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<br />iDECEMBER 1978
<br />
<br />LARRY VARDIMAN AND JAMES A. MOORE
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<br />1771
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<br />3. Study procedures
<br />
<br />L
<br />I,
<br />
<br />r.
<br />
<br />This study considered many meteorological variables
<br />that depend on the shape and size of the mountain
<br />barrier and on the characteristics of the clouds. The
<br />variables were grouped into four major physical
<br />categories: 1) the time available for nucleation, growth
<br />and fallout of the precipitation, 2) the water available
<br />for conversion to precipitation, 3) the natural nuclei
<br />available to start the precipitation process, and 4) the
<br />atmospheric mixing available to carry seeding material
<br />to the appropriate portion of the cloud,
<br />Using variables representative of these physical
<br />categories, stratifications were performed by project
<br />and groups of projects to determine seed/no-seed ratios
<br />and two-sided P values which suggest precipitation
<br />changes which might be attributed to seeding. A non-
<br />parametric test (the two-sample Wilcoxon test) was
<br />_ used since it required no prior assumptions about the
<br />distribution of the data (Duran and Mielke, 1968).
<br />Each of the variables was stratified to optimize positive
<br />and negative seeding effects. Many variables were calcu-
<br />lated from project sounding information but only four
<br />were chosen for intensive study. A detailed description
<br />of the four variables selected follows.
<br />The barrier trajectory index (BTI), a measure of the
<br />time available, is the difference in seconds between the
<br />time required for a snow crystal- to travel from the
<br />average generator location to the ridge crest and the
<br />time required for a crystal to fall from the cloud top
<br />to the height of the ridge crest, assuming a fall velocity
<br />of 0,5 ms-I. When BTI is zero, snow should fall on the
<br />~idge crest; when it is positive, snow should fall upwind,
<br />and when it is negative, snow should fall downwind
<br />of the crest.
<br />The saturated mixing ratio at cloud base (CBWS),
<br />a measure of the water available, indicates the maxi-
<br />mum amount of water vapor (grams per kilogram of
<br />dry air) the air can hold at the temperature and pres-
<br />
<br />sure of the cloud base. .Normally, high values of CBWS
<br />indicate warm, usually low, cloud bases.
<br />The lifted cloud-top temperature with respect to ice
<br />(LCTTI), a measure of the nuclei available, is an
<br />estimate of the coldest temperature in a cloud. LCTTI
<br />is an index of the concentration of ice crystals active in
<br />the precipitation process because of the relation between
<br />cloud temperature and nucleation of ice crystals. The
<br />greatest concentration of ice crystals is normally nu-
<br />cleated at cloud top where the temperature is coldest.
<br />Cloud-top temperature was established by locating the
<br />lowest level near the top of the cloud where the vapor
<br />pressure was less than the saturation vapor pressure
<br />with respect to ice. Checks were made to insure that
<br />no cloud existed within 50 rnb above, Lifting occurred
<br />by assuming a normal laminar flow over the barrier
<br />and applying a lifting routine developed at North
<br />American Weather Consultants (Elliott, 1977). No
<br />distinction was made between stable and unstable
<br />soundings for the calculation of LCTTI.
<br />The positive energy area (EPOS) represents buoyant
<br />energy (J g-I) and is a measure of the stability of the
<br />cloud or the potential for vertical motion and mixing
<br />in the cloud. It is determined from the sounding data
<br />plotted on a thermodynamic diagram (specifically, a
<br />skew T-Iog p diagram), and is the area between the
<br />level of free convection and the equilibrium level. In
<br />general, the larger the value of EPOS, the less stable
<br />the cloud. More vertical mixing of seeding material is
<br />expected with larger values of EPOS, During the
<br />calculation of EPOS, criteria were used whereby any
<br />negative energy area'50 mb thick or more would serve
<br />to cap the convection. No attempt was made to lift or
<br />heat a parcel to release potential instability. EPOS was
<br />used simply as an index of stability.
<br />
<br />4. Single-variable stratifications
<br />
<br />Variable means for the no-seed cases were calculated
<br />and are shown in Table 2. Bridger and Climax have the
<br />
<br />TABLE 2, Cases used from each project in the study and project means for barrier trajectory index (BTI), cloud-base saturation mixing
<br />ratio (CBWS), lifted cloud-top temperature with respect to ice (LCTTI) and the positive energy area (EPOS),
<br />
<br />r Project
<br /> Pyramid Santa
<br />1 Bridger Climax San Juan CENSARE Jemez Lake Barbara
<br />I
<br />I Cases 135 420 306 118 173 96 111
<br /> Time available
<br /> BTI (s) - 2855 1948 305 885.2 2634 -497 -1448
<br /> Water available
<br /> CBWS (g kg-I) 3,05 3.53 3,93 6,14 3,83 6,06 . 8,13
<br /> Nuclei available
<br /> LCTTI (OC) -23,3 -22,7 - 28.4 --16.5 -21.1 -18,0 -9,8
<br /> Mixing available
<br /> EPOS (J g-I) 0,0059 0,0406 0.0528 0.056 0.0516 0.112 0.138
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