Measurements of the ice nucleating ability of aerosol particles in air masses over Florida having sources from North Africa support the potential importance of dust aerosols for indirectly affecting ...cloud properties and climate. The concentrations of ice nuclei within dust layers at particle sizes below 1 pn exceeded 1/cu cm; the highest ever reported with our device at temperatures warmer than homogeneous freezing conditions. These measurements add to previous direct and indirect evidence of the ice nucleation efficiency of desert dust aerosols, but also confirm their contribution to ice nuclei populations at great distances from source regions.
Measurements from the US Department of Energy Atmospheric Radiation Measurement Program's 2004 Mixed‐Phase Arctic Cloud Experiment (M‐PACE) provide a unique opportunity to study poorly understood ice ...formation processes in mixed‐phase stratocumulus. Using meteorological, aerosol, and ice nucleus measurements to initialize large‐eddy simulations with size‐resolved microphysics, we compare predicted liquid and ice mass, number, and size distribution with observations from a typical flight. We find that ambient ice nuclei appear insufficient by a few orders of magnitude to explain observed ice, consistent with past literature. We also find that two processes previously hypothesized to explain the discrepancy, shatter of freezing drops and fragmentation during ice‐ice collisions, were not significant sources of ice based on parameterizations from existing studies. After surveying other mechanisms that have been hypothesized to explain ice formation in mixed‐phase clouds generally, we find two that may be strong enough: (1) formation of ice nuclei from drop evaporation residuals, a process suggested by sparse and limited measurements to date, and (2) drop freezing during evaporation, a process suggested only by inference at this time. The first mechanism can better explain the persistence of mixed‐phase conditions in simulations of less vigorous stratus observed during the Beaufort Arctic Storms Experiment (BASE). We consider conditions under which emission of nuclei from the ocean surface or activation through cloud‐phase chemistry could provide alternative explanations for M‐PACE observations. Additional process‐oriented measurements are suggested to distinguish among ice formation mechanisms in future field studies.
A recent field experiment in southern Florida using aircraft and polarization lidar shows that mineral dust particles transported from Saharan Africa are effective ice nuclei, apparently capable of ...glaciating a mildly supercooled (−5.2° to −8.8°C) altocumulus cloud. These results are similar to those from Asian dust storm particles observed over the western US, suggesting that in the northern hemisphere major dust storms play a role in modulating climate through the indirect aerosol effect on cloud properties. If this is true of desert dusts in general, then even minor aeolian emissions could have an effect on regional weather and climate.
A new approach is described for calculating the mass (m) and terminal velocity (Vt) of ice particles from airborne and balloon-borne imaging probe data as well as its applications for remote sensing ...and modeling studies. Unlike past studies that derived these parameters from the maximum (projected) dimension (D) and habit alone, the "two-parameter approach" uses D and the particle's projected cross-sectional area (A).
Gerber et al present a study that investigates the role of homogeneous ice nucleation in strong convection and its influences on the convectively generated cirrus microphysical composition of anvils. ...Among others, they claim that in deep convective updrafts, the anvil is the depository for the small and radiatively important ice particles (homogeneously nucleated) and the large particles from below (heterogeneously or secondarily produced, or recycled).
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