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<br />Reprinted from JOURNAL OF CLIMATE AND ApPLIED METEOROLOGY, Vol. 22, No.6, June 1983
<br />American Meteorological Society
<br />Printed in U.S.A.
<br />
<br />Bulk Parameterization of the Snow Field in a Cloud Model
<br />
<br />YUH-LANG LINI, RICHARD D. FARLEY AND HAROLD D. ORVILLE
<br />Institute of Atmospheric Sciences. South Dakota School of Mines and Technology. Rapid City 57701
<br />(Manuscript received I December 1982, in final form 5 February 1983)
<br />
<br />ABSTRACT
<br />
<br />A two-dimensional, time-dependent cloud model has been used to simulate a moderate intens~ty th~n-
<br />derstorm for the High Plains region. Six forms of water substance (water vapor, cloud water, ~Ioud Ice, ram,
<br />snow and hail, i.e., graupel) are simulated. The model utilizes the "bulk water" microphYSIcal par~met~r-
<br />ization technique to represent the precipitation fields which are all .assumed t? .follow exponentIal sIze
<br />distribution functions. Autoconversion concepts are used to parametenze the colhslOn-coa1escence and col-
<br />lision-aggregation processes. Accretion processes involving the various forms ofliquid and so~id hydromet~ors
<br />are simulated in this I model. The transformation of cloud ice to snow through autoconverslon (aggregatlO~)
<br />and Bergeron processes and subsequent accretional grov:th or a~~~tion to. form ha~l are simulated. ~all
<br />is also produced by various contact mechanisms and vIa probablhstJc freezmg of ral.ndro~s. ~vaporatlOn
<br />(sublimation) is con~idered for all precipitation parti~les outside ~he clou~. The mel~mg of .hall and snow
<br />are included in the model. Wet and dry growth of hall and sheddmg of ram from hall are SImulated.
<br />The simulations show that the inclusion of snow has improved the realism of the results compared to a
<br />model without snow: The formation of virga from cloud anvils is now modeled. Addition of the snow field
<br />has resulted in the inclusion of more diverse and physically sound mechanisms for initiating the hail field,
<br />yielding greater potential for distinguishing dominant embryo types characteristically different from warm-
<br />and cold-based clouds.
<br />
<br />1. Introduction
<br />
<br />The fact that ice particles play an important role
<br />in the formation of precipita:tion is firmly established,
<br />although details of ice formation and growth pro-
<br />cesses in clouds are poorly understood. Detailed
<br />knowledge of iceprocesses is complicated by the va-
<br />riety of nucleation mechanisms which may initiate
<br />the ice phase, the multitude of shapes and forms of
<br />the ice particles themselves, and the often complex
<br />nature of their motions. In attempting to bring order
<br />to the multiplicity of ice forms, several ice particle
<br />classification schemes have ,been proposed over the
<br />years; in general, ice particles may be grouped in!o
<br />four main classes: ice crystals, snow, graupel and had.
<br />The snow, graupel and hail: particles possess appre-
<br />ciable terminal velocities and thus fall relative to the
<br />air, and may be termed precipitating ice particles.
<br />Nearly 50 years ago, Bergeron (1935) theorized that
<br />precipitation formation almost invariably required
<br />the presence of ice particles, except in special,situa-
<br />tions. This theory is based. on the realization that
<br />water drops and ice crystals cannot coexist in equi-
<br />librium at subfreezing temperatures due to the fact
<br />that the saturation vapor pressure over ice is less than
<br />
<br />I Present .affiliation: Graduate St~dent, Department of Geology
<br />and Geophysics, Yale University, New Haven, CT 06520.
<br />
<br />! @ .1983 American Meteorological Society
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<br />that over water (Wegener, 1911). Bergeron suggested
<br />that ice crystals in supercooled clouds grow by vapor
<br />diffusion at the expense of the supercooled water
<br />drops until either all of the water drops are consumed
<br />or all of the ice has fallen out of the supercooled
<br />regions of the cloud. Findeisen (1939) provided sup-
<br />port to Bergeron's ideas and the theory has been re-
<br />ferred to variously as the Bergeron, Bergeron-Fin-
<br />deisen, or Wegener-Bergeron-Findeisen process; in
<br />this paper, we shall use the term Bergeron process.
<br />Observations at middle and high latitudes, begin-
<br />ning in the era of the Thunderstorm Project (Byers
<br />and Braham, 1949), have provided considerable sup-
<br />port for the importance of ice processes to precipi-
<br />tation formation in summertime convective clouds.
<br />Additional supportive observational studies have in-
<br />cluded Kuettner (1950), Project Whitetop (Koenig,
<br />1963; Braham, 1964), Dye et al. (1974) and Hallett
<br />et al. (1978), where the degree of sophistication in the
<br />observations has increased dramatically with time.
<br />These studies have established the role of capture
<br />mechanisms (riming) in the subsequent growth of ice
<br />particles after attaining certain size thresholds through
<br />diffusional growth. The riming size threshold varies
<br />with crystal habit (Hobbs, 1974) but, once attained,
<br />quickly dominates particle growth. The density ofthe
<br />rime deposit has been determined experimentally to
<br />be a function of temperature, water drop size and
<br />impact velocity (Macklin, 1962; Pflaum andPrup-
<br />
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