A CCN activation parameterization and its impacts on cloud and precipitation simulations

Abstract

A parameterization of cloud condensation nuclei (CCN) activation in a rising adiabatic air parcel is developed to link the activation rate to the temperature, vertical velocity, number concentrations of aerosols and cloud droplets, and radius of cloud droplets. The new parameterization considers the activation inside the cloud more precisely, by including the effects of preexisting cloud droplets on the maximum supersaturation of the air parcel. There is also a modification in the size distribution of aerosols by adding the number of CCN in cloud droplets to the potential aerosol number. The new parameterization is implemented in Thompson aerosol-aware microphysics scheme in Weather Research and Forecasting (WRF) model. An increase in the potential aerosol number leads to an increment in the cloud droplet number and consideration of the pre-existing cloud droplets on supersaturation results in the suppression of the activation process inside the cloud compared with the cloud edge. As a result, the cloud droplet number concentration is the highest in the entrance region of the cloud with the new parameterization, while the activation primarily occurs in the region of high vertical velocity with the previous parameterization. The new parameterization appears to decrease the cloud droplet number in the updraft-limited regime of CCN activation because the suppression of activation due to pre-existing cloud droplets through supersaturation is more significant than the increase in potential aerosol number. In the aerosol-limited regime, where the aerosol change is more important in activation, the new parameterization increases the cloud droplet number. In the idealized shallow cloud simulations (updraft-limited regime), a decrease in the cloud droplet number increases the cloud droplet radius, thus enhancing the rain formation. In the idealized deep cloud simulations (aerosol-limited regime), an increase in the cloud droplet number reduces the rain formation in the developing stage and an increase in the cloud droplet freezing enhances the latent heating, augmenting the vertical velocity in the mature stage. A precipitation event in the Korean Peninsula on 24 July 2014 is also simulated with climatological aerosol data. The system has both warm clouds and deep clouds with intense precipitation. The new parameterization induces a decrease in the cloud droplet number concentration, increasing the raindrop formation in the shallow cloud and decreasing the ice hydrometeor formation and the convective precipitation, as in the idealized simulations.