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Clouds change climate by modulating the radiative equilibrium of the Earth (Liou 1986; Stephens et al. 1990; Zeng et al. 2009; Heymsfield et al. 2017). They also change weather by modulating atmospheric stability and thus large-scale vertical circulations (Slingo and Slingo 1988, 1991; Raymond 2000; Raymond and Zeng 2000). Hence, clouds work as a key part in both the water and energy cycles of the atmosphere.
The current atmospheric models, however, do not represent clouds well. The global models have the biases of “excessive water vapor” and “too dense clouds” (Zhang et al. 2001; Nam et al. 2012; Jiang et al. 2012, 2015). The high-resolution models have similar biases even in different regions, such as the Arctic (Klein et al. 2009; Morrison et al. 2011), the midlatitudes (Zeng et al. 2007), and the tropics (Powell et al. 2012; Zeng et al. 2013; Franklin et al. 2016). All of the models perform imperfectly in cloud representation, because they overlook some cloud processes, such as the radiative effect on (cloud) microphysics (REM).
REM leads to two daily phenomena in unsaturated air: dew and frost. When radiation cools the ground surface, water vapor condenses/deposits on the ground surface even though the air is still unsaturated. Cloud particles near cloud top, just as dew/frost forms, can grow at the expense of water vapor, suggesting REM is a candidate to mitigate the bias of excessive water vapor.
REM has been studied for about six decades. Fuchs (1959) first introduced a radiative term into the diffusional drop growth equation by treating the atmosphere as a blackbody, and then concluded that REM is negligible except for large drops. In fact, the atmosphere is quasi-transparent for infrared radiation (IR) with wavelengths of 8–12 μm. Hence, REM is important in thin clouds or near the edge of thick clouds, which motivated the later studies (Heymsfield 1973; Roach 1976; Hall and Pruppacher 1976; Stephens 1983; Wu et al. 2000; Lebo et al. 2008; Zeng 2008, 2018a,b; Brewster et al. 2020).
REM was previously neglected partly because it is complicated for radiation transfer and ice crystal habit and orientation. Mathematically, it is a process in a geometric space with 10 dimensions (i.e., 4 for space and time; 2 for radiation direction; 1 for radiation wavelength;...