Classification of Aerosol-Cloud Interaction Regimes over Tokyo

Previous studies on the aerosol indirect effect have demonstrated that there are three regimes (aerosol-limited, updraft-limited, and transitional regimes). However, this classification of regimes has not been widely used in field observations as it requires measurements of updraft or supersaturation near the cloud base (S-max), which are not easy to perform. In this study, we attempted to classify the regimes using the effective supersaturation (S-eff) which is relatively easy to estimate by matching cloud droplet number concentration (N-c) and supersaturation spectra of cloud condensation nuclei. Parcel model simulations were performed to examine the suitability of Seff as a proxy for S-max. Further, 35 low-level cloud cases observed around 458 m over Tokyo were classified into three regimes. In the aerosol-limited regime, N-c increased more rapidly with an increment of aerosols than in the other two regimes, indicating that the regime classification using S-eff was sufficiently robust. The aerosol-limited regime in Tokyo appeared frequently when northerly winds prevailed, indicating the impact of local pollution sources around Tokyo on the aerosol cloud interaction. The results imply that anthropogenic aerosol emissions affect the climate of Tokyo.