Properties of subvisible cirrus clouds formed by homogeneous freezing

[摘要] Number concentrations and mean sizes of ice crystals and derived microphysical and opticalproperties of subvisible cirrus clouds (SVCs) formed by homogeneous freezing of supercooledaerosols are investigated as a function of temperature and updraft speed of adiabatically ascendingair parcels. The properties of such clouds are insensitive to variations of the aerosol number andsize distribution. Based on criteria constraining the optical extinction, sedimentation time, andexistence time of SVCs, longer-lived (>10min) clouds, capable of exerting a measurableradiative or chemical impact, are generated within a narrow range of updraft speeds below1-2cm s^-1 at temperatures below about 215K, with concentrations of ice crystalsnot exceeding 0.1cm^-3. The clouds do not reach an equilibrium state because the icecrystals sediment out of the formation layer typically before the supersaturation is removed. Twoimportant conclusions emerge from this work. First, the above characteristics of SVCs may providean explanation for why SVCs are more common in the cold tropical than in the warmer midlatitudetropopause region. Second, it seems likely that a limited number (<0.1cm^-3) ofeffective heterogeneous freezing nuclei that nucleate ice below the homogeneous freezing thresholdcan control the formation and properties of SVCs, although homogeneous freezing nuclei are far moreabundant.

[学科分类] 大气科学