Effects of ice-crystal structureon halo formation: cirrus cloudexperimental and ray-tracing modeling studies
Kenneth Sassen, Nancy C. Knight, Yoshihide Takano, and Andrew J. Heymsfield
During the 1986 Project FIRE (First International Satellite Cloud Climatology Project RegionalExperiment) field campaign, four 220 halo-producing cirrus clouds were studied jointly from a ground-based polarization lidar and an instrumented aircraft. The lidar data show the vertical cloud structureand the relative position of the aircraft, which collected a total of 84 slides by impaction, preserving the icecrystals for later microscopic examination. Although many particles were too fragile to surviveimpaction intact, a large fraction of the identifiable crystals were columns and radial bullet rosettes, withboth displaying internal cavitations, and radial plate-column combinations. Particles that were solid ordisplayed only a slight amount of internal structure were relatively rare, which shows that the usualmodel postulated by halo theorists, i.e., the randomly oriented, solid hexagonal crystal, is inappropriatefor typical cirrus clouds. With the aid of new ray-tracing simulations for hexagonal hollow-ended columnand bullet-rosette models, we evaluate the effects of more realistic ice-crystal structures on halo formationand lidar depolarization and consider why the common halo is not more common in cirrus clouds.
1. Introduction
Halos are sometimes a spectacular feature associatedwith the ubiquitous cirrus clouds of the upper tropo-sphere. Aside from the beauty of vividly coloredhalo displays, though, their geometries, when appliedto models, have been useful in demonstrating thepresence of light scatterers that display a fundamen-tal hexagonal symmetry and a preference for certainfall orientations. Thus cirrus cloud optical phenom-ena are not only aesthetically pleasing, but theseconcentrations of scattered light in the phase func-tions of cirrus cloud particles have also made themimportant passive remote sensingtools. The broaderinformation that describes the complete angular scat-tering properties of cirrus clouds is of considerableimportance for simulating the effects of these highclouds in radiative transfer and climate research.However, because of their inaccessibility for in situstudy, knowledge of the exact shapes of cirrus par-
K. Sassen and Y. Takano are with the Department of Meteorol-ogy, University of Utah, Salt Lake City, Utah 84112; N. C. Knightand A. J. Heymsfield are with the National Center for AtmosphericResearch, Boulder, Colorado 80303.
Received 27 September 1993; revised manuscript received 10January 1994.
tides is far from satisfactory, such that the relativelysimple hexagonal ice-crystal models that readily lendthemselves to theoretical simulation and that success-fully replicate halos may not yield angular scatteringpredictions appropriate for typical cirrus clouds.
What is currently known of the composition ofcirrus clouds comes largely from aircraft studies thatutilize various laser-based in situ size spectrometerprobes that, although capable of acquiring vastamounts of data, do not yield detailed information onparticle structure. The most commonly used two-dimensional ice-particle shadow probes, which dis-play minimum particle resolutions of 25 to 100 Am,depending on aircraft velocity and particle type, canprovide only limited data on ice-crystal shape andorientation. (This lower limit on detectable particlesize has led to controversy over the radiative transferimpact of undetected cirrus particles.') Furthermore,no information on the three-dimensional and internalstructures of ice particles can be recovered. Asexplored here, it is just such information that iscrucial to understanding the angular scattering behav-ior of cirrus clouds, especially with regard to haloformation.
Fortunately increasing effort is being directed to-ward the collection in situ of cirrus ice-crystal samplesby impaction for later microscopic examination.These cloud samples can be preserved either tempo-rarily or permanently by the collection of particles on
Fig. 1. Examples of 180 fish-eye photographs of cirrus cloud conditions at (a) 1556, 22 October; (b) 1750, 28 October; (c) 1950, 1November, (d) 2030, 2 November 1986. The 22° halos were often incomplete as a result of multiple scattering and attenuation effects inoptically dense particle fallstreaks, especially close to the horizon because of the increased scattering path lengths. The times above arein UTC.
slides that are oil coated (which are kept chilled untilexamined immediately once on the ground) or thatare coated with resins that rapidly evaporate toproduce permanent casts of the ice crystals.2 Theseinstruments can be borne on aircraft or balloons;their heritage can be traced from basic hand-heldslide airstream insertion samples collected from WorldWar II military aircraft3 to later mechanical anddecelerator-assisted (to reduce particle impact dam-age) slide insertion devices,4 and culminates in con-
tinuous aircraft 5 and balloonborne6 Formvar (i.e.,plastic resin) samplers. Although the techniques donot always produce reliable results, in that radial orfragile ice particles may break up and suffer compres-sion on imperfectly prepared substrates during impac-tion, unique records of the three-dimensional shapesand the internal structures of high-altitude cirrusparticles are often obtained.
