<br />NOVEMBER 1983
<br />
<br />HEGGLI ET AL.
<br />
<br />1885,
<br />
<br />~ -20
<br />~ -15
<br />::l
<br />~ -10
<br />ffi -5
<br />0..
<br />~ 0
<br />I-
<br />
<br />
<br />/\
<br />
<br />.2 ,2
<br />
<br />14
<br />
<br />12
<br />
<br />II 10 9 8 7 6 5 4
<br />TIME AFTER 700 mb, TROUGH PASSAGE (HRS,)
<br />
<br />2
<br />
<br />I~
<br />I
<br />I
<br />
<br />FIG. 12. Distribution of LWCjICC > 10 f.Lg per crystal shown in time section as a relative
<br />frequbncy of in cloud observations relative to the 700 mb trough passage time.
<br />
<br />
<br />
<br />perature zones, 0 to -5 oc Jereas ice crystals genernlly less io postfrontal segments than preftontal segments
<br />dominated the cloud phYs~cS at lower temperatures. of storms. The findings in the Sierra Nevada closely
<br />In particular echoes were t)jpically associated with ice follow the findings of Hobbs. As well, Lamb et al.,
<br />crystal concentrations up ~o an order of magnitude (1976) showed in Sierra storms that microphysical
<br />greater than concentrations lobserved in area-wide and seeding opportunities were generally related to intense
<br />cellular echo types at temperatures less than -50C. convection and that the greatest amounts ofLWC were
<br />Liquid water maxima w~re found near the freezing found 40-75 km upwind and to the west of the Sierra
<br />level irrespective of PET. ],his translated to a L WC/ Nevada crest. Our findings concur with those of Lamb
<br />Ice ratio> 10 ILg per crystal, having more than a 50% et al.
<br />observation frequency at terhperatures around OOC for This research has determined from aircraft obser-
<br />all three echo combination~. L WC maxima were gen- vations the distribution of water and ice within Sierra
<br />erally found 40-120 km west of the crest; beyond 120 Nevada winter storms. It is evident that these properties
<br />km, effects of the mountaid are unnoticeable. The re- are dependent upon storm conditions. Nevertheless,
<br />lease of convective instability or the forced lifting of the information clearly indicates that the cellular cloud
<br />the airmass over the barribr starts to introduce in- type represents, at least from the microphysical view-
<br />creasing amounts of L wd which are rapidly trans- point, the best candidate for a snowfall enhancement
<br />ported to supercooled environs. Between 40 an.d 120 project. The SCPP has used these results in part to
<br />km west of the crest, ice crrstals begin to form and design SCPP-l, a randomized CO2 experiment on the
<br />grow at the expense of the Lwc. As the precipitation cellular PET. '
<br />process continues, ice crys4U multiplication probably
<br />becomes a dominant fact0I1, causing a large decrease
<br />in L WC/I CC ratios as the ait moves closer to the crest.
<br />The barrier at 40 km roughly intersects the freezing
<br />level on average storm dayst so that east of this region
<br />the introduction of additional liquid water by con-
<br />vection is limited. The de~letion of the liquid water
<br />by accretion also probably lowers L WC/ICC ratios at
<br />distances closer than 40 kcl to the crest.
<br />, Large relative frequencies ofLWC/ICC > 10 ILg per
<br />crystal were well displaced Ifrom the time of the 700
<br />mb trough passage. These I were first noticeable 6 h
<br />after the trough passage, with the largest relative fre-
<br />quencies found 7 to 10 h i after the 700 mb trough
<br />passage. The available vertical depth for convection,
<br />I
<br />bounded below by the group.d and above by the front,
<br />appears to be an extremelyl important feature for the
<br />growth of clouds.
<br />The general conclusions reached in this study agree
<br />with previous work conducttd on west coast orographic
<br />storms. Hobbs (1975a,b) fobnd that ice particles dom-
<br />inate over liquid water inl prefrontal conditions in
<br />storms over the Cascade mountains of Washington
<br />State, and also found that i;atios of ice to water were
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<br />3
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<br />Acknowledgments, The authors wish to thank the
<br />many participants of the SCPP who made this study
<br />possible. Particular thanks are given to the University
<br />of Wyoming, Department of Atmospheric Science for
<br />collecting the aircraft data and providing comments.
<br />The concept ofLWC/ICC ratio was partially developed
<br />during discussions with Dr. John Marwitz. The authors
<br />appreciate the diligence exhibited by the typist J. Dut-
<br />ton and the drafting which was done by K. Dreher.
<br />
<br />REFERENCES
<br />
<br />Cooper, W. A., 1978: Cloud physics investigations by the University
<br />of Wyoming in HIPLEX 1977. Dept. of Atmos. Sci., University
<br />of Wyoming, 320 pp.
<br />-, and J. D. Marwitz, 1980: Winter storms over the San Juan
<br />Mountains. Part III: Seeding potential. J, Appl, Meteor,. 19,
<br />100-107.
<br />-, and C. P. R. Saunders, 1980: Winter storms over the San Juan
<br />Mountains, Part II: Microphysical processes. J, Appl, Meteor.,
<br />19, 927-941.
<br />Hobbs, P. Y., 1975a: The nature of winter clouds and precipitation
<br />in the Cascade Mountains and their modification by artificial
<br />seeding. Part I: Natural conditions. J, Appl, Meteor" 14,783-
<br />804. '
<br />- -, 1975b: The nature of winter clouds and precipitation in the
<br />Cascade Mountains and their modification by artificial seeding.
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