TECHNICAL NOTE

Ice-cloud depolarization of backscatter for CO2 and other infrared lidars: comment

Freeman F. Hall, Jr.

Multiple reflections from the front faces of ice crystals in clusters may lead to depolarization of backscatter when cirrus clouds are probed with infrared lidars.

Key words: lidar, cirrus backscatter, depolarization.

Eberhard1 points out correctly that the absorption in ice at 10.59 μm prevents much penetration and internal reflection from cirrus ice crystals when probed by a CO2 lidar. Then he states, "A depolarizing mechanism for reflection from the first surface is nonexistent, or weak at most," and later states, "The dominant mechanism for depolarization in backscat­ter in the short-wave, where absorption is negligible and scattering dominates, is refraction and internal reflection from crystal faces at nonnormal incidence." From these two premises he concludes that one should not expect much depolarization, δlc, from cirrus at 10.59 μm. The depolarization for the Na­tional Oceanic and Atmospheric Administration CO2 lidar is defined as

where Iper is measured by transmitting linear polariza­tion and detecting the orthogonal linear component, whereas Icir is measured by transmitting left-hand circular polarization and detecting right-hand circu­lar polarization. At 10.59 μm, δlc ≈ δl, the ratio of perpendicular to parallel polarization, which is the definition usually used in lidar research.

Next, Eberhard states, "The earlier observation by Gross et al.2 of δlc as high as 0.4 remains an enigma." However, using the terminology of Braithwaite,3 this conclusion does not follow logically from the conjunc­tion of Eberhard's two premises. Multicrystal ro­settes of ice are a common occurrence in convective cirrus clouds, as has been known since the pioneering investigations of Weickmann,4 50 years ago who

The author is with Harrier Consultants, 202 Ocean Street, Solana Beach, California 92075.

Received 28 June 1993; revision received 14 October 1993. 0003-6935/94/061079-02$06.00/0. © 1994 Optical Society of America.

found "hollow crystals in clusters" to be the typical ice form for such clouds (cirrus densus). A fre­quency distribution plot of the angles between cluster crystals shows a broad peak between 30° and 110° (Weickmann's Fig. 35A).

Similarly, Uyeda and Kikuchi5 found that angles between so-called bullet crystals in rosette clusters had a mean value at approximately 70°. From Brew­ster's law we find that maximum infrared depolariza­tion on reflection from ice would occur for an angle of incidence near 53°. Two or more corner-cube-like reflections from ice crystal clusters would involve angles near the Brewster value.

Cirrostratus clouds are typically composed of single ice crystals, and in this case Eberhard's first premise (a single reflection) would be inclusive. However, Gross et al.2 sampled cirrus spissatus (convective cloud tops, formerly termed cirrus densus6), where ice crystal clusters are common and backscatter can involve reflections (plural) from multiple crystal faces in a single cluster. Sassen7 has shown, using labora­tory-grown ice crystals, that backscatter from com­plex ice particles, such as radiating crystals (rosettes) and aggregates, can lead to depolarization ratios of δ ≈ 0.5.

Also, all Eberhard's data were taken with a verti­cally pointed lidar. If there were horizontally ori­ented, platelet crystals in the clouds, the specular and nondepolarized backscatter from such platelets may have influenced the results. Certainly the extremely narrow beam width of the coherent lidar would rule out multiple intercrystal scattering.

Finally, it is t rue that the data of Gross et al.2 were acquired at time intervals that were spaced by approxi­mately 1 min in time. Although the cloud bases did not vary in height significantly during this interval, it is desirable to minimize the time between measure­ments of the two polarizations measured to ensure that changes in cloud morphology did not contribute

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to the results. Eberhard's1 measurements were sepa­rated by only 1.25 s, a much more desirable condition.

Perhaps these are at least partial explanations for the differences in backscatter depolarization found in the two investigations.

References 1. W. L. Eberhard, "Ice-cloud depolarization of backscatter for

CO2 and other infrared lidars," Appl. Opt. 31, 6485-6490 (1992).

2. A. Gross, M. J. Post, and F. F. Hall, Jr., "Depolarization, backscatter, and attenuation of CO2 lidar by cirrus clouds," Appl. Opt. 23, 2518-2522 (1984).

3. R. B. Braithwaite, Scientific Explanation (University Press, Cambridge, 1955), Chap. III, pp. 50-55, 86-87.

4. H. Weickmann, Die Eisphase in der Atmosphare, Berichte des Deutschen Wetterdienstes in der US-Zone 6 (in German) (Ger­man Weather Service, Bad Kissingen, West Germany, 1949), pp. 3-54.

5. H. Uyeda and K. Kikuchi, "Observations of the three-dimensional configuration of snow crystals of combinations of bullet type," J. Meterorol. Soc. Jpn. 57, 488-492 (1979).

6. W. Bleeker and A. Viaut, eds., International Cloud Atlas (World Meteorological Organization, Paris, 1956), Vol. 1, pp. 19-24.

7. K. Sassen, "The polarization lidar technique for cloud research: A review and current assessment," Bull. Amer. Meteorol. Soc. 72, 1848-1866 (1991).

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