In this paper we provide numerous photomicro-graphic examples of the ice particles collected in situ
from four cirrus cloud systems that generated 220halos during periods of coordinated aircraft andground-based polarization lidar observations. Thestudies were conducted during the 1986 Project FIRE[First International Satellite Cloud Climatology Pro-gram (ISCCP) Regional Experiment] Intensive FieldObservation (IFO I) campaign near Wausau, Wis.,with the use of the University of Utah mobile polariza-tion lidar (MPL) system and the National Center forAtmospheric Research (NCAR) King Air aircraft,which was equipped with a slide injection device forparticle collections. Below, the instrumentation andfield project are briefly described, followed by the insitu and remote sensing findings for the four cirruscase studies. Although our chief purpose is to de-scribe the cirrus cloud particles associated with 220halo formation, in view of the scarcity of empiricaldata from high-altitude cirrus clouds we embellishthe photomicrograph collection with more detaileddescriptions of the vertical cloud structure and themicrophysical content of the cirrus. Then, with theaid of new geometric ray-tracing simulations thatdescribe the angular scattering patterns of morerealistic hexagonal ice-crystal models, we interpretthe in situ data in terms of halo-formation mecha-nisms in actual cirrus clouds.
2. Field Experiment
During a four-week period in October-November1986, a field campaign that utilized advanced remotesensing and airborne research instrumentation wasconducted over central and southern Wisconsin tocharacterize the properties of midlatitude cirrusclouds, which were concurrently examined from vari-ous satellite platforms.7 The purpose of this exer-cise was to enhance our basic knowledge of cirrusclouds, especially with regard to the Project FIREgoal of testing and improving ISCCP cirrus clouddetection and characterization algorithms routed inbispectral satellite radiance observations. One offour ground-based remote sensing sites8 was locatedat Wausau in central Wisconsin, which served as ahub for several coordinated University of Utah MPLand aircraft experiments. Four extensive cirrus cloudsystems were systematically studied by the lidar andKing Air. Each 2-h aircraft mission consisted of aseries of ascending or descending stepped leg profilesalong the mean cirrus wind direction and verticalspiral soundings in the immediate vicinity of theground site. The 20-40-km-long horizontal legs wereterminated over Wausau, and the spirals were cen-tered over the site such that the in situ data weregenerally representative of the cirrus cloud condi-tions probed by the vertically pointed MPL system.
King Air cloud microphysical instrumentation in-cluded both two-dimensional crystal (2D-C) and two-dimensional precipitation (2D-P) probes for character-izing the shapes and the sizes of ice crystals, fromwhich composite data algorithms to derive estimatesof ice-mass content have been applied.9 The ice-particle impaction device also deployed collected cir-rus ice-crystal samples at frequent intervals by using
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Fig. 2. Combined polarization lidar and in situ data displays thatcover the indicated 2-h period of the King Air mission on 22October 1986; they consist of lidar range-normalized returnedenergy (top left), linear depolarization ratio (top right; see 8 valuekey) HTI displays, and panels of aircraft-derived ice-crystal concen-tration Ni, ice-mass content M, temperature T, and relativehumidity RH,. Curves superimposed on the lidar HTI displaysshow the supporting aircraft flight track, where the heavy curvesegments indicate radial distances from Wausau of < 20 km, andthe open circles show the times the ice-crystal samples in Fig. 3 (asidentified by the letters at top) were collected. Note that the blankregions in the 6 display represent the rejection of inaccurate ratioscaused by noise in weak signals or off-scale signals in stronglyscattering parts of the cirrus cloud.
a relatively simple apparatus that consisted of analuminum rod with a spring-activated extension atone end. A coated glass slide mounted on that endwas exposed to the airstream at velocities of 100-125ms-' through the aircraft's navigation port. Thecoatings applied to the slides were most often viscoushexane oils, although other replication materialswere tested. Exact exposure times were obtainedthrough an event marker on the inboard end of therod so that ice-crystal concentration estimates couldlater be attempted. For the duration of the flight theexposed slides were stored in a dry-ice-cooled con-tainer in test tubes that contained a silicone solutionthat remained fluid at dry-ice temperature and didnot absorb water, characteristics essential to ice-crystal preservation. At the end of the flight thesamples were transported to a cold room and photo-graphed with an ordinary light microscope.
