Optical and microphysical properties of ice crystals

 

Key ingredients for modeling the radiative effect of cirrus clouds in climate are the radiative "effective size" of cirrus ice crystals, and how efficiently they forward rather than back-scatter light. Perhaps surprisingly, both these quantities remain very poorly known, or at least a topic of a fair bit of contention.


In past studies we placed an instrument, the Gerber Scientific Cloud Integrating Nephelometer, aboard the NASA WB-57 during the CRYSTAL-FACE and MidCiX field projects. The CIN measures the extinction coefficient (or fogginess) and asymmetry parameter (or directional scattering) of cloudy air. In combination with instruments that measure ice water content in clouds, the measurements consistently pointed to smaller, more reflective ice crystals than are normally assumed to characterize cirrus clouds.


If the measurements are correct, they would suggest that cirrus clouds are rather brighter than normally portrayed in climate models. That said, criticisms have been raised about the validity of the measurements due to concerns that large ice crystals shatter on instrument apertures at high velocity, creating bursts of artificially small ice crystals.


The issue isn't fully resolved yet. However, one approach to testing the measurements is to compare them with remote sensing data, which aren't affected by such shattering. The difficulty with remote sensing data is that assumptions are often required about cloud and ice crystal characteristics in order to do the retrievals, and very often these assumptions are more or less guess work.


With Research Professor Steve Cooper we have developed an infrared technique for roughly determining ice crystal size from space, using the MODIS instrument. The method requires an absolute bare minimum of assumptions: while it is not precise, unlike optically based techniques it is robust to assumptions made. For cases where comparisons could be made with in situ measurements we found that when aircraft said ice crystals were "small", so did remote sensing data.


This is encouraging, in that it suggests that prior in situ data may not be totally out to lunch. If so, it still begs the question of why measurements show ice crystals are individually so efficient at back-scattering light. In a nutshell, ice crystal scattering models that assume ice crystals are hexagonal prisms (a seemingly reasonable choice) show values of the cosine-weighted backscattering (1-g) fraction of ~0.2 or less, depending on ice crystal size and shape. By contrast, in situ measurements consistently show values of ~0.25, almost independent ice crystal size or ambient conditions.


Why is there this discrepancy between observations and idealized models? The argument we have made is that what matters for the angular scattering distribution is not the ice crystal size and shape, but rather the sub-micron texture of ice crystal surfaces. These small features are more likely to be strongly back-scattering, producing large values of (1-g) for the ice crystals as a whole. Other groups are starting to arrive at similar conclusions using a mixture of computer models and laboratory measurements.


Publications


van Diedenhoven, B., B. Cairns, A. M. Fridlind, A. S. Ackerman, and T. J. Garrett, 2012: Remote sensing of ice crystal asymmetry parameter using multi-directional polarization measurements – Part 2: Application to the Research Scanning Polarimeter. Atmos. Chem. Phys. (submitted).


Cooper, S. J. and T. J. Garrett, 2011: Application of infrared remote sensing to constrain in-situ estimates of ice crystal particle size during SPartICus Atmos. Meas. Tech., 4, 1593-1602


Cooper, S. J. and T. J. Garrett, 2010: Identification of small ice cloud particles using passive radiometric observations. J. Appl. Meteorol. Clim. 49, 2334–2347, doi: 10.1175/2010JAMC2466.1   


Garrett, T. J. Observational quantification of the optical properties of cirrus cloud. Chapter in Light Scattering Reviews, Vol. 3, Praxis, A. Kokhanovsky, ed., 2008


T. J. Garrett, 2007: Comments on "Effective radius of ice cloud particle populations derived from aircraft probes" J. Atmos. Oceanic. Technol. 24, 1492-1503 HTML


Garrett, T. J., M. B. Kimball, G. G. Mace, D. G. Baumgardner, 2007: Observing cirrus halos to constrain in-situ measurements of ice crystal size Atmos. Chem. Phys. Discuss, 7, 1295-1325


Garrett, T. J., B. C. Navarro, C. H. Twohy, E. J. Jensen, D. G. Baumgardner, T. P Bui, H. Gerber, R. L. Herman, A. J. Heymsfield, P. Lawson, P. Minnis, L. Nguyen, M. Poellot, S. K. Pope,  F. P. J. Valero,  and E. Weinstock 2005: Evolution of a Florida cirrus anvil, J. Atmos. Sci., 62, 2352–2372.   PDF


Garrett, T. J., H. Gerber, D. G. Baumgardner, D. G., C. H. Twohy, and E. M. Weinstock, 2003: Small, highly reflective ice crystals in low-latitude cirrus. Geophys. Res. Let., 30, 2132, doi:10.1029/2003GL018153. PDF


Garrett, T.J., P.V. Hobbs, and H. Gerber, 2001: Shortwave, single-scattering properties of arctic ice clouds, J. Geophys. Res., 106, 15,155-15,172. PDF