Ice-Phase Influence on Aerosol Susceptibility in Wintertime Marine Boundary-Layer Clouds over Northwest Pacific Ocean

Aerosol susceptibility (AS) is a parameter describing the sensitivity of cloud properties (such as increasing albedo and decreasing precipitation) to increasing aerosol concentrations. It has been shown that AS of shallow warm clouds decreases monotonically but remains positive with increasing condensation nuclei (CN) concentration. However, the presence of ice in the mixed-phase cloud may disrupt such susceptibility. We investigated the effect of ice-phase processes on AS by applying the WRF model with an aerosol-sensitive cloud microphysical scheme running in 1 km resolution. The type of low clouds investigated are the boundary-layer-topped clouds (BLTC) occurring frequently over the Northwest Pacific Ocean in winter. Aerosol sensitivity is examined by applying a wide range of CN concentration and ice nucleation rates, with the latter mimicking ice nuclei effects.

Our simulation results show that, with ice in the cloud, AS weakens and does not decrease monotonically with increasing CN concentration. The weakening of AS is due to the more efficient snow formation through the Wegener-Bergeron-Findeisen (WBF) process, which becomes stronger with more numerous cloud drops (increasing CN). Stronger snow production also enhanced graupel initiation. However, the riming growth of graupel tends to be inhibited under high CN conditions. Such a seesaw effect disrupted the monotonic trend of AS. We also found that CN and ice-phase processes can affect cloud coverage, which, in turn, gives feedback to the overall AS of all-sky albedo.