Because the characteristics of the lidar system asconfigured during the IFO I campaign have beendescribed previously,8 only a brief description is givenhere. The MPL unit utilized a giant-pulsed (1.5-J)ruby (0.694-pm wavelength) laser transmitter and a25-cm-diameter telescope receiver fitted with dualphotomultiplier tube detectors to measure laser back-scattering simultaneously in the planes of polariza-tion orthogonal and parallel to the transmitted verti-cally polarized pulse. From the two digitized (at7.5-m range resolution) signals, we derive the lineardepolarization ratio 8 from ratio of the orthogonal toparallel signals after adjusting for differences in the
Fig. 3. Representative ice-crystal photomicrographs obtained by impaction on slides (as identified in Fig. 2, top) at the following heights (inkilometers) and temperatures (in degrees centigrade), respectively: a, 7.32, -26.6; b, 7.63, -29.3; c, 7.32, -26.9; d, 7.32, -26.8; e, 7.94,-31.6; f, 8.54, -37.1; g, 6.86, -25.0; h, 6.34, -2 .. 28; i, 6.13, -20.0. Note the 250-pm reference scale in c.
gains of the two channels. The lidar returns werestored on magnetic disk at a rate of 0.5-1 shots perminute. Also collected at 30-min intervals were1800 black-and-white fish-eye photographs (a redfilter was used) showing the appearance of cirrusclouds and the optical displays they generated overthe ground site. Examples of the fish-eye photo-graphs for each case study are given in Fig. 1.
In Section 3 comprehensive data displays for thefour cirrus cloud case studies are presented. Thegraphics consist of height-versus-time (HTI) displaysof lidar range-normalized returned power (in arbi-trary logarithmic units) and linear depolarization
ratios over which are superimposed the King Airflight tracks in the vicinity of the ground site, accom-panied by in situ data panels of combined 2D-C and2D-P probe-derived ice-crystal concentration Ni andice-mass content Mi, as well as aircraft-measuredtemperature T and relative humidity RH, (withrespect to water).
Representative photomicrographs of the ice crys-tals sampled are given for each case study. Notethat all four of these 220 halo-producing studiesinvolved large-scale cirrus cloud systems associatedwith frontal activity. However, it should be men-tioned that these relatively deep cirrus clouds occasion-
ally produced dense fallstreaks and strong optical at-tenuation that variably obscured portions of the halos.
3. Halo-Producing Cirrus Cloud Case Studies
A. 22 October 1986
The cirrus cloud system studied on this occasion canbe characterized as an accumulation of 4-6 km-deepparticle fallstreaks. The fallstreaks emanated fromcloud-top-generating regions between 10 and 11 kmabove mean sea level (MSL) (all heights are MSL) andaccumulated in the lower cloud region where theyoften produced strong laser pulse attenuation. Dur-ing the 2-h period of coordinated King Air studiesfrom 1430-1630 (all times are UTC), the lidardisplays of the aircraft-sampled cloud region(Fig. 2,top) reveal the dominating effects of particle sedimen-tation from aloft, particularly at 1540, when amajor fallstreak that produced relatively strong(8 > 0.5) depolarization (see key at top right) at lowlevels was probed. According to the superimposedaircraft flight track position and the correspondingmicrophysical data, the King Air initially measuredvariable ice-cloud contents (see Mi and N, plots atlower left) during a rapid ascent of the cirrus, whichcorresponded to the sampling of the cirrus particlefallstreaks. (The low Mi values just after 1500, e.g.,can be seen to correspond to a weak laser-scatteringregion between fallstreaks.) At 1540 the aircraftclimbed to a maximum height of 8.7 km in a regionabove the optically dense sheared fallstreak (center)that produced range-limiting lidar attenuation effects.The subsequent spiral descent through the cloudlayer continued until the aircraft briefly emergedbelow cloud base at 1615. The lidar linear depolariza-tion display (top right) shows 8 values mostly between0.3 and 0.4, which are rather typical of many cirrus,but also sometimes reveals much lower ratios(8 < 0.15, particularly near cloud base at - 1530), aswell as 8 > 0.5 near the bases of the stronglyscattering fallstreaks. The low 8 values are gener-ally attributable to the effects of horizontally orientedplate crystals (note that the x symbols below the8-value HTI display represent data collected when thelidar was tipped by more than 2.50 off the zenithdirections), whereas the stronger 8 values indicateaccumulations of complex ice particles, perhaps in theform of aggregates. The fish-eye photograph of Fig.1(a), taken at 1556, depicts the upper half of a 220halo whose bottom is obscured by thicker cirrusfallstreaks closer to the horizon.
