The deposition coefficient and its role for cirrus clouds

Abstract

The potential of homogeneous freezing nucleation to produce large numbers of ice crystals in regions of gentle updraft has been underestimated so far. Small but realistic deposition coefficients for small ice crystals lead to longer periods with relative humidity staying near the critical value for nucleation, which results in much larger numbers of ice crystals than with deposition coefficients close to unity. This mechanism allows to reproduce number densities of crystals in the 10-30 μm size range obtained from recent measurement campaigns. Potential reasons for small deposition coefficients are discussed, and new laboratory experiments to measure them are recommended.