Ice growth driven by the Wegener-Bergeron-Findeisen process in expansion chamber simulations: roles of ascent rate and particle sedimentation

Abstract. Mixed-phase clouds often glaciate through the Wegener–Bergeron–Findeisen process (WBF), in which ice crystals grow at the expense of supercooled droplets. In finite-volume laboratory chambers, however, WBF signatures can be obscured by sedimentation losses and limited residence time within the mixed-phase temperature range. We use a Lagrangian super-droplet configuration of the babyEULAG model to simulate an idealized 2 m-high expansion cloud chamber with condensation/immersion freezing and externally mixed soluble aerosol and dust ice-nucleating particles. We vary imposed ascent rate, soluble aerosol concentration, and gravity to examine how particle growth time and sedimentation affect WBF. WBF is evaluated using a normalized state-dependent ice-deposition diagnostic and paired EVAP and NOEVAP simulations that isolate the effect of droplet evaporation. From each pair, we calculate two response metrics: dQ_ice​​, the relative enhancement of domain-mean ice mixing ratio in EVAP compared with NOEVAP, and Δf_ice, the corresponding increase in ice mass fraction. With gravity, both metrics show a turnover with ascent rate under polluted, Seoul-like conditions: they increase from low to intermediate ascent rates as faster cooling reduces sedimentation losses, but decrease at higher ascent rates as mixed-phase residence time becomes too short for sustained depositional growth. Under lower soluble-aerosol concentrations, fewer, larger droplets sediment more efficiently, weakening the WBF response. Without gravity and particle loss, WBF decreases monotonically with ascent rate, indicating dominant residence-time control. These results highlight the importance of finite-depth particle losses when interpreting chamber-based WBF measurements.