Photomicrograph examples from the impactionslides that yielded useful data on this mission aregiven in Fig. 3. (The locations of these samples areidentified on the aircraft flight track in Fig. 2, andtheir heights and temperatures are given in the Fig. 3caption). Considerable damage to the impacted par-ticles is evident, which indicates the general fragilityof ice particles encountered in this case. The easilyrecognizable elements are hollow bullets that wereonce members of radial bullet-rosette crytals (Figs.3a, 3b, and 3d), a hollow column (Fig. 3g, preserved
nearly end up), and relatively small solid, or mostlysolid, columns (Figs. 3a, 3c, and 3e). Much of thecrystal debris can be recognized as fractured hollowcolumns and bullets (Fig. 3f shows a shattered thin-walled rosette) and parts of hexagonal sector or platecrystals. As the compressed particles in Figs. 3c and3i resemble compressed radiating groups of sectorplates and prismatic crystals, fragile sector-columncombinations may also have been common.
B. 28 October 1986The period ofjoint ground-based and airborne obser-vations from 1600 to 1800 comprises a portion of thecirrus clouds intensively described in a FIRE IFO Icase-study special issue." The cloud system probedover Wausau was again composed of optically densefallstreaks that caused the 220 halo to become indis-tinct or incomplete (Fig. lb). Figure 4 provides thecombined lidar and aircraft data in the same formused above. What makes this thick cirrus cloudsystem unusual, however, is the occasional indicationof embedded supercooled liquid-water clouds. Thesepatches of altocumulus clouds are revealed by the 8 <0.15 values, mostly at altitudes of 6.7 and 7.8 km;one such patch that produced brilliant iridescentcolors can be seen near the center of the Fig. lbfish-eye photograph. Also in contrast to the abovecase, in situ relative humidity data were periodicallynear water saturation rather than near ice satura-tion, which is the minimum needed to maintain pureice cirrus clouds. Otherwise the lidar depolarizationratios are mostly in the 0.25-0.45 range, which mayindicate the presence of horizontally oriented planarice crystals mixed with other particle types and orien-tations. An obvious exception to these relatively low8 values occurs at 1615 at the bottom of a crystalfallstreak, where 8 > 0.55 values are prevalent.
The somewhat abbreviated King Air mission con-sisted of a single ascending step-up leg pattern (after1630) and a rapid spiral descent over Wausau.Considerable variability in cirrus cloud ice content
I
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Fig. 4. Combined lidar and aircraft data display as in Fig. 2, butshowing the results of the aircraft mission at the indicated times on28 October 1986.
(Fig. 4, bottom left) is associated with the aircraftfallstreak penetrations, and Mi and Nj tend to in-crease with decreasing altitude. (Note in particularthe vigorous fallstreak that descended, at 1715, toan altitude of 5.7 km, which is well below the usualcloud base.) The corresponding ice-crystal photomi-crograph collection given in Figs. 5a-5i shows thathollow bullet-rosette crystals and their derivatives,following impaction at the 100-125-ms-I airspeeds,are clearly dominant. Quite thin-walled bullets areparticularly evident in Figs. 6a, d, and 5g, shatteredhollow columns are evident in Fig. 5b, and mostlysolid bullet remnants are evident in Figs. 5e and 5i.
In addition, shattered sector-column and simple plate-crystal remnants are depicted in Fig. 5f. Also notethe rounded crystal forms in Fig. 5a, which werecollected near the cloud base in quite dry air.
C. 1 November1986
This third cirrus cloud system was also dominated bythe action of deep, sheared ice-particle fallstreaks,which created a strongly scattering cloud base regionbetween 5 and 6 km (Fig. 6). The occurrence of acomplete 220 halo was accordingly sporadic, but anenhancement of the upper-tangent arc region wasoften apparent (Fig. c). The King Air performed a
Fig. 5. Ice-crystal photomicrographs obtained (see Fig. 4, top) at the following heights (in kilometers) and temperatures (in degreescentigrade), respectively: a, 6.13, -20.0; b, c, 6.38, -22.2; d, 6.38, -22.2; e, 7.01, -26.9; f, 7.34, -298; g, 7.62, -32.2; h, 7.26, -29.2; i,6.72, -25.2. A 250- pm scale is provided in i.
