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An Airflow Case Study Over the San Juan Mountains of Colorado
Abstract
On 12 February 1973 an airflow case study was documented across the San
Juan Mountains in south-west Colorado. The main observation system was
an NCAR Queen Air aircraft. Several supplementary observations were
available from the weather modification project being conducted in the
area. The airflow data were synthesized and compared with previous
laboratory simulation results over the same area. The orographic cloud
contained a number of imbedded convective clouds which had an important
effect on the airflow and vertical diffusion processes. A precipitation
efficiency was derived using a technique which avoided most of the
critical assumptions of previous attempts.
... The effectiveness with which clouds perform this removal function depends both on the amount of air processed and on the particular set of microphysical processes responsible for cloud and precipitation formation. Determination of absolute removal efficiencies, as when trying to estimate precipitation efficiency, is particularly difficult because of the large uncertainties associated with measuring the fluxes of air through storm systems (Marwitz, 1974;Scott, 1982;Lesins and Lin, 1986;Rutledge et al., 1986). However, no matter how complicated or uncertain the kinematic structure of a storm may be, the air being processed simultaneously carries both the water vapor, from which the cloud particles and precipitation form, and the other trace substances in given proportions. ...
Lamb, D. and Chen, J.-P., 1990. A modeling study of the effects of ice-phase microphysical processes on trace chemical removal efficiencies. Atmos. Res., 25: 31-51. A simple cloud-chemistry model has been developed to study the effects of specific microphysical processes of precipitation formation and scavenging on the efficiency with which clouds remove trace chemicals form the atmosphere. The model is designed to let an isolated precipitation particle fall through the mixed-phase zone of a steady-state background cloud derived from the ascent of a Lagrangian parcel in which given mixing ratios of SO2, CO2 and NH3 are in equilibrium with the aqueous phase. The precipitation particle is allowed to acquire water mass by both vapor deposition and accretion of supercooled cloud water and solute content by gaseous entrapment in rime ice, as parameterized from prior laboratory experiments. The concept of a relative removal efficiency has been employed to identify the important microphysical fractionation processes within the cloud. The model calculations show that the removal efficiencies of the chemical constituents in the cloud nuclei and soluble trace gases are controlled largely by the accretional growth process. Even with modest solubilities, a trace gas such as SO2 is estimated to be removed much more efficiently than is water vapor via cold-cloud precipitation because of substantial entrapment of the gas during rime-ice formation.
... In addition to altering the microphysical processes of the orographic cloud system, the convective component may also function as a transport mechanism for seeding material which is dispersed from groundbased generators. This transport mechanism may be a help or a hindrance to successful cloud modification (Rhea, ~ al., 1969;Elliott, ~ al., 1971;Marwitz, 1974Cooper and Saunders, 1976). As is suggested above the presence or absence of the convective component strongly affects the potential for static and dynamic modification. ...
Includes bibliographical references. Sponsored by National Science Foundation FACFATMS100003BLUE
The winter orographic storms over the San Juan Mountains and the Sierra Nevada are compared. The topography of the San Juans is complex while the Sierra barrier is comparatively simple. The barrier jet is well developed upwind of the Sierra Nevada and its development is restricted upwind of the San Juans. The major difference between the storms on the two barriers is that the Sierra Nevada storms are typically maritime while the San Juan storms are continental. The implications for seeding are discussed.
When starting to read a chapter such as this, I have difficulty discerning where the author wants to lead. Usually, there are some generalities (this chapter describes some physics of wet and dry removal -- for chemists), some restrictions (there is insufficient space, here, to cover all the details), and some hazy outline of the approach (because of these space limitations, a deductive approach will be taken). Then, the author usually attempts to draw a road map with words. However, for me, the phrase “then, in the next section” has a definite mesmerizing quality. Some of the fault, I think, lies with editors and publishers who refuse to let the author list section and subsection headings. After all, that is what the author is trying to do; and for me, it’s so much easier to review the list, later, when I’m totally lost. Fortunately in the present case, the editors have been more receptive to rational arguments(!), and therefore I invite the reader to glance at the Table of Contents, now, to gain a glimpse of the chosen route.
ABSTRACT Precipitation efficiency is a single parameter for estimating the precipitation production ability of atmospheric moisture release mechanisms. Analysis of twenty cyclonic storms occurring in the eastern United States revealed distinct precipitation efficiency patterns with highest efficiency occurring in the cold sectors and at the cold fronts. These patterns can be explained by consideration of storm dynamics. The results for all storms were combined to produce a model of the precipitation efficiency distribution around cyclonic storms. The model allows variations in the pattern and efficiency for storms of differing size, age, rate of movement, and intensity, while topographic and advective effects can be estimated. Two storms demonstrate the applicability and limitations of the model, which should not be applied to individual storms, but can be used for a series of storms to give long-term estimates of rainfall amounts.
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