Height Dependence of Aerosol-cloud Interaction Regimes

Liu, Y., Brookhaven National Laboratory

Cloud-Aerosol-Precipitation Interactions

Warm Boundary Layer Processes, Aerosol Processes

Chen J, Y Liu, M Zhang, and Y Peng. 2018. "Height Dependency of Aerosol-Cloud Interaction Regimes." Journal of Geophysical Research: Atmospheres, 123(1), doi:10.1002/2017JD027431.


Schematic illustration of the main characteristics of aerosol-cloud interaction regimes. Note that the transitional regime defined by the ϵ behavior at the level of maximum supersaturation is a single line, i.e., for a fixed set of aerosol properties other than Na. The region of transitional regime in this figure is used to schematically illustrate the range of aerosol properties.


Vertical profiles of (a) parcel supersaturation (Sp), (b) cloud droplet number concentration (Nc), and (c) cloud droplet relative dispersion (ϵ) at different ratios of aerosol number concentration (Na) to vertical velocity (w). Aerosol-cloud interaction regimes change from aerosol-limited to transitional to updraft-limited regimes as Na/w increases.


Schematic illustration of the main characteristics of aerosol-cloud interaction regimes. Note that the transitional regime defined by the ϵ behavior at the level of maximum supersaturation is a single line, i.e., for a fixed set of aerosol properties other than Na. The region of transitional regime in this figure is used to schematically illustrate the range of aerosol properties.

Vertical profiles of (a) parcel supersaturation (Sp), (b) cloud droplet number concentration (Nc), and (c) cloud droplet relative dispersion (ϵ) at different ratios of aerosol number concentration (Na) to vertical velocity (w). Aerosol-cloud interaction regimes change from aerosol-limited to transitional to updraft-limited regimes as Na/w increases.

Science

This study found that the relationships of cloud droplet concentration and spectral shape to aerosol number and updraft velocity have strong height dependence, which differs for various aerosol-cloud interaction regimes.

Impact

The finding reconciles conflicting measurements, and has important implications for understanding and representation of aerosol-cloud interactions in large-scale models, especially for an updraft-limited regime where aerosol concentration is high relative to cloud updraft velocity.

Summary

This study investigates the height dependency of aerosol-cloud interaction regimes in terms of

the joint dependence of 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 the 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.

(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 an 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 was 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.