Fig. 6. Combined lidar and aircraft data display as in Fig. 2, butshowing the results of the aircraft mission at the indicated times on1 November 1986.
mission that consisted of a rapid descent, a steppedascent, and a spiral descent through the cirrus cloudsabove Wausau (see Fig. 6, top). Variability in icecontent that is attributable to aircraft fallstreakpenetrations is initially apparent, but during the finaldescent Mi and Ni are relatively high and constant,which indicates that the Eulerian spiral remainedwithin the domain of a fallstreak during descent.Laser depolarization was relatively strong ( - 0.5) inthe cloud region accessible to lidar probing (becauseof attenuation) during the spiral descent of the air-craft, as it also was at times near the lower bound-aries of the fallstreaks. Occasional evidence for hori-zontally oriented planar ice crystals is present.
The ice crystals photomicrographed on this occa-sion are quite well preserved and varied, as can beseen from Fig. 7. Although hollow columns thatdisplay intricate internal structures dominate, alsopresent are thick plates (Figs. 7a, 7c, and 7e), simpleplates (Fig. 7g), bullet rosettes (Fig. 7n), and aplate-capped column crystal that fortuitously sur-vived impaction (Fig. 7k). In addition, the remnantsof radial-sector or column-sector assemblages areevident in Figs. 7g, 7h, 7q, and 7t, which are perhapsalso responsible for the shattered debris obtainednear cloud top (Figs. 71 and 7m). Unusual ice-crystalreplicas include the seemingly identical, paired-column crystals'2 in Fig. 7b, and the spatial prismgrowths in Figs. 7f and 7h. Finally, ice crystalscollected near the cirrus cloud base (Figs. 7d, 7e, 7s,and 7t), where rapid particle evaporation was occur-ring in dry air, displayed rounded crystal forms thataffected the laser-scattering behavior of these par-ticles. Ice-crystal faces that lack sharp definition(because of evaporation) relative to the incident wave-length often generate decreased 8 values at the lowerboundary of cirrus clouds, as is apparent from Fig. 6.
D. 2 November 1986
The fish-eye photograph from 2030 in Fig. d showsan incomplete 220 halo, which was typical of the
conditions associated with the cirrus cloud systemstudied on the morning of 2 November. The KingAir, over Wausau, performed a mission consisting of astep-up leg pattern bracketed by two spiral descents(Fig. 8, top). Although the cloud microphysical datarecord is incomplete, the variability in ice content isagain in general agreement with the presence of thelidar-detected deep-precipitation fallstreaks, which attimes (especially at 2050) produced strong opticalattenuation. Lidar depolarization was typically inthe 0.4-0.5 range, although much lower 8 values weremeasured at - 2115 at an altitude of 6.7 km from anembedded supercooled (- - 31 C) altocumulus cloud.(Note that the < 0.15 values below the cloud baseand between the lower portions of the fallstreaksfrom 2100 to 2120 represent the effects of a mixtureof molecular and quite weak cirrus cloud returns.)
Figure 9 reveals that this deep cirrus cloud com-prised chiefly bullet-rosette crystals, which displayedconsiderable hollowness. There is again evidence,however, for sector-column combinations (Figs. 9dand 9p), and in Fig. 9k a large ( 800-[tm-long)shattered hollow column crystal is shown. Columnsthat display only a small amount of internal structureare also preserved (Figs. 9c and 9n), but obviouslythey constitute a relatively minor component of theice crystals sampled.
4. Halo Ray-Tracing Considerations
In recent years a significant advancement in ourability to treat the light-scattering properties of real-istically shaped ice crystals has resulted from the(albeit computer-intensive) programs based on geo-metric optics ray-tracing principles.'3"14 These nu-merical experiments have utilized models that displaythe widely accepted hexagonal symmetry of ice crys-tals through a range of realistic axial ratios, includingboth solid column and plate structures, but in view ofin situ evidence that indicates considerably morecomplicated cirrus cloud particle structures, it can bequestioned whether such simple models are appropri-ate. Fortunately ray-tracing predictions that in-volve the first step toward more realistic ice-crystalmodels are now available, as we show in Figs. 10and 1.
