Satellite based study on the role of Cloud Condensation Nuclei (CCN) and atmospheric thermodynamics in warm cloud evolution

The study presents observational evidence for the impact of Cloud Condensation Nuclei (CCN) concentration and atmospheric thermodynamic conditions on the diffusion growth and collision-coalescence processes in warm cloud evolution. We used multi-band optical data from the Visible Infrared Imaging Radiometer Suite (VIIRS) on board the Suomi National Polar-orbiting Partnership (NPP) satellite (2013–2018) to identify the convective cloud clusters and examine the warm cloud evolution under different thermodynamic and pollution conditions based on cloud top temperature (T) - cloud drop effective radius (r_e) relationships. Analysis revealed that diffusion growth is predominant under polluted and stable (Estimated Inversion Strength (EIS) > 1 K) atmospheric conditions. In the unstable regime (EIS < 1 K), relative dispersion (ϵ) reaches its maximum (∼0.39) at a CCN concentration of around 1000 cm⁻³ and supersaturation remains higher at lower CCN concentrations and decreases rapidly beyond ∼1000 cm⁻³ which marks the transitional point between the aerosol-limited and updraft-limited regimes. However in the stable regime (EIS > 1 K), ϵ remains relatively low (between 0.10 and 0.25) with low supersaturation. In unstable conditions the estimated cloud top entrainment index (ECTEI) is found to be −5.6 ± 1.5 indicating more favourable conditions for entrainment, while in stable conditions ECTEI is found to be −2 ± 3.3 reflecting entrainment suppression. Although the Twomey effect is found to be predominant under stable atmospheric conditions, a nonlinear relationship exists between droplet growth and CCN concentration. In the aerosol limited regime, an increase in growth rate ranging from ∼0.0026 $\mu$m/s to 0.0032 $\mu$m/s is observed while at high CCN concentrations (above ∼1000 cm⁻³) growth rates decreases to 0.0025 $\mu$m/s. These observations can help refine aerosol–cloud interaction (ACI) parameterizations in climate models, thereby reducing uncertainties in the estimation of aerosol effective radiative forcing under a warming climate.