Assessment of the corrected CCN activation parameterizations in simulating shallow cumulus using large-eddy simulations

Abstract

Cloud condensation nuclei (CCN) activation plays a crucial role in regional cloud-precipitation and global climate. However, inaccuracies in CCN activation parameterization, stemming from the presence of unactivated particles in CCN measurements that are mistakenly included in developing CCN activation parameterizations, can introduce biases in model predictions of cloud droplet number concentration, subsequently affecting cloud microphysics, precipitation initiation, and radiation. To address this issue, this study proposes correction coefficients for CCN activation parameterization using the Twomey power-law function and applies them in the large-eddy model to simulate continental shallow cumulus over the Southern Great Plains, USA. Results reveal that compared to simulations using uncorrected CCN parameterization, those using corrected parameterization decrease cloud droplet number concentration by 32.8 %, leading to an increase of cloud water autoconversion rate by 8.9 % and a decrease of cloud optical thickness by 17.3 %. This indicates a suppression of cloud-precipitation processes and an overestimation of cloud radiative cooling in the default scheme. Moreover, as aerosol loading increases, the differences between the corrected and uncorrected parameterization slightly diminish. Compared to uncorrected CCN parameterization, those using corrected parameterization exhibit stronger cloud sensitivity to aerosols, which partially mitigates the overestimation of cloud radiative cooling in the default scheme. The corrected CCN activation parameterization could help alleviate the overestimation of aerosol indirect effects, particularly in clouds with low supersaturation conditions, thereby contributing to reduced uncertainties in aerosol-cloud interaction simulations.