WMOD00509

• Title: Physics of Winter Orographic Precipitation and it's Modification - Summary of Presentations
• Date: 10/1/1985
• Weather Modification - Doc Type: Report

inefficiency of the natural precipitation processes, e.g.
when supercooloed water flows over t:he mountain barrier or
ice concentrations are low.
In this context, it appears that the stable case is not
seedable from the ground, because blocked flow generally
prevents the seeding material from being carried into the
clouds over the mountains. This stage also looks unfavorable
for airborne seeding, because the mountain wave is close to
the mountain. This storm stage was often deep, and as a
result ice crystal concentrations were high. Liquid water
contents were low as a result of the. high ice concentrations
and the limited deflection of the airflow by the mountain.
If the blocked airflow were not present (as in the "stable
orographic" case, not observed during the studies of Marwitz,
Cooper, and Saunders), the higher deflection might lead to
higher liquid water contents, greater upwind extent of the
liquid water cloud, and higher updrafts to provide
supercooled water. However, there is no documentation of
this storm type in the San Juan area..
In contrast, the unstable storm stage appears to present
the best seeding opportunity in these cloud systems. Seeding
is possible from the ground because of the common presence of
a surface convergence zone upwind of the barrier and
approximately over the generators, which carries the material
aloft~ Ice concentrations are generally lower, because of
lower cloud tops and embedded convective elements with warm
temperature roots. Liquid water contents are much higher
because of the greater lifting and lower ice concentrations.
There is often a stable capping layer, leading to warm cloud
tops and giving these clouds the appearance of an orographic
cloud. Although ice concentrations often remain high
(>lO/L), the updrafts are more than adequate to produce
condensation faster than it can be depleted by the ice.
Trajectories leading to the target a.rea appear plausible, but
have not been supported by detailed calculations of
precipitation growth rates.

5. Some "lessons"

The studies of the CRBPP have suggested the following
lessons to be considered in planning future programs in the
CRBPP:
(a). Forecasting or recognition of seeding criteria is
far more difficult than in post-project stratification. When
future projects are designed, it would be valuable to base
that design only on the past partitions which can be
recognized in time to permit seeding. Stratifications based
on data only available after the experiment are not useful in
determining how to limit seeding to seedable cases.

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