Height Dependency of Aerosol‐Cloud Interaction Regimes

Abstract

This study investigates the height dependency of aerosol‐cloud interaction regimes in terms of the joint dependence of the key cloud microphysical properties (e.g., cloud droplet number concentration and cloud droplet relative dispersion) on aerosol number concentration (Na) and vertical velocity (w). The three distinct regimes with different microphysical features are the aerosol‐limited regime, the updraft‐limited regime, and the transitional regime. The results reveal two new phenomena in updraft‐limited regime: (1) the “condensational broadening” of cloud droplet size distribution in contrast to the well‐known “condensational narrowing” in the aerosol‐limited regime and (2) above the level of maximum supersaturation; some cloud droplets are deactivated into interstitial aerosols in the updraft‐limited regime, whereas all droplets remain activated in the aerosol‐limited regime. Further analysis shows that the particle equilibrium supersaturation plays important role in understanding these unique features. Also examined is the height of warm rain initiation and its dependence on Na and w. The rain initiation height is found to depend primarily on either Na or w or both in different Na‐w regimes, suggesting a strong regime dependence of the second aerosol indirect effect.

Cloud droplet size distribution broadening during diffusional growth: ripening amplified by deactivation and reactivation

“…4a and c). These values are chosen to represent the conditions for clean clouds (10 2 cm −3 ) and polluted clouds (10 4 cm −3 ), which are consistent with previous studies (e.g., Xue and Feingold, 2004;Chen et al, 2018). Considering a vertical velocity of 0.5 m s −1 , they also represent the aerosollimited regime (the 10 2 cm −3 case leads to a w/n ratio of 5 × 10 −3 m s −1 cm 3 ) and the transition regime (the 10 4 cm −3 case leads to a w/n ratio of 4 × 10 −4 m s −1 cm 3 ).…”

“…The mechanism of CDSD broadening in this study requires the model to consider both solute and curvature effects all the time (i.e., before and after activation, deactivation and reactivation). Our results suggest the importance of solute and curvature effects to the deactivation and reactivation processes, which are consistent with previous studies (e.g., Andrejczuk et al, 2008;Hoffmann et al, 2015;Hoffmann, 2017;Chen et al, 2018). However the results are counter to some other studies where details of activation and deactivation are argued to be unimportant in the cloud simulation (e.g., Srivastava, 1991;Chuang et al, 1997;Grabowski et al, 2018).…”

“…Previous studies show that aerosol number concentration and vertical velocity are the two most important factors controlling cloud properties in an adiabatic cloud parcel model (e.g., McFiggans et al, 2006;Reutter et al, 2009;Chen et al, 2018). Two regimes are frequently considered: an aerosol-limited regime exists when there is an ample supply of water, and the cloud droplet number concentration is limited by the aerosol number concentration; and an updraftlimited regime exists when supersaturation is starved, and the cloud droplet number concentration is limited by the updraft velocity.…”

“…This is contrary to Çelik and Marwitz (1999), who found that supersaturation fluctuations are not responsible for CDSD broadening and the formation of large droplets. The curvature and solute effects on Ostwald ripening, activation and deactivation have been topics of study in recent years (e.g., Wood et al, 2002;Arabas and Shima, 2017;Chen et al, 2018;Sardina et al, 2018) but, to our knowledge, the relative roles of the curvature effect and solute effect on CDSD broadening have not been investigated.…”

Search for Microphysical Signatures of Stochastic Condensation in Marine Boundary Layer Clouds Using Airborne Digital Holography

“…Some in situ measurements (Lu et al, ; Zhao et al, ) show unclear signals for the relative dispersion, while many (Lu et al, , ; Miles et al, ; Pawlowska et al, ; Wang et al, ) show an increase in relative dispersion with decrease in number concentration, consistent with the findings presented here. Recent studies by Chen et al, (, , ) suggest that our flight segments could be in an updraft limited regime, and the relative dispersion should depend on the aerosol concentration. Turbulent fluctuations may lead to considerable variability in droplet number concentration thereby increasing the spectral width of the droplet size distribution, consistent with the systems theory approach by (Liu & Hallett, ; Liu et al, ).…”