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Optical Properties of Contrail-Induced Cirrus: Discussion of Unusual Halo Phenomena

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Optical Properties of Contrail-Induced Cirrus: Discussion of Unusual Halo Phenomena Optical properties of contrail-induced cirrus: discussion of unusual halo phenomena Ralf Sussmann Photographs of a 120° parhelion and a 22° parhelion within persistent contrails are presented. These phenomena result from hexagonal plate-shaped ice crystals oriented horizontally with diameters between 300 mm and 2 mm. From our observations and reinvestigation of previous reports, we conclude that a subset of the population in persistent contrails can consist of highly regular, oriented, hexagonal plates or columns comparable to the most regular crystals in natural cirrus clouds. This is explained by measured ambient humidities below the formation conditions of natural cirrus. The resulting strong azimuthal variability of the scattering phase function impacts the radiative transfer through persistent contrails. © 1997 Optical Society of America Key words: Aircraft, contrail, climatic impact, crystal growth, halo phenomena, ice crystals, optical properties, relative humidity, remote sensing, scattering phase function. 1. Introduction plied for this purpose,9 as well as advanced satellite Natural cirrus clouds that on average cover approx- multichannel imagery.5,10 In investigating the cli- imately 20% of the globe are known to play a major matic effect of contrails, one of the important ques- role in the radiation balance of the earth.1 In past tions we address is whether in persistent contrails years because of increased air traffic, research activ- there is any significant difference in radiative impact ities were extended strongly toward anthropogeni- compared with natural cirrus, i.e., whether there are cally induced cirrus ~contrails! and the possible differences in optical properties as ruled by particle impact on climate2,3 resulting from an additional sur- compositions, crystal shapes and orientations, and face coverage of at most 2% on long-term average.4,5 size distributions. ~By anthropogenically induced cirrus we refer to pri- Direct imaging of the crystal shape and orientation mary induced cirrus, which results from a rapid for- within contrails is sparse. Weickmann took micro- mation of ice particles, not to a possible secondary photographs from airborne particle sampling in a formation of ice particles late after the aircraft emis- young contrail ~aged 3–4 min! at an altitude between sion of aerosols.! This human-made enhance of 8 and 9 km and a temperature between 249 °C and cloud coverage has to be judged bearing in mind the 253 °C.11,12 He found small ~1-mm! primary parti- highly nonlinear dependence of solar backscattering cles difficult to resolve and few large ~100-mm! hollow and infrared absorption from cloud coverage and op- prisms typical for highly ice-supersaturated condi- tical thickness.6 Remote-sensing approaches for tions; however, as stated by Weickmann, the hollow studies of ice clouds were adapted to contrail re- prisms are “under no circumstances to be considered search; e.g., in our group a backscatter-depolarization as typical for contrails.” An ice replicator measure- lidar is implemented with a fast-scanning capability.7 ment on a young contrail ~2 6 1 min! was reported With this the evolution of geometry and optical thick- recently by Strauss showing nearly but not exactly ness can be monitored.8 An airborne lidar was ap- spherical particles ~droxtals! with sizes ranging from 1to5mm.13 Using a one-dimensional optical-array particle spectrometer, Knollenberg found mean par- ticle diameters of the order of 0.5 mm in old ~persis- R. Sussmann is with the Fraunhofer-Institut fu¨ r Atmo- tent! contrails.14 Similar measurements with spha¨rische Umweltforschung ~IFU!, Kreuzeckbahnstrasse 19, D-82467 Garmische-Partenkirchen, Germany. enhanced resolution give an indication of a compara- Received 14 August 1996; revised manuscript received 20 De- bly large fraction of small particles in young con- 9 cember 1996. trails. 0003-6935y97y184195-07$10.00y0 Remote-sensing with halo phenomena, i.e., the © 1997 Optical Society of America photographing of halos and comparison with simula- 20 June 1997 y Vol. 36, No. 18 y APPLIED OPTICS 4195 tions through ray-tracing methods of ice crystal re- upper left-hand side, Fig. 1 shows the aged contrail fraction and reflection phenomena, can give detailed ~lower part! that was advected by the northerly wind. information on ice crystal shapes and orienta- We briefly observed a white, brightly illuminated tions.15,16 However, there are only a few early notes spot-halo phenomenon seen in Fig. 1. The center of on halo observations within contrails, all without the halo was observed at a zenith angle of 71.1° and photographs and most with no unique scientific an azimuth angle of 315° at 12:25:18 UTC on 18 analysis.12,17–22 To our knowledge, hitherto only one January 