The Solute and Curvature Effects on the Broadening of Cloud Droplet Size Distribution

Yang, F., Brookhaven National Laboratory

Cloud Processes

Aerosol Processes

Yang F, P Kollias, R Shaw, and A Vogelmann. 2018. "Cloud droplet size distribution broadening during diffusional growth: ripening amplified by deactivation and reactivation." Atmospheric Chemistry and Physics, 18(10), 10.5194/acp-18-7313-2018.


Cloud droplet size distribution changes with time demonstrated for different conditions. Color bar is cloud droplet size distribution and y-axis is droplet radius. (a) Control with all effects: curvature, solute, activation, and deactivation. The gray region in (b)-(d) gives the full droplet size spectrum range in (a). (b) No solute or curvature effects. (c) Solute effect but no curvature effect. (d) Both solute and curvature effects but droplet deactivation and reactivation are not considered.


Cloud droplet size distribution changes with time demonstrated for different conditions. Color bar is cloud droplet size distribution and y-axis is droplet radius. (a) Control with all effects: curvature, solute, activation, and deactivation. The gray region in (b)-(d) gives the full droplet size spectrum range in (a). (b) No solute or curvature effects. (c) Solute effect but no curvature effect. (d) Both solute and curvature effects but droplet deactivation and reactivation are not considered.

Science

The size distribution of droplets in clouds is usually much wider than predicted from classical adiabatic theory. A broader size distribution benefits the collisional growth of cloud droplets, and thus allows for the rapid formation of rain. However, mechanisms that broaden the cloud droplet size distribution and lead to rain initiation are still not well understood, which limits our ability to accurately predict rainfall. Our modeling results examine the relative roles played by physical mechanisms that broaden the cloud droplet size distribution in a vertically oscillating air parcel. The broadening mechanisms involve the known size-dependent vapor deposition from small to large droplets (Ostwald ripening) amplified by the evaporation of small cloud droplets and the reactivation of the residual aerosol into cloud droplets as the air parcel nears (but does not leave) cloud base.

Impact

Physical mechanisms that broaden the cloud droplet size distribution affect cloud reflectivity and are important to rain formation. Our study examines the interplay among some key broadening mechanisms to indicate the potential need of explicitly representing the “haze state” that resides between the aerosol and cloud droplet states, as well as representing physiochemical properties of the droplets throughout their lifecycles. The study indicates that these mechanisms can generate a broader cloud droplet size distribution without necessarily requiring giant cloud condensation nuclei or mixing.

Summary

We investigate the condensational growth of cloud droplets in an adiabatic parcel using a moving-size-grid cloud parcel model. Cloud droplets are formed on polydisperse, sub-micrometer aerosol particles and the parcel undergoes vertical oscillations of prescribed configurations. The growth of all droplets considers the competition of the curvature effect, or the Kelvin effect in which evaporation increases for smaller droplets, and the solute effect, or Raoult’s law in which evaporation is suppressed by the solute concentration for smaller droplets, in vice versa. These two effects are considered before droplet activation (i.e., the “haze state”) and after droplet activation for the whole simulations, which is not typically done in models due to the computational cost. The cloud droplet size distribution can be broadened during condensation growth due to Ostwald ripening, where the size-dependent vapor deposition grows large droplets at the expense of small droplets, amplified by droplet deactivation and reactivation as the parcel nears (but does not leave) cloud base. We show that the size of the largest cloud droplet strongly depends on the time that droplet remains in the cloud and not on the variability of the in-cloud supersaturation. This is because the large number of smaller cloud droplets buffers the environmental air, where the environmental saturation ratio for an oscillating parcel is symmetric around the equilibrium saturation ratio over smaller cloud droplets. Our results suggest it is important to consider in a cloud model the curvature and solute effects on droplet growth before and after cloud droplet activation.