As the solid hexagonal column model has fre-quently been used to simulate cirrus cloud haloproperties,' 5" 6 we present for comparison ray-tracingphase functions for randomly oriented solid columns,which are denoted as the solid-dashed curves in Figs.10 and 11. Such idealistic particles generate a quitewell-defined 220 halo and, to a lesser but still notice-able extent, a 46° halo. However, it is obvious that ifone treats simple hollow-ended column structures(Fig. 10), then the inner edge of the 22° halo becomesconsiderably less distinct, whereas the 46° halo van-ishes. (Previously published 460 halo simulationsrelied on solid column-crystal end-on refractions. 5 )Column ice-crystal backscattering is also significantlydiminished by the hollow ends.
A comparison of ray-tracing phase functions forrandomly oriented solid columns and a solid (i.e.,
Fig. 8. Combined lidar and aircraft data display as in Fig. 2, butshowing the results of the aircraft mission at the indicated times on2 November 1986. Note that 2D-C and 2D-P probe data aremissing from 2025 to 2046 and after 2123.
idealistic) bullet-rosette ice-crystal model is given inFig. 11 (see figure caption for details). In this case,because of the unreasonably pristine nature of thepointed hexagonal bullet tips (compared with theprevious photomicrographs), a 8° halo emerges.However, with more irregular bullet tips the scat-tered intensity to the left of the 220 halo could still beexpected to fill in without the formation of the (rarelyobserved) 80 halo. Although the 460 halo and back-scattering are diminished, the use of hollow-endedbullets would no doubt have produced more dramaticeffects similar to those in Fig. 10 for hollow-endedcolumns. Also note that bullet-rosette sidescatter-ing increases markedly as a result of the complicatedray-tracing patterns induced by neighboring ice-crystal elements.
5. Conclusions
These detailed remote and in situ observations of four220 halo-producing cirrus cloud systems provide aunique data set that can be applied to explain theoccurrence of the common halo and, inversely, thenonoccurrence of optical displays in many, if notmost, cirrus clouds. It is apparent that in contrastto the traditional ice-particle model postulated by 220halo theorists, i.e., the randomly oriented solid hexago-nal crystal, the actual structures of halo-producingcirrus ice crystals are far more complex. It cangenerally be said that the intricacy of cirrus ice-crystal internal and radial structures violates thesimple assumptions usually applied to modeling thescattering behavior of these high clouds. Solid ormostly solid ice crystals, which could be expected to becomparatively well represented in the impacted par-ticle sample, are obviously uncommon. The majordistinctions between the traditionally modeled andthe cirrus ice-crystal shapes obviously lie in theinternal hollow structures and radial forms of thecloud particles. (Column and bullet-rosette crystalsare frequently so thin walled as to shatter on collec-
tion.) However, this finding should not be consid-ered as groundbreaking, as it has been previouslypointed out from aircraft observations that cirrusclouds composed of hollow or radial crystals arecapable of generating halos.3"17
How this diversity in cirrus ice-crystal shape re-lates to visible (0.69-[um) lidar linear depolarization isdifficult to judge because the sporadic impactionsamples were not obtained directly over the groundsite: a more continuous impaction device and betteraircraft navigation (both of which are now available)would have been better suited for this purpose.Nonetheless, we can attempt to make generalizedintercomparisons between the dominant ice-crystalhabits measured in situ and the distributed fields of 8values when the aircraft made close approaches.On 1 and 2 November, for example, relatively stronglidar depolarization (0.45-0.55) was widespread, and,as expected from basic scattering principles,'0 thecorresponding dominant crystal types were complexlyshaped. On 2 November bullet rosettes and theiraggregates (Figs. 9e-9i) appeared responsible for values of as high as 0.6 in the lower cirrus, whereason 1 November it was highly structured hollowcolumns (Figs. 7b-7f) and sector-column radial crys-tals (Figs. 7p-7s) that were indicated to have pro-duced strong depolarization. The cirrus clouds on22 and 28 October consistently produced lower values. Widespread cloud regions that displayed0.25-0.35 and 0.35-0.45 5 values were probed by theaircraft; we assume that these values represent amixture of prismatic crystals and horizontally ori-ented plate or capped-column crystals, which arechiefly preserved in the in situ samples as shatteredbits of sectors (Figs. 3 and 5). Oriented plate-crystallidar-scattering anisotropy was occasionally noted tobe present in both these cases, presumably from cloudregions that were dominated by plate specular reflec-tions. Finally, the tendency for 8 values to decreasesignificantly at the cirrus cloud base and perhaps insubsaturated regions within the clouds has beenlinked to evaporating crystals that lose their hexago-nal scattering faces. Although previous theoreticalresearch has shown lidar depolarization to be influ-enced by the basic hexagonal ice-crystal shape (i.e.,axial ratio) and orientation,' 3 it is clear that the shapecomplexities displayed by cirrus ice crystals need to besimulated more accurately to improve the interpreta-tion and the utility of polarization lidar returns.
