M.I.C.C.
Manchester Ice Cloud Chamber
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Fall Speed of Ice Crystals
A possible upcoming project involving the Manchester Ice Cloud Chamber (MICC) will investigate the fall
speed of small ice crystals. The rate at which an ice crystal falls through a cloud is dependent on its mass,
size and shape. The shape of ice
crystals is dependent on the
environment in which the crystals
grow, and is discussed in more
detail in the ice crystal light
scattering section. If the ice crystal
exists in an environment conducive
to growth for long enough, it can
become larger in size and more
massive (Fig. 1).
Climate models and related studies
have shown the fall speed of ice
crystals has the potential to
significantly affect the radiation
budget of the Earth and therefore is
important in predicting climate
change (Mitchell et al. 2008;
Sanderson et al. 2008). The
magnitude of the effect of crystal fall
speed is also very uncertain and
requires further study. In situations where ice crystal fall speed is low and ice remains for longer in the upper
atmosphere, the amount of water vapour in that region is increased. The increased water vapour presence
can act like a blanket and prevent long-wave radiation being emitted by the Earth from escaping into space,
enhancing warming (Fig. 2).
The theory of the fall speed of ice
crystals has been developed from
numerous laboratory studies (e.g.
Mitchell and Heymsfield, 2005;
Khvorostyanov and Curry, 2002);
however these experimental studies
concentrated on larger ice crystals
that were several hundred microns in
diameter. No experimental
confirmation of whether this theory
provides accurate fall speeds for
smaller crystals has been made and is
to be investigated in the work here.
The experimental approach to the
proposed experiments is to generate
ice crystals of various shapes and
sizes in the cloud chamber, with
emphasis on creating smaller crystals,
and sample them as they fall towards
the lower portions of the fall tube. To enable good measurements of their fall speed, a holographic probe is to
be used in which an individual particle can be tracked in three dimensions for long enough to be able to
determine its 3D velocity (Fugal et al., 2009; Fugal and Shaw, 2009; Pu et al., 2005; Sheng et al., 2007).
References
Fugal, J. P., and R. A. Shaw, 2009: Cloud particle size distributions measured with an airborne digital in-line holographic instrument. Atmos.
Meas. Tech. Discuss., 2, 659–688.
Fugal, J. P., T. J. Schulz, and R. A. Shaw, 2009: Practical methods for automated reconstruction and characterization of particles in digital in-
line holograms, submitted to Meas. Sci. Technol.
Khvorostyanov, V. I., and J. A. Curry, 2002: Fall velocities of droplets and crystals: power laws with continuous parameters over the size
spectrum. J. Atmos. Sci., 59, 1872–1884.
Mitchell, D., and A. J. Heymsfield, 2005: Refinements in the treatment of ice particle terminal velocities, highlighting aggregates. J. Atmos.
Sci., 62, 1637–1644.
Mitchell, D., P. Rasch, D. Ivanova, G. M. McFarquhar, and T. Nousiainen, 2008: Impact of small ice crystal assumptions on ice sedimentation
rates in cirrus clouds and gcm simulations. Geophys. Res. Lett., 35(doi:10.1029/2008GL033552), L09,806.
Pu, S. L., D. Allano, B. Patte-Rouland, M. Malek, D. Lebrun, and K. F. Cen, 2005: Particle field characterization by digital in-line holography:
3d location and sizing. Exp. Fluids, 39, 1–9.
Sanderson, B. M., C. Piani, W. J. Ingram, D. A. Stone, and M. R. Allen, 2008: Towards constraining climate sensitivity by linear analysis of
feedback patterns in thousands of perturbed-physics gcm simulations. Clim. Dyn., 30, 175–190.
Sheng, J., E. Malkiel, J. Katz, J. Adolf, R. Belas, and A. R. Place, 2007: Digital holographic microscopy reveals prey-induced changes in
swimming behavior of predatory dinoflagellates. Proc. Nat. Acad. Sci. USA, 104, 17,512–17,517.
Fig. 1. Smaller ice crystals with less mass and greater surface area take longer
to fall through clouds. When in the upper atmosphere, this can increase the
humidity, affecting the radiation budget of the planet and enhance warming.
Fig. 2. Ice crystal presence in the upper atmosphere increases the water vapour
content in that region, which then acts to maintain a lot of long wave radiation at
the Earth’s surface. This has a net warming effect.
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