Experimental determination of the deposition coefficient of small cirrus-like ice crystals near -50°C

Several small ice particles were observed to grow and evaporate in a quadrupole electrodynamic levitation cell at temperatures between -40 and -60°C and variable supersaturation. The particle mass was traced throughout each experiment by analysis of the electrical ("springpoint") boundaries for stable levitation. Measured growth of each particle was compared with that calculated from a standard particle-growth model. The small particle sizes and long durations of the experiments provided great sensitivity to the particle-mean deposition coefficient (α_m). Suitable agreement between the measured and modeled growth histories was obtainable only upon using very small deposition coefficients: e.g., 0.0045 < α_m < 0.0075, with α_m ∼ 0.006 being the most-likely value for a representative particle growing at -50°C. This finding of such low deposition coefficients suggests, that cirrus-cloud simulations need to account for the strong influence that the surface-kinetic resistance exerts on the vapor growth of ice at atmospherically low temperatures.