The geometric optics ray-tracing phase-functionpredictions given here for ice-crystal shapes thatbegin to approach the complexity of the particlescaptured in situ are noteworthy, despite their stillsimplistic nature. Through the inclusion of hollow-ended column and radial-bullet-scattering effects, anumber of interesting angular scattering featureshave been revealed. In agreement with observationsof many halos, the distinctiveness of the inner edge ofthe 220 halo, as well as the entire 460 halo, declinesnoticeably. Backscattering is also diminished,whereas sidescattering in the case of bullet rosettes is
Fig. 9. Ice-crystal photomicrographs obtained (see Fig. 8, top) at the following heights (in kilometers) and temperatures (in degreescentigrade), respectively: a,8.84, -46.3; b,7.70, -36.7; c,7.12, -31.9; d,6.34, -26.5; e,5.80, -23.8; f,5.27, -21.9; g,4.77, -18.9; h,4.76,-18.5; i, 5.49, -21.8;j, 5.71, -22.5; k, 6.10, -24.5; 1, 6.10, -24.8; m, 6.70, -29.3; n, 7.93, -39.6; o, 6.79, -30.9; p, 5.30, -20.2. Scale isprovided in p.
significantly enhanced. These findings not only haveimplications for halo formation (e.g., in explainingwhy the 460 halo is rarely seen), but may also improveour understanding of satellite-based radiance meth-ods for inferring cirrus cloud properties. In thisregard we point out that the increase in cirrus cloudsidescattering predicted for radial ice crystals helps toalleviate the necessity for evoking the presence ofnumerous small ice particles in cirrus.1
Finally, the question of why halos in cirrus cloudsare not more commonly observed can be addressed.According to previous ray-tracing simulations, the220 and the 460 halos should be prominent features as
long as simple hexagonal ice crystals, somewhatlarger than the wavelength, 8 are (mostly) randomlyoriented. 92 0 In contrast, in cirrus clouds the com-plexity and the diversity of particle shape interfereswith halo formation, unless conditions are appropri-ate for the generation of at least partially solidcrystals in sufficient numbers. As an examination ofthe more memorable historic halo displays reveals anassociation with low-altitude winter or polar ice-crystal clouds, rather than typical cirrus, it is appar-ent that cirrus particle growth processes often resultin unsuitable crystal habits. Not only does the insitu evidence indicate that typical cirrus particles are
Fig. 10. Comparison of ray-tracing-predicted phase functions Pifor randomly oriented hexagonal solid and hollow-ended columns(see inserts) computed for a 0.55-pm wavelength. Both columnmodels are 200 lm in length and 80 pAm in maximum width: thedepths d of the hollow ends are 50 pLm.
too hollow or complex to produce vivid halos, butmultiple scattering in optically thick cirrus and theevaporaton of cirrus particles, an integral part of thecirrus cloud growth-maintenance process,8 detractsfrom halo formation. As all cirrus clouds are precipi-tating and may not be actively renewing themselveslocally, decaying cirrus may not be capable of produc-ing halos as a result of the rounded crystal formsproduced during descent through subsaturated air.
We conclude that, to satisfy the requirement forrandomly oriented hexagonal particles, refracted raypaths through the solid portions of bullet crystals inradial rosettes are capable of generating (rather unex-ceptional) 220 halo displays. Mostly solid particlesmust comprise a sufficient proportion of the cirruscontent, however, to ensure that near forward scatter-ing from more complex internal reflections and refrac-tions does not dominate in the halo region. Simi-larly, during the 1 November 1986 case study, anoften indistinct upper-tangent arc to the 220 halo wasobserved in association with partially solid columncrystals large enough to maintain horizontal orienta-tions. Exceptionally vivid halo formation in cirrusclouds is thus reliant on the generation of solid icecrystals and the still largely unknown cloud micro-physical conditions responsible for their growth.
Support for the University of Utah participation inthe FIRE IFO project and partial support for theNCAR King Air was provided by National Science
10 2
IL
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i,a,
to
10 i
10 0
110 i
1-20 18060 120
Scattering Angle (deg.)
