Mineral Dust and its Microphysical Interactions with Clouds - Mineral Dust: A Key Player in the Earth System - page 285

and vertical velocity strengths of updraft parcels (Andreae et al. 2004 ). Enhanced
aerosol concentration may also increase the lifetime and vertical extent of clouds
with important feedbacks on the hydrological cycle (Penner et al. 2006 ; Lohmann
et al. 2007 ; Kirkevag et al. 2008 ; Menon et al. 2002 ; Andreae et al. 2004 ;Rosenfeld
2006 ; Rosenfeld et al. 2008 ). Increased concentration of large aerosol, termed giant
CCN (GCCN), may act as efficient collector drops and promote the formation of
precipitation (e.g., Woodcock 1950 ; Levin and Cotton 2008 ; Cheng et al. 2009 ).
GCCN may also deplete supersaturation (the relative humidity exceeding 100 %)
in the early stages of cloud formation and reduce the concentration of droplets that
eventually form (Ghan et al. 1998 ), and strongly impact the sensitivity of droplets
to submicron aerosol variations (Morales Betancourt and Nenes 2013 ). There is no
strict size definition for what constitutes a GCCN, with sizes ranging between 1 and
10 m radius (Mechem and Kogan 2008 ; Cheng et al. 2009 ). Increased aerosol may
also enhance evaporation rates of droplets at cloud fringes, which may result in a
negative buoyancy feedback from evaporative entrainment and reduce cloudiness
(Xue and Feingold 2006 ).
Besides influencing cloud properties through nucleation of cloud droplets, some
aerosol particles are capable of nucleating ice crystals. These ice-nucleating (IN)
aerosol particles are rare in comparison with CCN with around one aerosol particle
in a million capable of nucleating ice. This rarity has important implications for
clouds. For example, in a supercooled liquid stratus cloud, only a small fraction
of cloud droplets will or can freeze, but these ice crystals will then grow at the
expense of the supercooled water droplets. Hence, the introduction of ice nuclei to
a supercooled cloud therefore increases the hydrometeor size, reduces hydrometeor
concentration, increases precipitation rates, reduces cloud lifetime, and decreases
cloud shortwave reflectivity and longwave emissivity (Lohmann and Feichter 2005 ;
Andreae et al. 2004 ;Rosenfeld 2006 ; Rosenfeld et al. 2008 ). Increases of IN
can profoundly impact cirrus clouds, as IN form ice before the more populous
supercooled haze droplets. As a consequence, the ice from IN strongly competes
for water vapor with the supercooled drops, potentially reducing or completely
inhibiting their freezing, with profound impacts on ice crystal number and size and
hence cloud lifetime and longwave emissivity (e.g., Lohmann and Feichter 2005 ;
Barahona and Nenes 2007 , 2009 ; Barahona et al. 2010 ; Liu et al. 2012 ; Murray et al.
2010 ; Karcher and Lohmann 2003 ; Ren and Mackenzie 2005 ;Jensenetal. 2010 ;
Gettelman et al. 2012 ). However, our quantitative understanding of ice formation
in clouds remains poor. Apart from the technical difficulties in making accurate
measurements of ice nuclei in the atmosphere, we lack a fundamental understanding
of what makes an effective ice-nucleating particle; as a result, a general and practical
theoretical framework in which to place laboratory and field measurements of ice
nuclei is currently lacking and severely limits our predictive ability in aerosol-ice
cloud studies. Nevertheless, several approaches to describing ice nucleation have
been put forward (e.g., Barahona 2012 ; Khvorostyanov and Curry 2005 ; Hoose
et al. 2010 ; Niedermeier et al. 2011b ; Broadley et al. 2012 ; Connolly et al. 2009 ;
Va l i 1994 ; Herbert et al. 2014 ). A number of materials have been identified as
ice nuclei including mineral dust, soot, bacteria, fungal spores, pollen, crystalline