P1.4 LABORATORY GROWTH, SUBLIMATION, AND REGROWTH OF ICE CRYSTALS

Matthew P. Bailey* and J. HallettDesert Research Institute, Reno Nevada

Alexei Korolev and George IsaacMeteorological Service of Canada

1. INTRODUCTION

The habit of ice crystals in the atmosphereis not only a function of temperature and icesupersaturation but also depends on nucleationmode, fall velocity, radiation conditions, and particlehistory. Korolev et al. (1999 and 2000) concludefrom in situ observations that simple and complexpolycrystalline forms are the dominant habit in theatmosphere, typically constituting between 84-97%of crystals larger than approximately 40 :m is size.This has been confirmed by Bailey and Hallett(2002a) in laboratory ice studies at at all but thelowest ice supersaturations.

Aggregation can certainly lead to theappearance of complex forms, however Bailey andHallett (2002a and this conference) present imagesof individual laboratory grown polycrystals that arevirtually indistinguishable from many aggregatesobserved in situ, some additional examples of whichare shown in figure 1.

Figure 1. Complex polycrystalline forms grown in thelaboratory with a static diffusion chamber that do notresult from aggregation processes._________________________________________

Corresponding author address: Matthew Bailey,Desert Research Institute, 2215 Raggio Parkway,Reno, Nv 89512; emailL bailey@dri.edu.

While Bailey and Hallett observed singlepolycrystals with frequencies similar to thoseobserved by Korolev et al., the higher in situfrequencies observed at times may be indicative ofprocesses which increase the complexity of crystalpopulations, excluding riming and aggregation. Onepossible process that could accomplish this would bethe fallout and sublimation of ice crystals followed byre-uptake into clouds via updrafts with subsequentregrowth. Oraltay and Hallett (1989) and Dong et al.(1984) have demonstrated that sublimation canmodify crystal habit and lead to breakup and iceparticle multiplication. The subsequent regrowth ofthese partially sublimated crystals may lead to morecomplex habit forms depending on whether or notthe crystal has a memory of its previous orientation.To investigate this possibility, ice crystals weregrown in the laboratory, partially sublimated, andthen regrown in order to see if crystal orientation wasmodified or maintained. Both a static diffusionchamber and a dynamic diffusion chamber whichcan simulate fall velocity were used to investigatethis possibility.

2. EXPERIMENTAL METHOD

A schematic and a detailed discussion ofthe static diffusion chamber used in this study can befound in Bailey and Hallett (2002, presentation 3.3,this conference). A diffusion chamber for crystalgrowth basically consists of two plates coated withice at different temperatures and with a sufficientseparation so as to create an environment that issupersaturated with respect to ice. In a staticdiffusion chamber, the growing crystals areunventilated and the supersaturation is simply afunction of the height of the crystal in the chamberwith respect to the two ice coated plates.

Keller and Hallett (1982) describe thedynamic diffusion chamber used in this study andinvestigated the influence of air velocity on the habitof ice crystals growing from the vapor. The dynamicdiffusion chamber is a rectangular race track shapedchamber approximately 12 meters in total length thatemploys a low velocity fan to circulate air around thetrack at velocities up to 20 cm/s. On one side of thetrack, air flowing through the chamber flows over two

large pans of ice that are temperature controlled andare used to condition the air with water vapor. Onthe opposite side are two 2 meter long diffusionchamber sections hooked end to end which consistof long rectangular plates that are approximately 25centimeters in width with a separation of 2.5centimeters. In one of these sections, the plates arecoated with ice as with the static chamber previouslydescribed creating a supersaturated environment asa function of height between the two plates.