Fig. 11. Comparison of ray-tracing-predicted phase functions,again at a 0.55-pm wavelength between randomly oriented solidcolumns and bullet rosettes (see inserts). Each of the four solidbullets (aligned radially in the same plane) is 60 pAm in width and240 pm in total length, inclusive of the 48-pLm-long pointd bullettip. The 200-pm length and 100-pLm width of the columns arebased on a projected area that is equal to that of the randomlyoriented bullet rosettes.
Foundation grant ATM-8513975. Recent researchat the University of Utah has been funded by Na-tional Science Foundation grant ATM-8914348 andNASA grant NAG-1-868 for cirrus data analysis andby National Science Foundation grant ATM-9024217for scattering theory development. King Air dataanalysis at NCAR was funded under NASA contractL98110B. The National Center for AtmosphericResearch research is sponsored by the National Sci-ence Foundation.
References and Notes1. K. Sassen, A. J. Heymsfield, and D. O'C. Starr, "Is there a
cirrus small particle radiative anomaly?" in Preprints of theSeventh Conference on Atmospheric Radiation (American Me-teorological Society, Boston, Mass., 1990), pp. J91-J95.
2. V. J. Schaefer, "A method for making snowflake replicas,"Science 93, 239-240 (1941).
3. H. K. Weickman, "Die Eisphase in der Atmosphir," Lib.Trans. 273 (Royal Aircraft Establishment, Farnsborough, UK,1947).
4. A. J. Heymsfield and R. G. Knollenberg, "Properties of cirrusgenerating cells," J. Atmos. Sci. 29, 1358-1366 (1972).
5. P. A. Spyers-Duran and R. R. Braham, Jr., "An airbornecontinuous cloud particle replicator," J. Appl. Meteorol. 6,1108-1113 (1967).
6. C. Magono and S. Tazawa, "Design of snow crystal sondes," J.Atmos. Sci. 23, 618-625 (1966).
7. D. O'C. Starr, "A cirrus-cloud experiment: Intensive fieldobservations planned for FIRE," Bull. Am. Meteorol. Soc. 68,119-124 (1987).
8. K. Sassen, C. J. Grund, J. D. Spinhirne, M. Hardesty, and J. M.Alvarez, "The 27-28 October 1986 FIRE IFO cirrus casestudy: a five lidar overview of cloud structure and evolution,"Mon. Wea. Rev. 118, 2288-2311 (1990).
9. A. J. Heymsfield, K. M. Miller, and J. D. Spinhirne, "The27-28 October FIRE IFO cirrus case study: cloud microstruc-ture," Mon. Weather Rev. 118, 2313-2328 (1990).
10. K. Sassen, "The polarization lidar technique for cloud research:a review and current assessment," Bull. Am. Meteorol. Soc.72, 1848-1866 (1991).
11. The November 1990 (Vol. 118) issue of the Monthly WeatherReview compiles a number of related articles from this cirruscloud case study.
12. N. C. Knight, "No two alike," Bull. Am. Meteorol. Soc. 69,496(1988).
13. Y. Takano and K. N. Liou, "Solar radiative transfer in cirruscloud. Part I: single-scattering and optical properties ofhexagonal ice crystals," J. Atmos. Sci. 46, 3-19 (1989).
14. K. N. Liou and Y. Takano, "Light scattering by nonsphericalparticles: remote sensing and climatic implications," Atmos.Res. (to be published).
15. R. Greenler, Rainbows, Halos, and Glories (Cambridge U.Press, Cambridge, 1980).
16. R. A. R. Tricker, Ice Crystal Haloes (Optical Society of America,Washington, D.C., 1979).
17. M. Glass and D. J. Varley, "Observations of cirrus particlecharacteristics occurring with halos," in Preprints of theConference. on Cloud Physics and Atmospheric Electricity(American Meteorological Society, Boston, Mass., 1978), pp.126-128.
18. According to the laboratory studies reported in K. Sassen andK. N. Liou, "Scattering of polarized laser light by water drop-let, mixed phase and ice clouds. Part I: Angular scatteringpatterns," J. Atmos. Sci. 36, 838-851 (1979), minimum ice-crystal dimensions of 25 Lm are needed for generating halos.
19. K. Sassen, "Remote sensing of planar ice crystal fall atti-tudes," J. Meteorol. Soc. Jpn. 58, 422-429 (1980).
20. K. Sassen, "Polarization and Brewster angle properties of lightpillars," J. Opt. Soc. Am. A 4, 570-580 (1987).
