Measurements of the deposition coefficient for small cirrus-like ice crystals

The possibility of small deposition coefficients for cirrus crystals has been suggested on the basis of past theoretical arguments and in-situ observations. Now, new measurements with ice particles electrodynamically levitated in a climate-controlled cell indicate that a significant surface-kinetic resistance to growth and evaporation indeed exists, at least when the crystals are small (10-micron maximum dimension). The ice crystals were nucleated from supercooled water droplets at temperatures between -40 and -60 °C and subsequently exposed to varying degrees of subsaturation and supersaturation (to about 20%) over experimental periods exceeding an hour. The evolving crystals were followed optically, and the changing mass was determined through repeated measurements of the electrical parameters of the levitation system as the particles approached stability boundaries of the Mathieu function. Relative changes in particle mass were determined to within +/-1% precision. A particle-scale growth model based on a proven adaptive parameterization indicated that all measured crystals grew and evaporated far more slowly than predictions based on unity mass accommodation coefficient. Owing to the small sizes of the crystals, the large surface resistance, and the long duration of the levitation, the modeled growth and evaporation rates are very sensitive to the magnitude of the deposition coefficient, even at standard atmospheric pressure. The measurements of growth and evaporation for all of the experimental crystals between -40 and -60°C can only be explained by employing deposition coefficients less than 0.01, with best estimates ranging from 0.004 to 0.008 for individual particles. It is expected that measurements with larger crystals and/or higher supersaturations would result in modestly larger deposition coefficients, but it appears likely that significant surface-kinetic resistances would still be observed. These new measurements underscore the crucial importance of including kinetic resistance to particle growth and evaporation when modeling cirrus clouds.