Colloquium: Takanobu Yamaguchi

Uncertainty in radiative forcing associated with aerosol-cloud interactions (ACI) remains large and causes difficulty in assessing its contribution to climate change. Under-representation of marine low level clouds in global climate model significantly contributes to this uncertainty since the low clouds are the most abundant of all tropical clouds. ACI is a complicated problem. Aerosol scatters solar radiation, absorbs it, and becomes cloud condensation nuclei, which indirectly and directly influence clouds. Clouds, in turn, respond and adjust to aerosol effects and remove aerosol by precipitation. Efforts have been made to untangle ACI by utilizing various atmospheric models and satellite and in-situ observations. High resolution large eddy simulation is the most suitable modeling approach for process level understanding for low clouds. In this talk, two different ACI will be discussed with results obtained from process level numerical modeling. First, the focus will be a fast aerosol-cloud-precipitation interaction for stratocumulus-to-cumulus transition (SCT). Our large eddy simulations with prognostic cloud droplet number concentration (Nd) unveiled a new rapid SCT pathway which appeared only when Nd was predicted. Traditionally SCT is understood as a slow multi-day process and many process modeling studies with a fixed Nd did not observe the fast SCT. Second, a muted response to aerosol in trade cumulus will be discussed. In shallow cumulus clouds, cloud deepening as a dynamical response to increased Nd has recently been hypothesized to buffer the microphysical suppression of precipitation. This has been demonstrated with a process modeling study with a simple condensation, fixed Nd microphysics scheme. We revisited this hypothesis with a two-moment bin microphysics scheme. All simulations show that the aerosol effect is buffered, to first order; increased aerosol results in deeper clouds, a reduced cloud fraction, and an increase in the shortwave cloud radiative effect. In the last part of this talk, my ongoing research regarding ACI and model development to fulfill the scientific objectives will be discussed.