In both the static and dynamic cases,crystals are grown on thin glass filaments that arevertically suspended between the two plates. Thedynamic diffusion chamber has a supersaturationprofile as a function of height similar to that shown inBailey and Hallett (2002, presentation 3.3, thisconference). However, the effective supersaturationat the surface of a growing crystal suspended in theair flow is increased due to the ventilation providedby circulating moist air if the growth section isupstream from the other 2 meter long diffusionsection which is temperature controlled but is notcoated with ice. When the air flow is reversed sothat air flows through the dry section before reachingthe filament, the air can be made subsaturated withrespect to ice with proper temperature controlleading to sublimation of previously grown crystals.Another way to achieve subsaturated conditions is tomaintain the moist flow orientation and to increasethe air flow velocity above 20 cm/s at which timebreakthrough of unregulated upstream air occurs.This distributes warmer air between the plates and atthe position of a crystal on the filament, againresulting in subsaturated air at the position of thecrystal.

In the static diffusion chamber, theenvironment is always at least saturated with respectto ice which occurs when the top and bottom platesare made isothermal. Sublimation in this case wasachieved by directing a small flow of dry nitrogenonto a previously grown crystal via a small stainlesssteel tube inserted into the chamber from a sealedport in the chamber wall. In both the static anddynamic case, the experiments were conducted atlaboratory pressure of 850 hPa (mb).

3. RESULTS

Two growth/sublimation/regrowth cases forboth the static and dynamic diffusion chambers arepresented in the images which follow.

In series 1, crystals were grown at atemperature of -15 oC and an ice supersaturation ofapproximately F = 0.15. A flow velocity of 10 cm/swas sufficient to grow dendritic forms indicatingsupersaturation with respect to water, however, prior

to recording the sequence, subsaturated air wasintroduced to sublimate the dendrites formed on theupstream side of the flow (dendrites on thedownstream side of the filament support are visibleand are shadowed by crystals on the upstream sidereducing the effective supersaturation on that side ofthe thread which is not the region of interest). Thecrystals initially visible on the upstream side aresector plates which begin to sprout dendrites asgrowth occurs with a flow velocity of 10 cm/s in thefirst 5 frames. In the next 5 frames these crystalsare sublimated by increasing the flow velocity abovethe 20 cm/s limit at which breakthrough occurs. Thiscontinued for a several minutes until the originalsector plate kernel was reestablished. In the finalthree frames, the flow velocity is returned to a valueof 10 cm/s leading to subsequent regrowth. Dendritic forms again appear but with differentorientations. Since the dendritic growth regionrepresents an extreme in terms of growthcharacteristics which is usually exclusive of otherhabit forms in laboratory experiments, it is nosurprise that dendrites again appear. However achange in orientation of the branches has occurredwhich is likely due to minute surface structurechanges of the sector plate kernel that wereinfluenced by sublimation.

In series 2, crystals were initially grown at atemperature of approximately -8 oC and an icesupersaturation of approximately F = 0.12 acolumnar/sheath growth regime which is alsosupersaturated with respect to water. Flow velocitywas set at 10 cm/s. Thick sheaths initially grownunder moist flow conditions were then rotated to facea subsaturated reversed flow. After sublimating forseveral minutes, the flow was reversed again toprovide moist flow conditions and the crystals wererotated to face this flow. The process was repeatedthree times as seen in the images. Sheath typesreappeared each time but again with different andtypically more complex orientations.

Series 3 was obtained with the staticdiffusion at -10 oC and an ice supersaturation ofapproximately F = 0.12. This environment was alsosupersaturated with respect to water which results inmixed habits of cup-like crystals with extensivehopper patterns, sheaths, bundles of sheaths, scrollsand other complex forms. In the static diffusionchamber, the vapor field is symmetric with respect tothe filament support contrary to the ventilatedconditions of the dynamic diffusion chamber.However, defects in individual crystals lead to habitforms with very different characteristics as discussedby Hallett et al. (2002) so that uniformity

Series 1. Crystals grown, sublimated and regrown in a dynamic diffusion chamber at a temperature of -15 oC, an ice supersaturation of approximately F = 0.15, and a flow velocity of 10 cm/s.

Series 2. Crystals grown, sublimated and regrown in a dynamic diffusion chamber at a temperature of -8 oC, an ice supersaturation of approximately F = 0.12, and a flow velocity of 10 cm/s.

