Open AccessSensitivity studies of cirrus clouds formed by heterogeneous freezing in the ECHAM GCM
Author(s) -
Lohmann U.,
Kärcher B.,
Hendricks J.
Publication year - 2004
Publication title -
journal of geophysical research: atmospheres
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.67
H-Index - 298
eISSN - 2156-2202
pISSN - 0148-0227
DOI - 10.1029/2003jd004443
Subject(s) - cirrus , ice nucleus , clear ice , atmospheric sciences , homogeneous , ice crystals , supercooling , ice cloud , precipitation , nucleation , environmental science , aerosol , sea ice growth processes , freezing point , climatology , arctic ice pack , sea ice , meteorology , geology , thermodynamics , radiative transfer , sea ice thickness , physics , antarctic sea ice , quantum mechanics
Cirrus clouds can form by homogeneous and heterogeneous ice nucleation mechanisms at temperatures below 235 K. Here we evaluate the effectiveness of heterogeneous freezing versus homogeneous freezing using a newly developed parameterization of heterogeneous freezing that is restricted to immersion freezing as the most likely pathway for heterogeneous ice formation in cirrus conditions [ Kärcher and Lohmann , 2003]. In addition to a reference simulation considering homogeneous nucleation with temperature‐dependent freezing thresholds, we discuss two idealized model experiments. We conduct a scenario that hypothetically assumes that the aerosol particles available for homogeneous freezing could act as freezing nuclei commencing freezing at 130% with respect to ice and contrast that by a scenario that only considers black carbon and mineral dust as immersion nuclei with the same freezing relative humidity of 130%. These idealized simulations serve to delimit possible climate responses. If the number of freezing nuclei is limited by the number of black carbon and dust aerosols, then heterogeneous freezing results in fewer ice crystals than formed by homogeneous freezing. These fewer ice crystals grow more readily to precipitation size and with that increase the global mean precipitation, decrease the ice water path, and trap less outgoing longwave radiation at the top of the atmosphere.