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

1.14

1.

The Moisture-Driven Convective Field OVer Snow.

The moist convective plume model of Chai and Telford (1983) is used
here to demonstrate the moisture-driven convection over a snow-covered sur-
face under a temperature inversion. The entrainment of overlyinq inversion
air into the convective layer is not taken into account in the model since
we postulate that when instability at the layer top occurs, the entrainment
is very rapid until the inversion is aqain stable. Hence the time for this
staqe is so short that it can be neqlected compared to the time needed for
convective chanqes from the surface. We should note that the effect of en-
trainment is important on droplet spectra development and on the resultinq
temperature and mixinq ratio of the convective layer. However, in our
plume model the values of temperature and mixinq ratio (or the liquid water
content) are qiven at the top boundary. Therefore, the entrainment effect
will be reflected on those qiven values of the meteoroloqical cloud-top
parameters. Since the boundary layer is under entrainment-free conditions
durinq the majority of the time, our model can be used to represent the
boundary layer, between entrainment episodes, which is sufficient for the
whole process if entrainment occurs rapidly by suddenly imposinq rapid mix-
inq at appropriate, well-spaced intervals. We assume that the earth is a
heat reservoir and the heat can be conducted so effectively that the eva-
poration of, snow at the surface will not chanqe the snow-surface temperature.
We also assume that the condensation at the top of the layer produces
super-cooled water only and sublimation is not initiated to form ice.

This model consists of two reqimes, a plume convective layer and a
surface layer. From requirements due to the conservation of volume and
kinetic energy, the depth of the surface is about one-half of the plume
radius. Above the surface layer, the buoyant turbulent elements become
orqanized to form plume convection. The model consists of ten conservation,
equations, five for the plumes and five for the surroundinq downdrafts.
These equations describe the quantities such as volume, total mass, mass
of water substances, vertical momentum and turbulent kinetic energy (Chai
and Telford, 1983 and Telford and Chai, 1984).

The controllinq parameters of the field are: the heiqht of the con-
vective layer, the wind velocity, the temperature difference between the
bottom of the convective layer and the snow surface and the liquid water
content at the top of the convective layer. Given the values of these
controllinq parameters toqether with the temperature at the top boundary,
and applyinq appropriate boundary conditions (detailed descriptions of the
boundary conditions can be found in Chai and Telford, 1983), the conser-
vation equations can then be inteqrated to find the vertical structures
of the meteoroloqical parameters in the convective field.

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