Series 3. Crystals grown, sublimated and regrown in a static diffusion chamber at a temperature of -10 oC and an ice supersaturation of approximately F = 0.12.

Series 4. Crystals grown, sublimated and regrown in a static diffusion chamber at a temperature of -10 oC and an ice supersaturation of approximately F = 0.06.

is not expected or observed. In this sequence, thefilament support was rotated so that crystals atvarious positions were faced toward the dry nitrogenflow which caused sublimation to occur. Twocomplete cycles with a final growth sequence areshown. Once again a pronounce change in crystalhabit and orientation are observed.

The final set of images, series 4, showcrystals growing in the static diffusion chamber at atemperature of approximately -10 oC and an icesupersaturation of F = 0.06 which is subsaturatedwith respect to water. This is a habit regime wherethick plates are the most common form observed butin which columns may occasionally appear. In thissequence focus should be placed on theassemblage of thick plates growing from the bottomof the filament and the column growing from the sideapproximately 500 :m above this point. In the firstseven images, the column and assemblage of platesat the tip is observed to grow. In the next sevenimages partial sublimation occurs followed by threeimages showing subsequent regrowth. Additionalsublimation in the next three frames sublimates thecolumn to its base but not the assemblage at the tipof the filament. The column does not appear in thefinal images but several smaller columns at differentpositions in the vicinity of the original column areapparent. The assemblage of thick plates at the tipwas essentially regenerated each time though withmodified facets and additional faceted componentsthat are not visible at the resolution of the images. Acluster of plates of this type would be subject tovapor competition amongst the neighboringcomponent plates that probably helps to maintain thehabit through the cyclical process described whilespatially extended crystals like columns are morestrongly effected.

4. CONCLUSION

Growth, sublimation, and regrowth of icecrystals have been investigated in the laboratorywith both a static and a dynamic diffusion chamber.The results show that such a process modifies andchanges the habit in complex ways that appear toindicate that crystals do not retain memory of theirprevious habits or orientations under conditionsranging from moderate ice supersaturation tosupersaturations at or above water saturation. Fromthese results it may be concluded that the extent ofmodification due to this cyclical process is relatedto the degree of supersaturation, subsaturation andthe duration of these conditions. Growth,sublimation, and regrowth of ice crystals underconditions of low ice supersaturation andsubsaturation probably do not lead to the degree of

habit modification observed in this study thoughsome modification is likely.

ACKNOWLEDGEMENTS

This research was supported by National ScienceFoundation, Physical Meteorology Program, GrantATM-9900560 and National Aeronautics and SpaceAdministration Grant NAG-1-2046.

REFERENCES

Bailey, M. and J. Hallett 2000: Nucleation, growthand habit distribution of cirrus type crystals undercontrolled laboratory conditions. 13th InternationalConference on Clouds and Precipitation, August 14-18, 2000, Reno, Nevada, pp. 629-632.

Bailey, M. and J. Hallett, 2002a: Nucleation effectson the habit of vapour grown ice crystals from -18 oC to -42 oC. Quart. J.R. Met. Soc. (accepted forpublication) and this conference.

Dong, Y.Y., R.G. Oraltay and J. Hallett, 1994: Iceparticle generation during evaporation. J. Atmos.Res., 32, 45-53.

Hallett, J., W.P. Arnott, M.P. Bailey and J.T. Hallett,2001: Ice crystals in cirrus. in Cirrus, Editors D.K.Lynch, K. Sassen, D.O. Starr, G. Stephens, OxfordUniversity Press, 2002, Ch. 3, pp 41-77.

Keller, V.W. and J. Hallett, 1982: Influence of airvelocity on the habit of ice crystal growth from thevapor. J. Cryst. Growth, 60, 91-106.

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Oraltay, R.G. and J. Hallett, 1989: Evaporation andmelting of ice crystals: a laboratory study. J. Atmos.Res., 24, 169-189.