1996. The apparent angles of the sun at unique photograph of a halo ~refraction and reflec- this time are calculated by a ray-tracing computer tion! phenomenon in contrails was reported: Sassen program ~Sun-zenith angle of 69.39°, Sun-azimuth presented the photograph of an old and persistent angle of 194.82°!. contrail that clearly shows an upper 22° tangent-arc The observed halo phenomenon is unambiguously halo component together with a weak 22° halo.23 identified to be a 120° parhelion according to the Related to halo phenomena are optical phenomena following threefold criteria: ~1! white color, ~2! ap- caused by diffraction effects that point to small crys- pearance at the same zenith angle as the Sun, and ~3! tals. Such a diffraction ~iridescence! phenomenon is appearance at an azimuth angle difference of 120° also observed in the photograph of Ref. 23. From relative to the Sun. All three criteria are matched this the author derived a growth of the spherical and for our observation of Fig. 1. The white color was monodispersed particles from 2 mm ~200 m behind uniquely recognized from our visual observation, and the aircraft! to 3 mm ~400 m behind the aircraft!. the angle criteria were matched within the accuracy Furthermore, Sassen observed corona ~diffraction! ef- of our angle determination ~61°! from the video pic- fects within contrails that point to a considerable ture. For completeness, note that our observation fraction of relatively small ~;10-mm! ice particles in also might have been attributed initially to the phe- medium-aged ~;10-min! contrails.24 nomenon of a parhelic circle15 resulting from less In this paper we present first photographs of two specific crystal properties; we exclude this possibility. different further halo phenomena within persistent The only criterion for this circle-type halo is a zenith ~aged .0.5 h! contrails, including one halo component angle equal to the actual Sun-zenith angle. Our ob- rarely observed even in natural cirrus. With this we served bright spot would then be caused by a crossing derive an unambiguous characterization of the ice point between the contrail and the circle given by the crystal shape, size range, and orientation within the parhelic circle criterion. We exclude this possibility contrails responsible for the observed phenomena. because in this case the halo spot should have been Together with the discussion of the previous obser- observable for a longer time. Additionally, during vations, we draw some conclusions on typical ice crys- this time it should have moved along the contrail tal properties in persistent contrails. while the contrail was drifting; this was not the case. Finally, note again that the 120° parhelion appeared 2. Photographic Observations only for the short time when the contrail passed the The video image of our first contrail halo observation twofold angle criterion ~see above!. This proved that ~Fig. 1! was performed with a commercial CCD cam- the observed halo was indeed caused by the contrail; era ~Sony XC-77CE! with a video processor ~AEG! i.e., it was not caused by a possible weak or subvisible and a frame-grabber ~Matrox!. The camera is char- natural cirrus. Typical ray paths responsible for the acterized by focal lengths of f 5 12.5–75 mm, a max- occurrence of the 120° parhelia are discussed, e.g., in imum field of view of 40°, and a resolution of 756 3 Ref. 16. Our observation of a 120° parhelion within 581 picture elements. The observation direction is the persistent contrail results unambiguously from computer controlled with a two axis scanning mount. ice crystals of the shape of hexagonal plates that are On 18 January 1996 all of Europe was under the oriented horizontally ~oriented plates, c-axis vertical, influence of an extended high-pressure system with one rotational degree of freedom!.15,16 weak northerly winds, free from significant synoptic Another halo phenomenon caused by persistent disturbances. Persistent contrails were formed at contrails was photographed close to sunset on 17 approximately noon above the IFU @47.7 °N, 11.1 °E, June 1996 with a 35-mm format camera ~ f 5 35–70 730 m above sea level ~asl!# at 8700 m asl ~height mm!~Fig. 2!. The observation was in a main air measured by lidar!. From the Munich radiosonde corridor 30 km west of Augsburg, Germany. We data @100 km to the north, 12 Coordinated Universal have no lidar information on the contrail height. Time ~UTC!# we find a temperature of T 5246.5 °C However, the typical flight levels in this corridor are at this height and calculate a relative humidity above in the range of 9300–10,500 m asl. Because of the ice of RHi 5 79%. These conditions are close to the northerly winds at this height we observe the nearest Appleman–Schmidt threshold for initial contrail for- radio soundings to the north of the observation mation ~for kerosene and overall propulsion efficiency ~Stuttgart 12 UTC radio soundings, 100 km to the of 0.3!.25,26 Note that the radiosonde humidity mea- northwest!.
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