Cloud Physics Research - Fall Speed of Small Ice Crystals
by Dr. Christopher Emersic
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).
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.