The dependence of ice microphysics on aerosol concentration in arctic mixed‐phase stratus clouds during ISDAC and M‐PACE

Cloud and aerosol data acquired by the National Research Council of Canada (NRC) Convair‐580 aircraft in, above, and below single‐layer arctic stratocumulus cloud during the Indirect and Semi‐Direct Aerosol Campaign (ISDAC) in April 2008 were used to test three aerosol indirect effects hypothesized to act in mixed‐phase clouds: the riming indirect effect, the glaciation indirect effect, and the thermodynamic indirect effect. The data showed a correlation of R = 0.78 between liquid drop number concentration, N liq inside cloud and ambient aerosol number concentration N PCASP below cloud. This, combined with increasing liquid water content LWC with height above cloud base and the nearly constant vertical profile of N liq , suggested that liquid drops nucleated from aerosol at cloud base. No evidence of a riming indirect effect was observed, but a strong correlation of R = 0.69 between ice crystal number concentration N i and N PCASP above cloud was noted. Increases in ice nuclei (IN) concentration with N PCASP above cloud for 2 flight dates combined with the subadiabatic LWC profiles suggest possible mixing of IN from cloud top consistent with the glaciation indirect effect. The lower N ice and lower effective radius r el for the more polluted ISDAC cases compared to data collected in cleaner single‐layer stratocumulus conditions during the Mixed‐Phase Arctic Cloud Experiment is consistent with the operation of the thermodynamic indirect effect. However, more data in a wider variety of meteorological and surface conditions, with greater variations in aerosol forcing, are required to identify the dominant aerosol forcing mechanisms in mixed‐phase arctic clouds.