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<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 <br />I <br /> <br />" <br /> <br /> <br /> <br />3 <br /> <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. <br />