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<br />:Of '~
<br />
<br />the case during the King Air flight on lS DecE~-
<br />ber. These clouds have the most supercooled ~Ta-
<br />ter and the least ice crystal concentrations (If
<br />all clouds over the Sierras.
<br />
<br />Downwind of the post-frontal COnVE!C-
<br />tive cloud walls and yet upwind of the crest the
<br />clouds contained light turb'ulence, were essen..
<br />tially free of supercooled water and contained
<br />moderate concentrations of ice crystals.
<br />
<br />The post-frontal convective cloud
<br />walls are the purest form of orographic clouds
<br />in that the upwind valley is clear, there is no
<br />cirrus and downwind of the Sierra crest the
<br />clouds dissipate. It is clear that under these
<br />conditions, were it not for the Sierra Nevadas
<br />there would be no clouds.
<br />
<br />4.
<br />
<br />STORM MICROPHYSICS
<br />
<br />One of the significant microphysi<:al
<br />observations was the presence of high liquid wa-
<br />ter content in association with post-frontal
<br />convection over the foothills area. Values up
<br />to 4.0 gm/m3 were measured for droplets up to
<br />45 pm in diameter in this convection at eleva-
<br />tions from 2500-3500 m MSL. In contrast, the
<br />stable pre-frontal situation typically contained
<br />water contents less than 0.5 gm/m3 These re-
<br />sults agree qualitatively with the findings of
<br />Lamb et al. (1976) and Rangno et al. (1977) over
<br />the Sierras, and with the results obtained by
<br />Marwitz et al. (l976) in flights over the San
<br />Juan Mountains of southwestern Colorado. How-
<br />ever, in this latter case, peak values of liquid
<br />water content were normally less than 1.0 gm/m3.
<br />
<br />The droplet size distribution also
<br />depended upon atmospheric stability. In convec-
<br />tion, the distributions were nonnally bimodal
<br />with peaks typically occurring between 3-6 pm
<br />and 25-35 ~m. In more stable conditions monomo-
<br />dal distributions were observed with peaks typ-
<br />ically occurring between 8 and IS ~m. Again this
<br />observation is qualitatively in agreement with
<br />results obtained over the San Juans by Marwitz et
<br />al. (1976). See Walsh (1977) for the most up-to-
<br />date cloud droplet data on San Juan storms.
<br />
<br />Ice crystal concentrations in post-
<br />frontal convection over the foothills were low
<br />with concentrations often being less than lOt
<br />liter. A consequence of the few ice crystals
<br />and the abundance of cloud droplets in post-
<br />frontal convection was that on a few occasiotl.s
<br />liquid water contents as high as 2 gm/m3 were
<br />being observed while the radars would only be
<br />observing intermittent small echoes. In morE,
<br />stable conditions, concentrations were much
<br />higher and lOO/liter wele sometimes observed at
<br />these same locations.
<br />
<br />Atmospheric ~tability also affected .
<br />the crystals observed near the Sierra crest by a
<br />ground observer and by the instruments aboard: the
<br />aircraft. The more unstable conditions produced
<br />rimed ice crystals, and sometimes aggregates and
<br />graupel, as opposed to more stable conditions
<br />which caused less riming and a larger fraction
<br />of dendrites to fall on the ground.
<br />
<br />1;0 ~.:
<br />
<br />These few preliminary ob:servations
<br />reveal that the microphysical nature of Sierra
<br />winter storms is dependent upon atmospheric sta-
<br />bility. Moreover, since a storm traversing the
<br />mountains may evolve through several stability
<br />changes, precipitation processes may undergo
<br />marked changes even within a single storm.
<br />
<br />S.
<br />
<br />SEEDABILITY
<br />
<br />The preliminary.indication is that the
<br />greatest seeding potential occurs in association
<br />with the convective clouds. The convective clouds
<br />have the highest updrafts and hence highest con-
<br />densation rates and hence greatest supercooled
<br />liquid water contents. They also seem to have the
<br />lowest natural ice crystal concentrations compar-
<br />ed to stable clouds at comparable temperatures.
<br />
<br />6.
<br />
<br />REFERENCES
<br />
<br />Lamb, D., K. W. Nielsen, H. E. Klieforth, and
<br />J. Hallett, 1976: Measurements of liquid
<br />water content in winter cloud systems over
<br />the Sierra Nevadas. J. Appl. Meteor., IS,
<br />763-775.
<br />
<br />Marwitz, J. D., W. A. Cooper and C. :P. R. Saun-
<br />ders, 1976: Structure and seedability of
<br />San Juan storms. Report No. ASl18, Depart-
<br />ment of Atmospheric Science, University.of
<br />Wyoming, Laramie, Wyoming, 329 pp. .
<br />
<br />Moore, J. A., 1978: A microphysics and synoptic
<br />study of the 14-IS December, 1977 Sierra
<br />storm. Proc. Conf. Sierra Nevada Meteor.,
<br />L. Tahoe, California.
<br />
<br />Rangno, A. L., P. V. Hobbs, and L. F. Radke, 1977:
<br />Tracer and diffusion and microphysical stu-
<br />dies in the American River Basin. Contribu-
<br />tions from the Cloud Physics Group, Univer-
<br />sity of Washington, Seattle, Washington,
<br />64 pp. .
<br />
<br />Walsh, P. A., 1977: Cloud droplet measurements
<br />in wintertime clouds. M.S. thesis, Univer-
<br />sity of Wyoming, Laramie, Wyoming, l70 pp.
<br />
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