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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1.19

When the super-cooled cloud baSEl is lower than the limiting hedght,
the moisture in the air will be supersaturated with respect to the snow
surface, and so vapor deposition will begin onto the snow. This will reduce
the vapor mixing ratio in the air, and perhaps warm the snow surface with
the latent heat released. If buoyancy from heating exceeds loss of buoyancy
from the vapor removed, as it will do if the snow does not absorb ~luch heat,
convection will begin and remove vapor until the vapor mixing ratiCI in the
whole layer again matches the saturated ice vapor pressure at the siurface
with the saturated water vapor pressnre at cloud base. In these cClnditions
the condensation latent heat driven convection will take the water vapor
out of the air layer, raise the cloud base and warm the surface until the
vapor is again just saturated with nspect to ice at the surface. 'l~us, this
process will prevent the formation of a supercooled water fog' over the snow
surface, whereas fogs commonly form c)ver the sea. An ice fog' may perhaps
form in appropriate conditions since the layer will be slightly sU'ler-
saturated with respect to ice right up to cloud base.

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Thus, if the supersaturation near a snow surface nucleates sneM cry-
stals in the air which do not fallout, the convection driven. by tiLe latent
heat released from sublimation may then bring ice crystals in,to thE! over-
head super-cooled water cloud and seed the cloud. In this ca,se thE! above-
mentioned restriction on cloud base height is removed. This is bec:ause
instead of now being saturated with respect to water at cloud base the air
is saturated with respect to ice, and subsaturated lower down" so 1~e snow
will begin to evaporate.

When a moister air layer is advected over colder snow it, will also
make the air near the snow surface supersaturated so moisture. again sublimes
onto the snow. In this case the latent heat released by the cooling will
only partly cancel the cooling due to the negative heat flux from 1~e air
to the colder snow, and the air will still cool and no convection will
result. OVer water the air would again reach saturation and condensation
would begin, and buoyant parcels could again rise and cool the air down to
surface temperature. Over snow ice crystals would need to Hll thE! air for
this process to occur. The likelihood of ice being released to modify the
process in this way is hard to assess.

Another mechanism which can also make the surface air saturatE!d is
when the snow is so loosely packed that the heat conductivity is slIl\all.
Therefore, the evaporation at the snow surface will cool the snow !surface
temperature. This will slow down the vapor-driven convection as the rate
of snow evaporation decreases and conduction of heat into the snow from
below will limit the process.