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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1.13

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Thus the SP6 tracing program has proved to be a viable operating scheme
and the initial results have been of interest in that, on very convective
days with little wind the SF6 was found only within about a mile of the
release point. When wind is present the SF6 is found several hund:red feet
above the ground in convective conditions but in a comparatively n!rrow
band. The SF6 flowed around the 1500 ft hill we investigated but <experi-
enced a clear change in direction after passing the hill, pol!sibly due to
the wind funneling down a valley upstream.

(c) The Study of Convection over a Snow-'Covered Surfaces

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The convective transfer of seeding agents into clouds is a subject
of continuing concern. While seeding winter orographic cloucis, t.h,e qround-
released seeding materials can be carried up into the clouds if th,e plane-
tary boundary layer has an upward heat flux from the surface. However,
cloudy conditions need to be considered. While the sky is co'lTered by clouds
the sun has little effect on heating the snowy surface to drive thermal
convection. Since, however, water vapor is less dense than dry air, the
evaporation of the snow can initiate vapclr-driven convection and such a
mechanism may be important.

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It has been demonstrated by Telford and Chai (1984) that convection
occurs over oceans at a significant rate, even without surface heating to
drive the process. When the interface temperatures for air and water are
the same this process can, still generate and maintain stratui!l cloud layers.
In these conditions the evaporation from the surface increases the water
vapor in the air so that the air leaving the surface is moister and less
dense than the descending air which replaces it. This process gives buoy-
ant motion which can moisten a layer several hundred meters 'thick in a few
hours, and stratus clouds can form at itsl top, which also leclds to a tem-
perature inversion being formed at cloud top.

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It seems unlikely that this form of convection driven ~1 water vapor
would make much of a contribution when the sun is available to heat the
surface, since the velocities are only some tens of centimeters per second,
compared to a meter per second or so found when sunshine is present over
land. However, over the sea, the sun does not increase the temperature of
the water surface appreciably on the timEI scale of a few hou:rs as it does
over land, so this water vapor process dc)minates.

Under overcast skies the sun simila:dy has little effect in heating
a snow surface. Thus the evaporation from snow, on snow-covered surfaces,
may be significant in providing convecticm to disperse pollutants, and this
mechanism may also provide a way for ground releases of seed.ing agents to
be carried upwards in the absence of the!~l convection.