|Working Group I: The Scientific Basis|
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5.3.6 Ice Formation and Indirect Forcing
Formation of ice in the atmosphere has long been recognised to be a topic of great importance due to its key role in the precipitation process. However, progress in elucidating this role has been plagued by a host of complex issues such as the precise mode of action of ice nuclei (IN) (e.g., Cooper, 1980), in situ modification of IN activity by various substrate coatings, including residual ice (Borys, 1989; Curry et al., 1990; Rosinski and Morgan, 1991; Beard, 1992; Vali, 1992), secondary ice production (e.g., Beard, 1992) and a lack of consistency in measurement techniques (cf., Bigg, 1990; Vali, 1991; Rogers, 1993; Pruppacher and Klett, 1997).
Because of these issues, it is premature to quantitatively assess the impact of ice formation on indirect forcing. Instead, the potential importance of such formation is given a preliminary assessment by addressing a set of four fundamental and serial questions whose answers would, in principle, yield the desired assessment. A summary of what is known with respect to these questions is given below.
Does ice formation have an impact on radiative forcing?
Is formation of the ice phase modulated by aerosols?
For lower level, warmer (though still supercooled) clouds, in principle the answer to our query is necessarily positive. In this vast, liquid-water reservoir, temperatures are simply too warm for homogeneous freezing of cloud drops to occur and aerosol surfaces of some sort must provide the substrate for ice initiation. However, prolific secondary ice formation due to such processes as the Hallett-Mossop mechanism renders the establishment of a clear relationship between measured IN concentrations and ice particle concentrations quite difficult (cf., Beard, 1992; Rangno and Hobbs, 1994). Nevertheless, in some instances relationships between IN concentrations and ice formation in lower-tropospheric clouds have been found (e.g., Stith et al., 1994; DeMott et al., 1996). This lends credibility to the view that the actual ice initiation process must be modulated by aerosols.
For both upper and lower level clouds in the troposphere, it seems clear that the ice initiation process is dependent on aerosols, though the nucleation process can proceed via different pathways and from a variety of different nucleating chemical species.
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