CCN Spectral Shape and Cumulus Cloud and Drizzle Microphysics

Abstract

The shapes of high-resolution cloud condensation nucleus (CCN) spectra are compared with cloud and precipitation characteristics observed in the Ice in Clouds Experiment-Tropical. These high-resolution spectra often revealed bimodality that is usually caused by in-cloud processing. Bimodal CCN spectra were associated with clouds that had a third of the droplet concentrations, twice as broad droplet spectra and 2 to 3 orders of magnitude more drizzle than clouds associated with unimodal CCN spectra. These findings are opposite of a similar earlier study of stratus clouds that suggested enhancement of the indirect aerosol effect. The observations were consistent over the full range of cloud droplet liquid water content (LWC c ) thresholds and bands. But the largest differences between clouds associated with bimodal and unimodal CCN were at intermediate LWC c bins where drizzle and droplet spectral width was greatest in the clouds associated with bimodal CCN. Key Points:• Bimodal CCN were associated with lower cloud droplet concentrations • Bimodal CCN were associated with greater drizzle • Intermediate cloud droplet liquid water bins contained the most drizzle and the broadest cloud droplet spectraSupporting Information:• Supporting Information S1

Cumulus Cloud and Drizzle Microphysics Relationships With Complete CCN Spectra

“…These results indicate that larger cloud droplets had grown on lower S c CCN and these droplets were more likely to undergo autoconversion and coalescence to larger droplets and ultimately to drizzle. Drizzle should not increase low S CCN concentrations because drizzle should preferentially remove by collection larger cloud droplets that had more likely grown on low S c CCN (Hudson & Noble, 2020; Yum & Hudson, 2001). Combining the two good drizzle altitudes is appropriate because these altitudes are adjacent.…”

“…Many papers have demonstrated effects of CCN on clouds (Fitzgerald & Spyers‐Duran, 1973; Guibert et al., 2003; Hudson, 1983; Hudson & Li, 1995; Hudson & Mishra, 2007; Hudson & Noble, 2009, 2014a, 2014b; Hudson & Noble, 2020; hereafter HN20; Hudson & Svensson, 1995; Hudson et al., 1998, 2009, 2010a, 2010b, 2012, 2015, hereafter H15; Hudson et al., 2018, hereafter H18; Hudson & Yum, 2001, 2002; Leaitch et al., 1996; Rosenfeld et al., 2008; Snider & Brenguier, 2000; Snider et al., 2003, 2017, Squires, 1958; Twomey, 1959; Twomey & Warner, 1967; Warner, 1969; Yum & Hudson, 2001, 2002, 2004; Yum et al., 1998). And many papers have demonstrated effects of clouds on atmospheric aerosol (Clarke et al., 1996, 1998; 1999, 2004; 2013; Covert et al., 1996; Hoffmann, 1993; Hoppel et al., 1985, 1986, 1990, 1994, 1996; Jensen et al., 1996; Kleinman et al., 2012; Modini et al., 2015; Noble & Hudson, 2019; Tomlinson et al., 2007; Tunved et al., 2003; Van Dingenen et al., 1995; Weber et al., 1997; Weingartner et al., 1999).…”

CCN Spectral Modality Compared to Droplet Spectra and Drizzle in RICO Cumuli

“…CCN modality characterizes the entirety of CCN spectra. The present study is thus analogous to Hudson and Noble (2020; hereafter HN20), which considered CCN modality comparisons with cloud and drizzle microphysics in another low‐altitude warm (non‐freezing) maritime small cumulus study of the Ice in Clouds Experiment‐Tropical (ICE‐T) project. Because of the common subject of CCN modality and analyses of the same RICO project, much of this and the next section were also presented in HN21.…”

A Note on Aerosol Processing by Droplet Collision‐Coalescence

“…Furthermore, cloud droplets can collect unactivated aerosol particles by Brownian capture (e.g., Svenningsson et al, 1997). The subject of this study will, however, be the collision and subsequent coalescence of cloud droplets, merging the dissolved aerosol masses inside them to form larger particles upon evaporation (e.g., Flossmann et al, 1985;Hudson & Noble, 2020Noble & Hudson, 2019). As these processes shape the aerosol size distribution, they alter the ability of aerosol particles to activate to cloud droplets and to act as drizzle embryos, and thus feed back on the aforementioned effects of aerosol particles on clouds and the climate.Fundamentally, the aerosol size distribution is defined asdescribing the number of aerosol particles exhibiting sizes in an infinitesimal aerosol radius range between r a and r a + dr a , where N a is the cumulative aerosol concentration.…”