratio over the snl:)w on the
surface and the required surface evaporation ceases, and so does the con-
vection.

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This condition is illustrated in Figure 1.10. The dot-dashed lines in
this figure show the relationships, at different cloud base height, between
the snow-surface temperature and the difference between the saturation mix-
ing ratio over the snow surface and the d.owndraft vapor mixing ratio at the
snow surface temperature there is a limiting cloud base height bel,:)w which
. the vapor flux at the surface is zero or negative. TherefOrE!, the cloud
base cannot be lowered below this limiting height. For example, if the
snow surface temperature is -lOoe, the lowest cloud base height at which
the saturated water vapor mixing ratio is less than at the snow covered
surface is about 150 m. As the cloud base lowers to 150 m, the 'rapor flux
reduces to zero and the vapor-driven convection stops.

Thus when the snow surface is cooler than oOe, the vapor-driven con-
vection can only occur when the cloud base is above a certaill limiting height,
below this limit the usual convective structure with buoyant plumes driven
by water vapor cannot exist, although forced convection due to she,ar is
possible, and this limiting height increases with decreasing snow-surface
temperature. Thus, beyond this limit convection can take place only when
some other mechanism is working, such as advecting over a warmer snow sur-
face or entrainment cooling by evaporation of wind borne snow particles at
the top of the layer, or by cooling at cl.oud top by radiation or entrainment.