WMOD00509

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

3. Some characteristics of the storms

The following summarize some results from the physical
studies, as reported in Hobbs et ale {1975}, Marwitz {1980},
Cooper and Saunders {1980}, and Cooper and Marwitz {1980}:
~ Ice concentrations were freauently hiqh. generally
much higher than could be explained by nucleation or known
ice multiplication mechanisms. About half of the cloud
regions examined by the aircraft had ice concentrations
>20/L. The major exceptions were the cases of cumulus clouds
or regions of embedded convection.
~ Liauid water contents were relativelY low,
especially in areas far enough upwind of the barrier to
appear "seedable". Typical maximum liquid water contents
encountered over at least 1 km were ~0.1 g/m3 {stable}, 0.3
g/m3 {neutral}, and >0.5 g/m3 {unstable stage}.
~ Storms evolved from stable to unstable stages, and
liquid water contents increased during this evolution.
~ The stability was best characterized Qy potential
or convective instability. not conditional stability. It was
often the case that these two measures of stability were out
of phase. Early storm stages were often periods of high
conditional stability, but because of the absence of a
surface heat source or other means to release the instability
and because the low-level flow was often parallel to or away
from the barrier, this instability was not released. Such
periods were generally characterized by potential stability.
Conversely, the later storm periods were generally periods
when potential instability could be released because of
airflow toward the mountain and associated lifting, but it
was common that such layers were conditionally stable upwind
of the barrier. This has led to much confusion in the
differing claims of different groups: Elliott et ale {1978}
used a parameter based on conditional instability, as did
Hjermstad {1975}, while Marwitz {1980} used potential
instability.
~ Precipitation mechanisms in the storm chanqed from
predominantly diffusional growth in the early stable stage to
mixed diffusional and accretional growth in the later
unstable stages.

4. Considerations of seedability

Cooper and Marwitz {1980} considered the seedability of
these storms in relationship to the needs for a practical
seeding experiment. Supercooled water is needed in an area
where it can be converted to precipitation falling on a
desirable target, in meaningful amounts, and there should be
a way to recognize and/or forecast seedable conditions if
they are limited. supporting evidence for seedability is the

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